Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press ({OUP})  

<jats:title>Abstract</jats:title>
<jats:p>We report on the determination of the mass of TOI-519 b, a transiting substellar object around a mid-M dwarf. We carried out radial velocity measurements using Subaru/InfraRed Doppler (IRD), revealing that TOI-519 b is a planet with a mass of $0.463^{+0.082}_{-0.088}\, M_{\rm Jup}$. We also found that the host star is metal rich ([Fe/H] = 0.27 ± 0.09 dex) and has the lowest effective temperature (Teff = 3322 ± 49 K) among all stars hosting known close-in giant planets based on the IRD spectra and mid-resolution infrared spectra obtained with NASA Infrared Telescope Facility/SpeX. The core mass of TOI-519 b inferred from a thermal evolution model ranges from 0 to ∼30 M⊕, which can be explained by both core accretion and disk instability models as the formation origins of this planet. However, TOI-519 is in line with the emerging trend that M dwarfs with close-in giant planets tend to have high metallicity, which may indicate that they formed in the core accretion model. The system is also consistent with the potential trend that close-in giant planets around M dwarfs tend to be less massive than those around FGK dwarfs.</jats:p>

DOI： 10.1093/pasj/psad031

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Publishing type：Research paper (scientific journal)  

Context. Exoplanets with orbital periods of less than one day are known as ultra-short period (USP) planets. They are relatively rare products of planetary formation and evolution processes, but especially favourable for characterisation with current planet detection methods. At the time of writing, 125 USP planets have already been confirmed. Aims. Our aim is to validate the planetary nature of two new transiting planet candidates around M dwarfs announced by the NASA Transiting Exoplanet Survey Satellite (TESS), registered as TESS Objects of Interest (TOIs) TOI-1442.01 and TOI-2445.01. Methods. We used TESS data, ground-based photometric light curves, and Subaru/IRD spectrograph radial velocity (RV) measurements to validate both planetary candidates and to establish their physical properties. Results. TOI-1442 b is a validated exoplanet with an orbital period of P = 0.4090682 ± 0.0000004 day, a radius of Rp = 1.15 ± 0.06 R☉, and equilibrium temperature of Tp,eq = 1357+−4942 K. TOI-2445 b is also validated with an orbital period of P = 0.3711286 ± 0.0000004 day, a radius of Rp = 1.33 ± 0.09 R☉, and equilibrium temperature of Tp,eq = 1330+−6156 K. Their physical properties align with current empirical trends and formation theories of USP planets. Based on the RV measurements, we set 3σ upper mass limits of 8 M☉ and 20 M☉, thus confirming the non-stellar, sub-Jovian nature of both transiting objects. More RV measurements will be needed to constrain the planetary masses and mean densities, and the predicted presence of outer planetary companions. These targets extend the small sample of USP planets orbiting around M dwarfs up to 21 members. They are also among the 20 most suitable terrestrial planets for atmospheric characterisation via secondary eclipse with the James Webb Space Telescope, according to a widespread emission spectroscopy metric.

DOI： 10.1051/0004-6361/202243592

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Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

The Subaru telescope is currently performing a strategic program (SSP) using the high-precision near-infrared (NIR) spectrometer IRD to search for exoplanets around nearby mid/late M dwarfs via radial velocity (RV) monitoring. As part of the observing strategy for the exoplanet survey, signatures of massive companions such as RV trends are used to reduce the priority of those stars. However, this RV information remains useful for studying the stellar multiplicity of nearby M dwarfs. To search for companions around such “deprioritized” M dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 with pyramid wave-front sensing at NIR wavelengths, leading to high sensitivity to substellar-mass companions within a few arcseconds. We detected two new companions (LSPM J1002+1459 B and LSPM J2204+1505 B) and two new candidates that are likely companions (LSPM J0825+6902 B and LSPM J1645+0444 B), as well as one known companion. Including two known companions resolved by the IRD fiber injection module camera, we detected seven (four new) companions at projected separations between ∼2 and 20 au in total. A comparison of the colors with the spectral library suggests that LSPM J2204+1505 B and LSPM J0825+6902 B are located at the boundary between late M and early L spectral types. Our deep high-contrast imaging for targets where no bright companions were resolved did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions (∼10–75 M_Jup) at 10 au or further. The failure with Keck/NIRC2 around the IRD-SSP stars having significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with the James Webb Space Telescope to search for smaller-mass companions below the NIRC2 detection limits.

DOI： 10.3847/1538-3881/acbf37

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Other Link： https://iopscience.iop.org/article/10.3847/1538-3881/acbf37/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

<jats:title>Abstract</jats:title>
<jats:p>We report on the discovery of an Earth-sized transiting planet (<jats:italic>R</jats:italic>
<jats:sub>
<jats:italic>p</jats:italic>
</jats:sub> = 1.015 ± 0.051 <jats:italic>R</jats:italic>
<jats:sub>⊕</jats:sub>) in a <jats:italic>P</jats:italic> = 4.02 day orbit around K2-415 (EPIC 211414619), an M5V star at 22 pc. The planet candidate was first identified by analyzing the light-curve data obtained by the K2 mission, and it is here shown to exist in the most recent data from TESS. Combining the light curves with the data secured by our follow-up observations, including high-resolution imaging and near-infrared spectroscopy with IRD, we rule out false-positive scenarios, finding a low false-positive probability of 2 × 10<jats:sup>−4</jats:sup>. Based on IRD’s radial velocities of K2-415, which were sparsely taken over three years, we obtain a planet mass of 3.0 ± 2.7 <jats:italic>M</jats:italic>
<jats:sub>⊕</jats:sub> (<jats:italic>M</jats:italic>
<jats:sub>
<jats:italic>p</jats:italic>
</jats:sub> &lt; 7.5 <jats:italic>M</jats:italic>
<jats:sub>⊕</jats:sub> at 95% confidence) for K2-415b. Being one of the lowest-mass stars (≈0.16 <jats:italic>M</jats:italic>
<jats:sub>⊙</jats:sub>) known to host an Earth-sized transiting planet, K2-415 will be an interesting target for further follow-up observations, including additional radial velocity monitoring and transit spectroscopy.</jats:p>

DOI： 10.3847/1538-3881/acb7e1

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Other Link： https://iopscience.iop.org/article/10.3847/1538-3881/acb7e1/pdf

Publishing type：Research paper (scientific journal)  

Remote sensing of the Earth has demonstrated that photosynthesis is traceable as the vegetation red edge (VRE), which is a steep rise in the reflection spectrum of vegetation, and as solar-induced fluorescence. This study examines the detectability of biological fluorescence from two types of photosynthetic pigments, chlorophylls (Chls) and bacteriochlorophylls (BChls), on Earthlike planets with oxygen-rich/poor and anoxic atmospheres around the Sun and M dwarfs. Atmospheric absorption, such as H2O, CH4, O2, and O3, and the VRE obscure the fluorescence emissions from Chls and BChls. We find that the BChl-based fluorescence for wavelengths of 1000-1100 nm, assuming the spectrum of BChl b-bearing purple bacteria, could provide a suitable biosignature, but only in the absence of water cloud coverage or other strong absorbers near 1000 nm. The Chl fluorescence is weaker for several reasons, e.g., spectral blending with the VRE. The apparent reflectance excess is greatly increased in both the Chl and BChl cases around TRAPPIST-1, due to the fluorescence and stellar absorption lines. This could be a promising feature for detecting the fluorescence around ultracool red dwarfs using follow-up ground-based observations at high spectral resolution; however, this would require a long time around Sunlike stars, even for a LUVOIR-like space mission. Moreover, the simultaneous detection of fluorescence and the VRE is the key to identifying traces of photosynthesis, because absorption, reflectance, and fluorescence are physically connected. For further validation of the fluorescence detection, the nonlinear response of biological fluorescence as a function of light intensity could be considered.

DOI： 10.3847/1538-4357/aca3a5

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Publishing type：Research paper (scientific journal)   Publisher：EDP Sciences  

Context. In the age of JWST, temperate terrestrial exoplanets transiting nearby late-type M dwarfs provide unique opportunities for characterising their atmospheres, as well as searching for biosignature gases. In this context, the benchmark TRAPPIST-1 planetary system has garnered the interest of a broad scientific community.

Aims. We report here the discovery and validation of two temperate super-Earths transiting LP 890-9 (TOI-4306, SPECULOOS-2), a relatively low-activity nearby (32 pc) M6V star. The inner planet, LP 890-9 b, was first detected by TESS (and identified as TOI-4306.01) based on four sectors of data. Intensive photometric monitoring of the system with the SPECULOOS Southern Observatory then led to the discovery of a second outer transiting planet, LP 890-9 c (also identified as SPECULOOS-2 c), previously undetected by TESS. The orbital period of this second planet was later confirmed by MuSCAT3 follow-up observations.

Methods. We first inferred the properties of the host star by analyzing its Lick/Kast optical and IRTF/SpeX near-infrared spectra, as well as its broadband spectral energy distribution, and Gaia parallax. We then derived the properties of the two planets by modelling multi-colour transit photometry from TESS, SPECULOOS-South, MuSCAT3, ExTrA, TRAPPIST-South, and SAINT-EX. Archival imaging, Gemini-South/Zorro high-resolution imaging, and Subaru/IRD radial velocities also support our planetary interpretation.

Results. With a mass of 0.118 ± 0.002 M_⊙, a radius of 0.1556 ± 0.0086 R_⊙, and an effective temperature of 2850 ± 75 K, LP 890-9 is the second-coolest star found to host planets, after TRAPPIST-1. The inner planet has an orbital period of 2.73 d, a radius of 1.320 _−0.027^+0.053R_⊕, and receives an incident stellar flux of 4.09 ± 0.12 S_⊕. The outer planet has a similar size of 1.367 _−0.039^+0.055R_⊕ and an orbital period of 8.46 d. With an incident stellar flux of 0.906 ± 0.026 S_⊕, it is located within the conservative habitable zone, very close to its inner limit (runaway greenhouse). Although the masses of the two planets remain to be measured, we estimated their potential for atmospheric characterisation via transmission spectroscopy using a mass-radius relationship and found that, after the TRAPPIST-1 planets, LP 890-9 c is the second-most favourable habitable-zone terrestrial planet known so far (assuming for this comparison a similar atmosphere for all planets).

Conclusions. The discovery of this remarkable system offers another rare opportunity to study temperate terrestrial planets around our smallest and coolest neighbours.

DOI： 10.1051/0004-6361/202244041

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

We report the near-infrared radial velocity (RV) discovery of a super-Earth planet on a 10.77 d orbit around the M4.5 dwarf Ross 508 (J(mag) = 9.1). Using precision RVs from the Subaru Telescope IRD (InfraRed Doppler) instrument, we derive a semi-amplitude of 3.92(-0.58)(+0.60) m s(-1), corresponding to a planet with a minimum mass msin i = 4.00(-0.55)(+0.53) M-circle plus. We find no evidence of significant signals at the detected period in spectroscopic stellar activity indicators or MEarth photometry. The planet, Ross 508 b, has a semi-major axis of 0.05366(-0.00049)(+0.00056) au. This gives an orbit-averaged insolation of approximate to 1.4 times the Earth's value, placing Ross 508 b near the inner edge of its star's habitable zone. We have explored the possibility that the planet has a high eccentricity and its host is accompanied by an additional unconfirmed companion on a wide orbit. Our discovery demonstrates that the near-infrared RV search can play a crucial role in finding a low-mass planet around cool M dwarfs like Ross 508.

DOI： 10.1093/pasj/psac044

Web of Science

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

We present the direct-imaging discovery of a substellar companion in orbit around a Sun-like star member of the Hyades open cluster. So far, no other substellar companions have been unambiguously confirmed via direct imaging around main-sequence stars in Hyades. The star HIP 21152 is an accelerating star as identified by the astrometry from the Gaia and Hipparcos satellites. We detected the companion, HIP 21152 B, in multiple epochs using the high-contrast imaging from SCExAO/CHARIS and Keck/NIRC2. We also obtained the stellar radial-velocity data from the Okayama 188 cm telescope. The CHARIS spectroscopy reveals that HIP 21152 B’s spectrum is consistent with the L/T transition, best fit by an early T dwarf. Our orbit modeling determines the semimajor axis and the dynamical mass of HIP 21152 B to be 17.5${}_{-3.8}^{+7.2}$ au and 27.8${}_{-5.4}^{+8.4}$M_Jup, respectively. The mass ratio of HIP 21152 B relative to its host is ≈2%, near the planet/brown dwarf boundary suggested by recent surveys. Mass estimates inferred from luminosity-evolution models are slightly higher (33–42 M_Jup). With a dynamical mass and a well-constrained age due to the system’s Hyades membership, HIP 21152 B will become a critical benchmark in understanding the formation, evolution, and atmosphere of a substellar object as a function of mass and age. Our discovery is yet another key proof of concept for using precision astrometry to select direct-imaging targets.

DOI： 10.3847/2041-8213/ac772f

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

We present the discovery and validation of a temperate sub-Neptune around the nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of 3.09 ± 0.11 R_⊕ and an orbital period of 2.5 days, using a combination of Transiting Exoplanets Survey Satellite (TESS) and follow-up observations using ground-based telescopes. Joint analysis of multiband photometry from TESS, Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT), MuSCAT3, Sinistro, and KeplerCam confirmed the transit signal to be achromatic as well as refined the orbital ephemeris. High-resolution imaging with Gemini/’Alopeke and high-resolution spectroscopy with the Subaru InfraRed Doppler (IRD) confirmed that there are no stellar companions or background sources to the star. The spectroscopic observations with IRD and Infrared Telescope Facility SpeX were used to determine the stellar parameters, and it was found that the host star is an M4 dwarf with an effective temperature of T_eff = 3185 ± 76 K and a metallicity of [Fe/H] = 0.336 ± 0.060 dex. The radial velocities measured from IRD set a 2σ upper limit on the planetary mass to be 48.8 M_⊕. The large radius ratio (R_p/R_⋆ ∼ 0.1) and the relatively bright near-infrared magnitude (J = 12.2 mag) make this planet an attractive target for further follow-up observations. TOI-1696 b is one of the planets belonging to the Neptunian desert with the highest transmission spectroscopy metric discovered to date, making it an interesting candidate for atmospheric characterizations with JWST.

DOI： 10.3847/1538-3881/ac6bf8

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

The increasing number of super-Earths close to their host stars have revealed a scarcity of close-in small planets with 1.5–2.0 R_⊕ in the radius distribution of Kepler planets. The atmospheric escape of super-Earths by photoevaporation can explain the origin of the observed “radius gap.” Many theoretical studies have considered the in situ mass loss of a close-in planet. Planets that undergo atmospheric escape, however, move outward due to the change in the orbital angular momentum of their star–planet systems. In this study, we calculate the orbital evolution of an evaporating super-Earth with a H₂/He atmosphere around FGKM-type stars under stellar X-ray and extreme-UV irradiation (XUV). The rate of increase in the orbital radius of an evaporating planet is approximately proportional to that of the atmospheric mass loss during a high stellar XUV phase. We show that super-Earths with a rocky core of ≲10 M_⊕ and a H₂/He atmosphere at ≲0.03–0.1 au (≲0.01–0.03 au) around G-type stars (M-type stars) are prone to outward migration driven by photoevaporation. Although the changes in the orbits of the planets would be small, they would rearrange the orbital configurations of compact, multiplanet systems, such as the TRAPPIST-1 system. We also find that the radius gap and the so-called “Neptune desert” in the observed population of close-in planets around FGK-type stars still appear in our simulations. On the other hand, the observed planet population around M-type stars can be reproduced only by a high stellar XUV luminosity model.

DOI： 10.3847/1538-4357/ac558c

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：arXiv e-prints  

The NASA space telescope $TESS$ is currently in the extended mission of its all-sky search for new transiting planets. Of the thousands of candidates that TESS is expected to deliver, transiting planets orbiting nearby M dwarfs are particularly interesting targets since they provide a great opportunity to characterize their atmospheres by transmission spectroscopy. We aim to validate and characterize the new sub-Neptune sized planet candidate TOI-2136.01 orbiting a nearby M dwarf ($d = 33.36 \pm 0.02$ pc, $T_{eff} = 3373 \pm 108$ K) with an orbital period of 7.852 days. We use TESS data, ground-based multi-color photometry, and radial velocity measurements with the InfraRed Doppler (IRD) instrument on the Subaru Telescope to validate the planetary nature of TOI-2136.01 and estimate the stellar and planetary parameters. We also conduct high-resolution transmission spectroscopy to search for helium in its atmosphere. We confirmed that TOI-2136.01 (now named as TOI-2136b) is a bona fide planet with a planetary radius of $R_p = 2.2 \pm 0.07$ $R_{Earth}$ and a mass of $M_p = 4.7^{+3.1}_{-2.6}$ $M_{Earth}$. We also search for helium 10830 Å absorption lines and place an upper limit on the equivalent width of $&lt;$ 7.8 mÅ (95 % confidence) and on the absorption signal of $&lt;$ 1.44 % (95 % confidence). TOI-2136b is a sub-Neptune transiting a nearby and bright star (J=10.8) and is a potential hycean planet, making it an excellent target for atmospheric studies to understand the formation, evolution, and habitability of the small planets....

DOI： 10.1051/0004-6361/202243381

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing Ltd  

Detailed chemical analyses of M dwarfs are scarce but necessary to constrain the formation environment and internal structure of planets being found around them. We present elemental abundances of 13 M dwarfs (2900 < T (eff) < 3500 K) observed in the Subaru/IRD planet search project. They are mid- to late-M dwarfs whose abundance of individual elements has not been well studied. We use the high-resolution (similar to 70,000) near-infrared (970-1750 nm) spectra to measure the abundances of Na, Mg, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Sr by the line-by-line analysis based on model atmospheres, with typical errors ranging from 0.2 dex for [Fe/H] to 0.3-0.4 dex for other [X/H]. We measure radial velocities from the spectra and combine them with Gaia astrometry to calculate the Galactocentric space velocities UVW. The resulting [Fe/H] values agree with previous estimates based on medium-resolution K-band spectroscopy, showing a wide distribution of metallicity (-0.6 < [Fe/H] < +0.4). The abundance ratios of individual elements [X/Fe] are generally aligned with the solar values in all targets. While the [X/Fe] distributions are comparable to those of nearby FGK stars, most of which belong to the thin-disk population, the most metal-poor object, GJ 699, could be a thick-disk star. The UVW velocities also support this. The results raise the prospect that near-infrared spectra of M dwarfs obtained in the planet search projects can be used to grasp the trend of elemental abundances and the Galactic stellar population of nearby M dwarfs.

DOI： 10.3847/1538-3881/ac3ee0

Web of Science

arXiv

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Publisher：arXiv e-prints  

Context. Exoplanets with orbital periods of less than one day are know as Ultra-short period (USP) planets. They are relatively rare products of planetary formation and evolution processes, but especially favourable to current planet detection methods. At the time of writing, 120 USP planets have already been confirmed. Aims. We aim to confirm the planetary nature of two new transiting planet candidates announced by the NASA's Transiting Exoplanet Survey Satellite (TESS), registered as TESS Objects of Interest (TOIs) TOI-1442.01 and TOI-2445.01. Methods. We use the TESS data, ground-based photometric light-curves and Subaru/IRD spectrograph radial velocity (RV) measurements to validate both planetary candidates and to establish their physical properties. Results. TOI-1442 b is a hot super-Earth with an orbital period of $P = 0.4090682 \pm 0.0000004 \, d$, a radius of $R_{\mathrm{p } } = 1.15 \pm 0.06 \, R_{\oplus}$, equilibrium temperature of $T_{\mathrm{p,eq } } = 1357_{-42}^{+49} \, K$, and a mass $M_{\mathrm{p } } &lt; 18 \, M_{\oplus}$ at 3$\sigma$. TOI-2445 b is also a hot super-Earth/mini-Neptune with an orbital period of $P = 0.3711286 \pm 0.0000004 \, d$, a radius of $R_{\mathrm{p } } = 1.33 \pm 0.09 \, R_{\oplus}$, equilibrium temperature of $T_{\mathrm{p,eq } } = 1330_{-56}^{+61} \, K$, and a mass $M_{\mathrm{p } } &lt; 38 \, M_{\oplus}$ at 3$\sigma$. Their physical properties align with current empirical trends and formation theories of USP planets. More RV measurements will be useful to constrain the planetary masses and mean densities, as well as the predicted presence of outer planetary companions....

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

<title>Abstract</title>
We report the discovery of TOI-2285b, a sub-Neptune-sized planet transiting a nearby (42 pc) M dwarf with a period of 27.3 d. We identified the transit signal from the Transiting Exoplanet Survey Satellite photometric data, which we confirmed with ground-based photometric observations using the multiband imagers MuSCAT2 and MuSCAT3. Combining these data with other follow-up observations including high-resolution spectroscopy with the Tillinghast Reflector Echelle Spectrograph, high-resolution imaging with the SPeckle Polarimeter, and radial velocity (RV) measurements with the InfraRed Doppler instrument, we find that the planet has a radius of $1.74 \pm 0.08\, R_\oplus$, a mass of $\lt \!\!19.5\,M_\oplus$ ($95\%$ c.l.), and an insolation flux of 1.54 ± 0.14 times that of the Earth. Although the planet resides just outside the habitable zone for a rocky planet, if the planet harbors an H2O layer under a hydrogen-rich atmosphere, then liquid water could exist on the surface of the H2O layer depending on the planetary mass and water mass fraction. The bright host star in the near-infrared (Ks = 9.0) makes this planet an excellent target for further RV and atmospheric observations to improve our understanding of the composition, formation, and habitability of sub-Neptune-sized planets.

DOI： 10.1093/pasj/psab106

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: J = 9.5-9.6) hosting ultra-short-period (USP) planets identified by the TESS mission. The two stars are similar in temperature, mass, and radius (T_eff ≍ 3500 K, M_⋆ ≍ 0.45-0.46 M_⊙, and R_⋆ ≍ 0.45-0.46 R_⊙), and the planets are both super-Earth size (1.25 R_⊕ &lt; R_p &lt; 2.0 R_⊕). For both systems, light curves from ground-based photometry exhibit planetary transits, whose depths are consistent with those from the TESS photometry. We also refine the transit ephemerides based on the ground-based photometry, finding the orbital periods of P = 0.9893436 ± 0.0000020 days and P = 0.6691416 ± 0.0000019 days for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using the InfraRed Doppler (IRD) instrument on the Subaru 8.2 m telescope, we confirm the planetary nature of the TOIs and measure their masses: 10.14 ± 0.95 M_⊕ and 3.43 ± 0.93 M_⊕ for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model. Combining those with the planet radii of R_p = 1.749 ± 0.079 R_⊕ (TOI-1634b) and 1.459 ± 0.065 R_⊕ (TOI-1685b), we find that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets. TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of discussing the size limit of rocky planet cores as well as testing the formation scenarios for USP planets. Excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities. * Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan....

DOI： 10.3847/1538-3881/ac0fdc

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

We obtained high-resolution spectra of the ultracool M-dwarf TRAPPIST-1 during the transit of its planet "b" using two high-dispersion near-infrared spectrographs, the Infrared Doppler (IRD) instrument on the Subaru 8.2m telescope, and the Habitable Zone Planet Finder (HPF) instrument on the 10 m Hobby-Eberly Telescope. These spectroscopic observations are complemented by a photometric transit observation for planet "b" using the APO/ARCTIC, which assisted us in capturing the correct transit times for our transit spectroscopy. Using the data obtained by the new IRD and HPF observations, as well as the prior transit observations of planets "b," "e" and "f" from IRD, we attempt to constrain the atmospheric escape of the planet using the He I triplet 10830 Å absorption line. We do not detect evidence for any primordial extended H-He atmospheres in all three planets. To limit any planet-related absorption, we place an upper limit on the equivalent widths of &lt;7.754 mÅ for planet "b," &lt;10.458 mÅ for planet "e," &lt;4.143 mÅ for planet "f" at 95% confidence from the IRD data, and &lt;3.467 mÅ for planet "b" at 95% confidence from HPF data. Using these limits along with a solar-like composition isothermal Parker wind model, we attempt to constrain the mass-loss rates for the three planets. For TRAPPIST-1b, our models exclude the highest possible energy-limited rate for a wind temperature &lt;5000 K. This nondetection of extended atmospheres with low mean-molecular weights in all three planets aids in further constraining their atmospheric composition by steering the focus toward the search of high-molecular-weight species in their atmospheres. ^*Based on data collected at Subaru Telescope, operated by the National Astronomical Observatory of Japan, Hobby-Eberly Telescope operated by The University of Texas McDonald Observatory, and ARC 3.5m Telescope at Apache Point Observatory....

DOI： 10.3847/1538-3881/ac0d57

arXiv

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Publishing type：Research paper (scientific journal)  

Seven Earth-sized planets orbiting an ultracool dwarf TRAPPIST-1, two or three of which are potentially habitable, were discovered in 2017. Adjacent pairs of planets around TRAPPIST-1 are locked into a（near-）mean-motion resonance. The mass-radius relation of TRAPPIST-1 planets suggests that they are likely to be rocky. Earth-sized planets around TRAPPIST-1 can be good sample for a better understanding of the orbital evolution and formation of terrestrial planets around a low-mass star. Also, transmission spectroscopy in the upper atmosphere of the TRAPPIST-1 planets reveals that they may have either no atmospheres or tenuous secondary atmospheres, such as volcanic gases. TRAPPIST-1 planets are promising follow-up objects for the James-Webb space telescope, which is scheduled for launch in 2021, toward atmospheric characterization of terrestrial planets outside the Solar System. In this article, I review our current understanding of the planetary system around TRAPPIST-1....

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Publishing type：Research paper (scientific journal)   Publisher：EDP Sciences  

<italic>Context.</italic> According to planetary interior models, some giant planets contain large metal masses with large metal-mass fractions. HD 149026b and TOI-849b are characteristic examples of these giant planets. It has been suggested that the envelope mass loss during giant impacts plays a key role in the formation of such giant planets.


<italic>Aims.</italic> The aim of the present Letter is to propose a mechanism that can explain the origin of such giant planets.


<italic>Methods.</italic> We investigate the formation of giant planets in a rapidly dissipating disk using <italic>N</italic>-body simulations that consider pebble accretion.


<italic>Results.</italic> The results show that although the pebble isolation mass is smaller than the metal mass (≳30 Earth masses) in some giant planets, the interior metal mass can be increased by giant impacts between planets with the isolation mass. Regarding the metal fraction, the cores accrete massive envelopes by runaway gas accretion during the disk-dissipation phase of 1−10 Myr in a disk that evolves without photoevaporation. Although a large fraction of the envelope can be lost during giant impacts, the planets can reaccrete the envelope after impacts in a slowly dissipating disk. Here, we demonstrate that, by photoevaporation in a rapidly dissipating disk, the runaway gas accretion is quenched in the middle, resulting in the formation of giant planets with large metal-mass fractions.


<italic>Conclusions.</italic> The origins of HD 149026b and TOI-849b, which are characterized by their large metal-mass fractions, can be naturally explained by a model that considers a disk evolving with photoevaporation.

DOI： 10.1051/0004-6361/202140464

arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing Ltd  

Close-in gas giants are expected to have a strong magnetic field of ∼10-100 G. Magnetic fields in extrasolar giant planets are detectable by future radio observations in ⪆10 MHz and the spectropolarimetry of atomic lines. In contrast, the elusive interiors of exoplanets remain largely unknown. Here we consider the possibility of inferring the existence of the innermost cores of extrasolar giant planets through the detection of planetary magnetic fields. We simulated the long-term thermal evolution of close-in giant planets with masses of 0.2-10 M Jup to estimate their magnetic field strengths. A young, massive gas giant tends to have a strong magnetic field. The magnetic field strength of a hot Jupiter is insensitive to its core mass, whereas the core strongly affects the emergence of a planetary dynamo in a hot Saturn. No dynamo-driven magnetic field is generated in a hot Saturn with no core or a small one until ∼10-100 Myr if metallization of hydrogen occurs at ⪆1-1.5 Mbar. The magnetic field strength of an evolved gas giant after ∼100 Myr is almost independent of the stellar incident flux. Detecting the magnetic field of a young, hot Saturn as a good indicator of its core may be challenging because of the weakness of radio signals and the shielding effect of plasma in Earth's ionosphere. Hot Jupiters with ⪆0.4 M Jup can be promising candidates for future ground-based radio observations.

DOI： 10.3847/1538-4357/abd8d1

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arXiv

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Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

<title>Abstract</title>
We report on a radial-velocity search for short-period planets in the Pleiades open cluster. We observed 30 Pleiades member stars at the Okayama Astrophysical Observatory with the High Dispersion Echelle Spectrograph. To evaluate and mitigate the effects of stellar activity on radial-velocity (RV) measurements, we computed four activity indicators (full width at half maximum, Vspan, Wspan, and SHα). Among our sample, no short-period planet candidates were detected. Stellar intrinsic RV jitter was estimated to be 52 m s−1, 128 m s−1, and 173 m s−1 for stars with $v$ sin i of 10 km s−1, 15 km s−1, and 20 km s−1, respectively. We determined the planet occurrence rate from our survey and set the upper limit to 11.4% for planets with masses 1–13 MJUP and period 1–10 d. To set a more stringent constraint on the planet occurrence rate, we combined the result of our survey with those of other surveys targeting open clusters with ages in the range 30–300 Myr. As a result, the planet occurrence rate in young open clusters was found to be less than 7.4%, 2.9%, and 1.9% for planets with an orbital period of 3 d and masses of 1–5, 5–13, and 13–80 MJUP, respectively.

DOI： 10.1093/pasj/psaa105

arXiv

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Publishing type：Research paper (scientific journal)   Publisher：EDP Sciences  

We present the confirmation and characterisation of GJ 3473 b (G 50–16, TOI-488.01), a hot Earth-sized planet orbiting an M4 dwarf star, whose transiting signal (<italic>P</italic> = 1.1980035 ± 0.0000018 d) was first detected by the Transiting Exoplanet Survey Satellite (TESS). Through a joint modelling of follow-up radial velocity observations with CARMENES, IRD, and HARPS together with extensive ground-based photometric follow-up observations with LCOGT, MuSCAT, and MuSCAT2, we determined a precise planetary mass, <italic>M</italic>_b = 1.86 ± 0.30 <italic>M</italic>_⊕, and radius, <italic>R</italic>_b = 1.264 ± 0.050 <italic>R</italic>_⊕. Additionally, we report the discovery of a second, temperate, non-transiting planet in the system, GJ 3473 c, which has a minimum mass, <italic>M</italic>_c sin <italic>i</italic> = 7.41 ± 0.91 <italic>M</italic>_⊕, and orbital period, <italic>P</italic>_c = 15.509 ± 0.033 d. The inner planet of the system, GJ 3473 b, is one of the hottest transiting Earth-sized planets known thus far, accompanied by a dynamical mass measurement, which makes it a particularly attractive target for thermal emission spectroscopy.

DOI： 10.1051/0004-6361/202038967

arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Super-Earths possess low-mass H-2/He atmospheres (typically less than 10% by mass). However, the origins of super-Earth atmospheres have not yet been ascertained. We investigate the role of rapid disk clearing by photoevaporation during the formation of super-Earths and their atmospheres. We perform unified simulations of super-Earth formation and atmospheric evolution in evolving disks that consider both photoevaporative winds and magnetically driven disk winds. For the growth mode of planetary cores, we consider two cases in which planetary embryos grow with and without pebble accretion. Our main findings are summarized as follows. (i) The time span of atmospheric accretion is shortened by rapid disk dissipation due to photoevaporation, which prevents super-Earth cores from accreting massive atmospheres. (ii) Even if planetary cores grow rapidly by embryo accretion in the case without pebble accretion, the onset of runaway gas accretion is delayed because the isolation mass for embryo accretion is small. Together with rapid disk clearing, the accretion of massive atmospheres can be avoided. (iii) After rapid disk clearing, a number of high-eccentricity embryos can remain in outer orbits. Thereafter, such embryos may collide with the super-Earths, leading to efficient impact erosion of accreted atmospheres. Therefore, we find that super-Earths with low-mass H-2/He atmospheres are naturally produced byN-body simulations that consider realistic disk evolution.

DOI： 10.3847/1538-4357/aba75e

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A substantial number of super-Earths have been discovered, and atmospheres of transiting super-Earths have also been observed by transmission spectroscopy. Several lines of observational evidence indicate that most super-Earths do not possess massive H-2/He atmospheres. However, accretion and retention of less massive atmospheres on super-Earths challenge planet formation theory. We consider the following three mechanisms: (i) envelope heating by pebble accretion, (ii) mass loss during giant impacts, and (iii) atmospheric loss by stellar X-ray and EUV photoevaporation. We investigate whether these mechanisms influence the amount of the atmospheres that form around super-Earths. We develop a code combining an N-body simulation of pebble-driven planetary formation and an atmospheric evolution simulation. We demonstrate that the observed orbital properties of super-Earths are well reproduced by the results of our simulations. However, (i) heating by pebble accretion ceases prior to disk dispersal, (ii) the frequency of giant impact events is too low to sculpt massive atmospheres, and (iii) many super-Earths having H-2/He atmospheres of greater than or similar to 10 wt% survive against stellar irradiation for 1 Gyr. Therefore, it is likely that other mechanisms, such as suppression of gas accretion, are required to explain less massive atmospheres (less than or similar to 10 wt%) of super-Earths.

DOI： 10.3847/1538-4357/ab7fa7

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arXiv

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Publishing type：Research paper (scientific journal)  

Recently, transmission spectroscopy in the atmospheres of the TRAPPIST-1 planets revealed flat and featureless absorption spectra, which rule out cloud-free, hydrogen-dominated atmospheres. Earth-sized planets orbiting TRAPPIST-1 likely have either a clear or a cloudy/hazy, hydrogen-poor atmosphere. In this paper, we investigate whether a proposed formation scenario is consistent with expected atmospheric compositions of the TRAPPIST-1 planets. We examine the amount of hydrogen-rich gas that TRAPPIST-1-like planets accreted from the ambient disk until disk dispersal. Since TRAPPIST-1 planets are trapped into a resonant chain, we simulate disk gas accretion onto a migrating TRAPPIST-1-like planet. We find that the amount of accreted hydrogen-rich gas is as small as 10-2 wt% and 0.1 wt% for TRAPPIST-1 b and 1 c, 10-2 wt% for 1 d, 1 wt% for 1 e, a few wt% for 1 f and 1 g and 1 wt% for 1 h, respectively. We also calculate the long-term thermal evolution of TRAPPIST-1-like planets after disk dissipation and estimate the mass loss of their hydrogen-rich atmospheres driven by stellar X-ray and UV irradiation. We find that all the accreted hydrogen-rich atmospheres can be lost via hydrodynamic escape. Therefore, we conclude that TRAPPIST-1 planets should have no primordial hydrogen-rich gases but secondary atmospheres such as a Venus-like one and water vapor, if they currently retain atmospheres.

DOI： 10.3847/1538-4357/ab6168

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arXiv

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Publishing type：Research paper (scientific journal)  

We present $\kappa$ Andromeda b's photometry and astrometry taken with
Subaru/SCExAO+HiCIAO and Keck/NIRC2, combined with recently published
SCExAO/CHARIS low-resolution spectroscopy and published thermal infrared
photometry to further constrain the companion's atmospheric properties and
orbit. $\kappa$ And b's Y/Y-K colors are redder than field dwarfs, consistent
with its youth and lower gravity. Empirical comparisons of its Y-band
photometry and CHARIS spectrum to a large spectral library of isolated field
dwarfs reaffirm the conclusion from Currie et al. (2018) that it likely has a
low gravity but admit a wider range of most plausible spectral types (L0-L2).
Our gravitational classification also suggests that the best-fit objects for
$\kappa$ And b may have lower gravity than those previously reported.
Atmospheric models lacking dust/clouds fail to reproduce its entire 1--4.7 $\mu
m$ spectral energy distribution, cloudy atmosphere models with temperatures of
$\sim$ 1700--2000 $K$ better match $\kappa$ And b data. Most well-fitting model
comparisons favor 1700--1900 $K$, a surface gravity of log(g) $\sim$ 4--4.5,
and a radius of 1.3--1.6\,$R_{\rm Jup}$; the best-fit model (DRIFT-Phoenix)
yields the coolest and lowest-gravity values: $T_{\rm eff}$=1700 K and $\log
g$=4.0. An update to $\kappa$ And b's orbit with ExoSOFT using new astrometry
spanning seven years reaffirms its high eccentricity ($0.77\pm0.08$). We
consider a scenario where unseen companions are responsible for scattering
$\kappa$ And b to a wide separation and high eccentricity. If three planets,
including $\kappa$ And b, were born with coplanar orbits and one of them was
ejected by gravitational scattering, a potential inner companion with mass
$\gtrsim10M_{\rm Jup}$ could be located at $\lesssim$ 25 au.

DOI： 10.3847/1538-3881/ab5afa

arXiv

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Other Link： http://arxiv.org/pdf/1911.09758v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R-circle dot, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1R(circle plus), planet on a 0.95 day orbit and a 2.3R(circle plus), planet on a 5 day orbit. Because the host star is small the decrease in light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets.

DOI： 10.3847/2041-8213/ab3d30

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arXiv

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Publishing type：Research paper (scientific journal)  

The Juno mission¹ has provided an accurate determination of Jupiter's gravitational field², which has been used to obtain information about the planet's composition and internal structure. Several models of Jupiter's structure that fit the probe's data suggest that the planet has a diluted core, with a total heavy-element mass ranging from ten to a few tens of Earth masses (about 5 to 15 per cent of the Jovian mass), and that heavy elements (elements other than hydrogen and helium) are distributed within a region extending to nearly half of Jupiter's radius^3,4. Planet-formation models indicate that most heavy elements are accreted during the early stages of a planet's formation to create a relatively compact core^5-7 and that almost no solids are accreted during subsequent runaway gas accretion^8-10. Jupiter's diluted core, combined with its possible high heavy-element enrichment, thus challenges standard planet-formation theory. A possible explanation is erosion of the initially compact heavy-element core, but the efficiency of such erosion is uncertain and depends on both the immiscibility of heavy materials in metallic hydrogen and on convective mixing as the planet evolves^11,12. Another mechanism that can explain this structure is planetesimal enrichment and vaporization^13-15 during the formation process, although relevant models typically cannot produce an extended diluted core. Here we show that a sufficiently energetic head-on collision (giant impact) between a large planetary embryo and the proto-Jupiter could have shattered its primordial compact core and mixed the heavy elements with the inner envelope. Models of such a scenario lead to an internal structure that is consistent with a diluted core, persisting over billions of years. We suggest that collisions were common in the young Solar system and that a similar event may have also occurred for Saturn, contributing to the structural differences between Jupiter and Saturn^16-18....

DOI： 10.1038/s41586-019-1470-2

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arXiv

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Publishing type：Research paper (scientific journal)  

This white paper reviews the scientific community's ability to use data from future telescopes to search for life on exoplanets. It summarizes products from the Exoplanet Biosignatures Workshop Without Walls (EBWWW). This effort led to papers that constituted the Exoplanet Analysis Group's (ExoPAG) 16th Science Assessment Group (SAG 16)....

arXiv

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Publishing type：Research paper (scientific journal)  

Context. In clustered environments, stellar encounters can liberate planets from their host stars via close encounters. Although the detection probability of planets suggests that the planet population in open clusters resembles that in the field, only a few dozen planet-hosting stars have been discovered in open clusters. <BR /> Aims: We explore the survival rates of planets against stellar encounters in open clusters similar to the Pleiades, Hyades, and Praesepe and embedded clusters. <BR /> Methods: We performed a series of N-body simulations of high-density and low-density open clusters, open clusters that grow via mergers of subclusters, and embedded clusters. We semi-analytically calculated the survival rate of planets in star clusters up to 1 Gyr using relative velocities, masses, and impact parameters of intruding stars. <BR /> Results: Less than 1.5% of close-in planets within 1 AU and at most 7% of planets with 1-10 AU are ejected by stellar encounters in clustered environments after the dynamical evolution of star clusters. If a planet population from 0.01-100 AU in an open cluster initially follows the probability distribution function of exoplanets with semi-major axis (a_p) between 0.03 and 3 AU in the field discovered by RV surveys (∝ a_p^-0.6), the PDF of surviving planets beyond 10 AU in open clusters can be slightly modified to ∝ a_p^-0.76. The production rate of free-floating planets (FFPs) per star is 0.0096-0.18, where we have assumed that all the stars initially have one giant planet with a mass of 1-13 M_Jup in a circular orbit. The expected frequency of FFPs is compatible with the upper limit on that of FFPs indicated by recent microlensing surveys. Our survival rates of planets in open clusters suggest that planets within 10 AU around FGKM-type stars are rich in relatively-young (≲10-100 Myr for open clusters and 1-10 Myr for embedded clusters), less massive open clusters, which are promising targets for planet searches....

DOI： 10.1051/0004-6361/201834677

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

During the past five years, 6, 7, and 26 transit observations were carried out for the HAT-P-9b, HAT-P-32b, and HAT-P-36b systems, respectively, through the Transiting Exoplanet Monitoring Project network. Combined with the published photometric data and radial-velocity measurements, our new photometry allows us to revisit the system parameters and search for additional close-in planetary companions in these hot Jupiter systems. We measure an updated R P /R ∗ = 0.1260 ±0.0011 for HAT-P-36 system in the R band, which is 4.5σ larger than the published i-band radius ratio of 0.1186 ±0.0012. We also perform a transit timing variation (TTV) analysis for each system. Because no significant TTVs were found, we place an upper mass limit on an additional planet for each system.

DOI： 10.3847/1538-3881/aaf6b6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The core-accretion model predicts that planetary cores as massive as super-Earths undergo runaway gas accretion to become gas giants. However, the exoplanet census revealed the prevalence of super-Earths close to their host stars, which should have avoided runaway gas accretion. In fact, mass-radius relationships of transiting planets suggest that some close-in super-Earths possess H-2/He atmospheres of similar to 0.1%-10% by mass. Previous studies indicated that properties of a disk gas such as metallicity and the inflow/outflow cycle of a disk gas around a super Earth can regulate accumulation of an H-2/He atmosphere onto itself. In this paper, we propose a new mechanism for which radial mass accretion in a disk can limit the gas accretion onto super-Earth cores. Recent magnetohydrodynamic simulations found that magnetically driven disk winds can drive a rapid gas flow near the disk surface. Such a rapid gas flow may slip out of a planetary core and regulate gas supply to an accreting gas onto the core. We performed N-body simulations for formation of super-Earths with accretion of atmospheres in a viscous accretion disk including effects of wind-driven accretion. We found that even super-Earth cores can avoid triggering runaway gas accretion if the inflow of a disk gas toward the cores is limited by viscous accretion. Our model predicts that super-Earths having an H2/He atmosphere of similar to 0.1-10 wt% form within. 1 au of the central star, whereas gas giants are born in the outer region. This mechanism can explain the radial dependence of observed giant planets beyond the solar system.

DOI： 10.3847/1538-4357/aae534

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arXiv

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Publishing type：Research paper (scientific journal)  

Context. The G-type star GJ504A is known to host a 3-35 M_Jup companion whose temperature, mass, and projected separation all contribute to making it a test case for planet formation theories and atmospheric models of giant planets and light brown dwarfs. <BR /> Aims: We aim at revisiting the system age, architecture, and companion physical and chemical properties using new complementary interferometric, radial-velocity, and high-contrast imaging data. <BR /> Methods: We used the CHARA interferometer to measure GJ504A's angular diameter and obtained an estimation of its radius in combinationwith the HIPPARCOS parallax. The radius was compared to evolutionary tracks to infer a new independent age range for the system. We collected dual imaging data with IRDIS on VLT/SPHERE to sample the near-infrared (1.02-2.25 μm) spectral energy distribution (SED) of the companion. The SED was compared to five independent grids of atmospheric models (petitCODE,Exo-REM, BT-SETTL, Morley et al., and ATMO) to infer the atmospheric parameters of GJ 504b and evaluate model-to-model systematic errors. In addition, we used a specific model grid exploring the effect of different C/O ratios. Contrast limits from 2011 to 2017 were combined with radial velocity data of the host star through the MESS2 tool to define upper limits on the mass of additional companions in the system from 0.01 to 100 au. We used an MCMC fitting tool to constrain the companion'sorbital parameters based on the measured astrometry, and dedicated formation models to investigate its origin. <BR /> Results: We report a radius of 1.35 ± 0.04 R_⊙ for GJ504A. The radius yields isochronal ages of 21 ± 2 Myr or 4.0 ± 1.8 Gyr for the system and line-of-sight stellar rotation axis inclination of 162.4_-4.3^+3.8 degrees or 186.6_-3.8^+4.3 degrees. We re-detect the companion in the Y2, Y3, J3, H2, and K1 dual-band images. The complete 1-4 μm SED shape of GJ504b is best reproduced by T8-T9.5 objects with intermediate ages (≤ 1.5Gyr), and/or unusual dusty atmospheres and/or super-solar metallicities. All atmospheric models yield T_eff = 550 ± 50 K for GJ504b and point toward a low surface gravity (3.5-4.0 dex). The accuracy on the metallicity value is limited by model-to-model systematics; it is not degenerate with the C/O ratio. We derive log L/L_⊙ = -6.15 ± 0.15 dex for the companion from the empirical analysis and spectral synthesis. The luminosity and T_eff yield masses of M = 1.3_-0.3^+0.6 M_Jup and M = 23_-9⁺¹⁰ M_Jup for the young and old age ranges, respectively. The semi-major axis (sma) is above 27.8 au and the eccentricity is lower than 0.55. The posterior on GJ 504b's orbital inclination suggests a misalignment with the rotation axis of GJ 504A. We exclude additional objects (90% prob.) more massive than 2.5 and 30 M_Jup with semi-major axes in the range 0.01-80 au for the young and old isochronal ages, respectively. <BR /> Conclusions: The mass and semi-major axis of GJ 504b are marginally compatible with a formation by disk-instability if the system is 4 Gyr old. The companion is in the envelope of the population of planets synthesized with our core-accretion model. Additional deep imaging and spectroscopic data with SPHERE and JWST should help to confirm the possible spin-orbit misalignment and refine the estimates on the companion temperature, luminosity, and atmospheric composition. <P />Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programs 093.C-0500, 095.C-0298, 096.C-0241, and 198.C-0209, and on interferometric observations obtained with the VEGA instrument on the CHARA Array....

DOI： 10.1051/0004-6361/201832942

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report the photometry of six transits of the hot Jupiter HAT-P-29b obtained from 2013 October to 2015 January. We analyze the new light curves, in combination with the published photometric, Doppler velocimetric, and spectroscopic measurements, finding an updated orbital ephemeris for the HAT-P-29 system, T-C[0] = 2456170.5494(15)[BJD(TDB)] and P = 5.723390(13) days. This result is 17.63 s (4.0 sigma) longer than the previously published value, amounting to errors exceeding 2.5 hr at the time of writing (on UTC 2018 June 1). The measured transit mid-times for HAT-P-29b show no compelling evidence of timing anomalies from a linear model, which rules out the presence of perturbers with masses greater than 0.6, 0.7, 0.5, and 0.4M(circle plus) near the 1:2, 2:3, 3:2, and 2:1 resonances with HAT-P-29b, respectively.

DOI： 10.3847/1538-3881/aadcfc

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Mary Ann Liebert Inc.  

Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond the Solar System. Accumulating experiences in astrophysical, chemical, and climatological characterization of uninhabitable planets are paving the way to characterization of potentially habitable planets. In this paper, we review our possibilities and limitations in characterizing temperate terrestrial planets with future observational capabilities through the 2030s and beyond, as a basis of a broad range of discussions on how to advance "astrobiology" with exoplanets. We discuss the observability of not only the proposed biosignature candidates themselves but also of more general planetary properties that provide circumstantial evidence, since the evaluation of any biosignature candidate relies on its context. Characterization of temperate Earth-sized planets in the coming years will focus on those around nearby late-type stars. The James Webb Space Telescope (JWST) and later 30-meter-class ground-based telescopes will empower their chemical investigations. Spectroscopic studies of potentially habitable planets around solar-type stars will likely require a designated spacecraft mission for direct imaging, leveraging technologies that are already being developed and tested as part of the Wide Field InfraRed Survey Telescope (WFIRST) mission. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and a larger survey that are envisioned beyond 2030. The broad outlook this paper presents may help develop new observational techniques to detect relevant features as well as frameworks to diagnose planets based on the observables.

DOI： 10.1089/ast.2017.1733

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

We search for signatures of a distant planet around the two million-year-old classical T-Tauri star CI Tau hosting a hot-Jupiter candidate (Mp sin i ∼ 8.1 MJupiter) in an eccentric orbit (e ∼ 0.3). To probe the existence of an outer perturber, we reanalyzed 1.3 mm dust continuum observations of the protoplanetary disk around CI Tau obtained by the Atacama Large Millimeter/submillimeter Array (ALMA). We found a gap structure at ∼0 8 in CI Taus disk. Our visibility fitting assuming an axisymmetric surface brightness profile suggested that the gap is located at a deprojected radius of 104.5 ± 1.6 au and has a width of 36.9 ± 2.9 au. The brightness temperature around the gap was calculated to be ∼2.3 K lower than that of the ambient disk. Gap-opening mechanisms such as secular gravitational instability (GI) and dust trapping can explain the gap morphology in the CI Tau disk. The scenario that an unseen planet created the observed gap structure cannot be ruled out, although the coexistence of an eccentric hot Jupiter and a distant planet around the young CI Tau would be challenging for gravitational scattering scenarios. The mass of the planet was estimated to be between ∼0.25 MJupiter and ∼0.8 MJupiter from the gap width and depth (0.41-0.06+0.04) in the modeled surface brightness image, which is lower than the current detection limits of high-contrast direct imaging. The young classical T-Tauri CI Tau may be a unique system for exploring the existence of a potential distant planet as well as the origin of an eccentric hot Jupiter.

DOI： 10.3847/2041-8213/aac6d2

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press  

We have newly developed a parallelized particle-particle particle-tree code for planet formation, PENTACLE, which is a parallelized hybrid N-body integrator executed on a CPU-based (super)computer. PENTACLE uses a fourth-order Hermite algorithm to calculate gravitational interactions between particles within a cut-off radius and a Barnes-Hut tree method for gravity from particles beyond. It also implements an open-source library designed for full automatic parallelization of particle simulations, FDPS (Framework for Developing Particle Simulator), to parallelize a Barnes-Hut tree algorithm for a memory-distributed supercomputer. These allow us to handle 1-10 million particles in a high-resolution N-body simulation on CPU clusters for collisional dynamics, including physical collisions in a planetesimal disc. In this paper, we show the performance and the accuracy of PENTACLE in terms of Rcut and a time-step Δt. It turns out that the accuracy of a hybrid N-body simulation is controlled through Δt/Rcut and Δt/Rcut ∼ 0.1 is necessary to simulate accurately the accretion process of a planet for ≥10 6 yr. For all those interested in large-scale particle simulations, PENTACLE, customized for planet formation, will be freely available from https://github.com/PENTACLE-Team/PENTACLE under the MIT licence.

DOI： 10.1093/pasj/psx073

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We present 10. R-band photometric observations of eight different transits of the hot Jupiter HAT-P-33b, which has been targeted by our Transiting Exoplanet Monitoring Project. The data were obtained by two telescopes at the Xinglong Station of National Astronomical Observatories of China (NAOC) from 2013 December through 2016 January, and exhibit photometric scatter of 1.6-3.0 mmag. After jointly analyzing the previously published photometric data, radial-velocity (RV) measurements, and our new light curves, we revisit the system parameters and orbital ephemeris for the HAT-P-33b system. Our results are consistent with the published values except for the planet to. star radius ratio (RP/R-*), the ingress/egress duration (tau) and the total duration (T-14), which together indicate a slightly shallower and shorter transit shape. Our results are based on more complete light curves, whereas the previously published work had only one complete transit light curve. No significant anomalies in Transit Timing Variations (TTVs) are found, and we place upper mass limits on potential perturbers, largely supplanting the loose constraints provided by the extant RV data. The TTV limits are stronger near mean-motion resonances, especially for the low-order commensurabilities. We can exclude the existence of a perturber with mass larger than 0.6, 0.3, 0.5, 0.5, and 0.3 M-circle plus near the 1: 3, 1: 2, 2: 3, 3: 2, and 2: 1 resonances, respectively.

DOI： 10.3847/1538-3881/aa7519

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

We report on the confirmation that the candidate transits observed for the star EPIC 211525389 are due to a short-period Neptune-sized planet. The host star, located in K2 campaign field 5, is a metal-rich ([Fe/H] = 0.26 +/- 0.05) G-dwarf (T-eff = 5430 +/- 70 K and log g = 4.48 +/- 0.09), based on observations with the High Dispersion Spectrograph (HDS) on the Subaru 8.2m telescope. High spatial resolution AO imaging with HiCIAO on the Subaru telescope excludes faint companions near the host star, and the false positive probability of this target is found to be &lt; 10(-6) using the open source vespa code. A joint analysis of transit light curves from K2 and additional ground-based multicolor transit photometry with MuSCAT on the Okayama 1.88mtelescope gives an orbital period of P = 8.266902 +/- 0.000070 d and consistent transit depths of Rp/R-* similar to 0.035 or (Rp/R-*)(2) similar to 0.0012. The transit depth corresponds to a planetary radius of R-p = 3.59(-0.39)(+0.44) R-circle plus indicating that EPIC 211525389 b is a short-period Neptune-sized planet. Radial velocities of the host star, obtained with the Subaru HDS, lead to a 3 sigma upper limit of 90M. (0.00027M(circle dot)) on the mass of EPIC 211525389 b, confirming its planetary nature. We expect this planet, newly named K2-105 b, to be the subject of future studies to characterize its mass, atmosphere, and spin-orbit (mis) alignment, as well as investigate the possibility of additional planets in the system.

DOI： 10.1093/pasj/psx002

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arXiv

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K2-19 (EPIC201505350) is an interesting planetary system in which two transiting planets with radii similar to 7 R-circle plus (inner planet b) and similar to 4 R-circle plus (outer planet c) have orbits that are nearly in a 3:2 mean-motion resonance. Here, we present results of ground-based follow-up observations for the K2-19 planetary system. We have performed high-dispersion spectroscopy and high-contrast adaptive-optics imaging of the host star with the HDS and HiCIAO on the Subaru 8.2 m telescope. We find that the host star is a relatively old (&gt;= 8 Gyr) late G-type star (T-eff similar to 5350 K, M-s similar to 0.9M(circle dot) and R-s similar to 0.9 R-circle dot). We do not find any contaminating faint objects near the host star that could be responsible for (or dilute) the transit signals. We have also conducted transit follow-up photometry for the inner planet with KeplerCam on the FLWO 1.2 m telescope, TRAPPISTCAM on the TRAPPIST 0.6 m telescope, and MuSCAT on the OAO 1.88 m telescope. We confirm the presence of transit timing variations (TTVs), as previously reported by Armstrong and coworkers. We model the observed TTVs of the inner planet using the synodic chopping formulae given by Deck & Agol. We find two statistically indistinguishable solutions for which the period ratios (P-c/P-b) are located slightly above and below the exact 3:2 commensurability. Despite the degeneracy, we derive the orbital period of the inner planet P-b similar to 7.921 days and the mass of the outer planet M-c similar to 20 M-circle plus. Additional transit photometry (especially for the outer planet) as well as precise radial-velocity measurements would be helpful to break the degeneracy and to determine the mass of the inner planet.

DOI： 10.1088/0004-637X/815/1/47

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

Mini-Neptunes and volatile-poor super-Earths coexist on adjacent orbits in proximity to host stars such as Kepler-36 and Kepler-11. Several post-formation processes have been proposed for explaining the origin of the compositional diversity between neighboring planets: mass loss via stellar XUV irradiation, degassing of accreted material, and in situ accumulation of the disk gas. Close-in planets are also likely to experience giant impacts during the advanced stage of planet formation. This study examines the possibility of transforming volatile-rich super-Earths/mini-Neptunes into volatile-depleted super-Earths through giant impacts. We present the results of three-dimensional hydrodynamic simulations of giant impacts in the accretionary and disruptive regimes. Target planets are modeled with a three-layered structure composed of an iron core, silicate mantle, and hydrogen/helium envelope. In the disruptive case, the giant impact can remove most of the H/He atmosphere immediately and homogenize the refractory material in the planetary interior. In the accretionary case, the planet is able to retain more than half of the original gaseous envelope, while a compositional gradient suppresses efficient heat transfer as the planetary interior undergoes double-diffusive convection. After the giant impact, a hot and inflated planet cools and contracts slowly. The extended atmosphere enhances the mass loss via both a Parker wind induced by thermal pressure and hydrodynamic escape driven by the stellar XUV irradiation. As a result, the entire gaseous envelope is expected to be lost due to the combination of those processes in both cases. Based on our results, we propose that Kepler-36b may have been significantly devolatilized by giant impacts, while a substantial fraction of Kepler-36c's atmosphere may remain intact. Furthermore, the stochastic nature of giant impacts may account for the observed large dispersion in the mass-radius relationship of close-in super-Earths and mini-Neptunes (at least to some extent).

DOI： 10.1088/0004-637X/812/2/164

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics Publishing  

We report detections of new exoplanets from a radial-velocity (RV) survey of metal-rich FGK stars by using three telescopes. By optimizing our RV analysis method to long time-baseline observations, we have succeeded in detecting five new Jovian planets around three metal-rich stars, HD 1605, HD 1666, and HD 67087, with masses of 1.3 M⊙, 1.5 M⊙, and 1.4 M⊙, respectively. A K1 subgiant star, HD 1605 hosts two planetary companions with minimum masses of Mp sin i = 0.96MJup and 3.5MJup in circular orbits with the planets periods P = 577.9 and 2111 days, respectively. HD 1605 shows a significant linear trend in RVs. Such a system consisting of Jovian planets in circular orbits has rarely been found and thus HD 1605 should be an important example of a multiplanetary system that is likely unperturbed by planetplanet interactions. HD 1666 is an F7 main-sequence star that hosts an eccentric and massive planet of Mp sin i = 6.4MJup in an orbit with ap = 0.94 AU and eccentricity e = 0.63. Such an eccentric and massive planet can be explained as a result of planetplanet interactions among Jovian planets. While we have found large residuals of rms = 35.6 m s -1, the periodogram analysis does not support any additional periodicities. Finally, HD 67087 hosts two planets of Mp sin i = 3.1MJup and 4.9MJup in orbits with P = 352.2 and 2374 days, and e = 0.17 and 0.76, respectively. Although the current RVs do not lead to accurate determinations of its orbit and mass, HD 67087 c can be one of the most eccentric planets ever discovered in multiple systems.

DOI： 10.1088/0004-637X/806/1/5

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report detections of new exoplanets from a radial-velocity (RV) survey of metal-rich FGK stars by using three telescopes. By optimizing our RV analysis method to long time-baseline observations, we have succeeded in detecting five new Jovian planets around three metal-rich stars, HD 1605, HD 1666, and HD 67087, with masses of 1.3 M-circle dot, 1.5 M-circle dot, and 1.4 M-circle dot, respectively. A K1 subgiant star, HD 1605 hosts two planetary companions with minimum masses of M-p sin i = 0.96M(Jup) and 3.5M(Jup) in circular orbits with the planets' periods P = 577.9 and 2111 days, respectively. HD 1605 shows a significant linear trend in RVs. Such a system consisting of Jovian planets in circular orbits has rarely been found and thus HD 1605 should be an important example of a multi-planetary system that is likely unperturbed by planet-planet interactions. HD 1666 is an F7 main-sequence star that hosts an eccentric and massive planet of M-p sin i = 6.4M(Jup) in an orbit with a(p) = 0.94 AU and eccentricity e = 0.63. Such an eccentric and massive planet can be explained as a result of planet-planet interactions among Jovian planets. While we have found large residuals of rms = 35.6 m s(-1), the periodogram analysis does not support any additional periodicities. Finally, HD 67087 hosts two planets of M-p sin i = 3.1M(Jup) and 4.9M(Jup) in orbits with P = 352.2 and 2374 days, and e = 0.17 and 0.76, respectively. Although the current RVs do not lead to accurate determinations of its orbit and mass, HD 67087 c can be one of the most eccentric planets ever discovered in multiple systems.

DOI： 10.1088/0004-637X/806/1/5

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arXiv

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The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shock loading. The pressure and density of compressed hydrogen were determined by impedance matching to a quartz standard. The shock temperature was independently measured from the brightness of the shock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensed matter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, and this results in shock-compressed hydrogen having a higher compression and higher temperature than deuterium at the same shock pressure. © 2011 American Physical Society.

DOI： 10.1103/PhysRevB.83.054117

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Other Link： http://orcid.org/0000-0002-3978-8427

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Hydrogen at high pressure in the fluid state is of great interest for target design of inertial confinement fusion and understanding the interior structure of gas giant planets. In this work, we successfully obtained the Hugoniot data for liquid hydrogen up to 55 GPa under laser-driven shock loading using impedance matching to a quartz standard. The shocked temperature was determined simultaneously by the brightness temperature. The compression and temperature along the principal Hugoniot are in good agreement with theoretical models. High reflectivity of hydrogen was observed at 40 GPa, which suggests the fluid becomes conducting

DOI： 10.1088/1742-6596/244/4/042018

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Language：English   Publisher：Springer Netherlands  

The origin and evolution of planetary protoatmospheres in relation to the protoplanetary disk is discussed. The initial atmospheres of planets can mainly be related via two formation scenarios. If a protoplanetary core accretes mass and grows inside the gas disk, it can capture H2, He and other gases from the disk. When the gas of the disk evaporates, the core that is surrounded by the H2/He gas envelope is exposed to the high X-ray and extreme ultraviolet flux and stellar wind of the young host star. This period can be considered as the onset of atmospheric escape. It is shown that lower mass bodies accrete less gas and depending on the host stars radiation environment can therefore lose the gaseous envelope after tens or hundreds of million years. Massive cores may never get rid of their captured hydrogen envelopes and remain as sub-Neptunes, Neptunes or gas giants for their whole life time. Terrestrial planets which may have lost the captured gas envelope by thermal atmospheric escape, or which accreted after the protoplanetary nebula vanished will produce catastrophically outgassed steam atmospheres during the magma ocean solidification process. These steam atmospheres consist mainly of water and CO2 that was incorporated into the protoplanet during its accretion. Planets, which are formed in the habitable zone, solidify within several million years. In such cases the outgassed steam atmospheres cool fast, which leads to the condensation of water and the formation of liquid oceans. On the other hand, magma oceans are sustained for longer if planets form inside a critical distance, even if they outgassed a larger initial amount of water. In such cases the steam atmosphere could remain 100 million years or for even longer. Hydrodynamic atmospheric escape will then desiccate these planets during the slow solidification process.

DOI： 10.1007/s11214-016-0280-1

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Language：English   Publisher：The Japanese Society for Planetary Sciences  

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WASP-80b is a warm Jupiter transiting a bright late-K/early-M dwarf, providing a good opportunity to extend the atmospheric study of hot Jupiters toward the lower temperature regime. We report multi-band, multi-epoch transit observations of WASP-80b by using three ground-based telescopes covering from optical (g', R-c, and I-c bands) to near-infrared (NIR; J, H, and K-s bands) wavelengths. We observe 5 primary transits, each in 3 or 4 different bands simultaneously, obtaining 17 independent transit light curves. Combining them with results from previous works, we find that the observed transmission spectrum is largely consistent with both a solar abundance and thick cloud atmospheric models at a 1.7 sigma discrepancy level. On the other hand, we find a marginal spectral rise in the optical region compared to the NIR region at the 2.9 sigma level, which possibly indicates the existence of haze in the atmosphere. We simulate theoretical transmission spectra for a solar abundance but hazy atmosphere, finding that a model with equilibrium temperature of 600 K can explain the observed data well, having a discrepancy level of 1.0 sigma. We also search for transit timing variations, but find no timing excess larger than 50 s from a linear ephemeris. In addition, we conduct 43 day long photometric monitoring of the host star in the optical bands, finding no significant variation in the stellar brightness. Combined with the fact that no spot-crossing event is observed in the five transits, our results confirm previous findings that the host star appears quiet for spot activities, despite the indications of strong chromospheric activities.

DOI： 10.1088/0004-637X/790/2/108

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arXiv

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Language：English   Publisher：EDP SCIENCES S A  

Context. Recent progress in transit photometry opened a new window to the interior of super-Earths. From measured radii and masses, we can infer constraints on planetary internal compositions. It has been recently revealed that super-Earths orbiting close to host stars (i.e., hot super-Earths) are diverse in composition. This diversity is thought to arise from diversity in volatile content.
Aims. The stability of the volatile components, which we call the envelopes, is to be examined, because hot super-Earths, which are exposed to strong irradiation, undergo photo-evaporative mass loss. While several studies investigated the impact of photo-evaporative mass loss on hydrogen-helium envelopes, there are few studies as to the impact on water-vapor envelopes, which we investigate in this study. To obtain theoretical prediction to future observations, we also investigate the relationships among masses, radii, and semi-major axes of water-rich super-Earths and also sub-Earths that have undergone photo-evaporative mass loss.
Methods. We simulate the interior structure and evolution of highly-irradiated sub/super-Earths that consist of a rocky core surrounded by a water envelope, which include mass loss due to the stellar XUV-driven energy-limited hydrodynamic escape.
Results. We find that the photo-evaporative mass loss has a significant impact on the evolution of hot sub/super-Earths. With a widely-used empirical formula for XUV flux from typical G-stars and the heating efficiency of 0.1 for example, the planets of less than 3 Earth masses orbiting 0.03 AU have their water envelopes completely stripped off. We then derive the threshold planetary mass and radius below which the planet loses its water envelope completely as a function of the initial water content and find that there are minimums of the threshold mass and radius.
Conclusions. We constrain the domain in the parameter space of planetary mass, radius, and the semi-major axis in which sub/super-Earths never retain water envelopes in 1-10 Gyr. This would provide an essential piece of information for understanding the origin of close-in, low-mass planets. The current uncertainties in stellar XUV flux and its heating efficiency, however, prevent us from deriving robust conclusions. Nevertheless, it seems to be a robust conclusion that Kepler planet candidates contain a significant number of rocky sub/super-Earths.

DOI： 10.1051/0004-6361/201322258

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arXiv

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Several exoplanets have recently been imaged at wide separations of &gt; 10 AU from their parent stars. These span a limited range of ages (&lt;50 Myr) and atmospheric properties, with temperatures of 800-1800 K and very red colors (J - H &gt; 0.5 mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct-imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS survey. The system is older than all other known directly imaged planets; as a result, its estimated mass remains in the planetary regime independent of uncertainties related to choices of initial conditions in the exoplanet modeling. Using the most common exoplanet cooling model, and given the system age of 160(-60)(+350) Myr, GJ 504b has an estimated mass of 4(-1.0)(+4.5) Jupiter masses, among the lowest of directly imaged planets. Its projected separation of 43.5 AU exceeds the typical outer boundary of similar to 30 AU predicted for the core accretion mechanism. GJ 504b is also significantly cooler (510(-20)(+30) K) and has a bluer color (J - H = -0.23 mag) than previously imaged exoplanets, suggesting a largely cloud-free atmosphere accessible to spectroscopic characterization. Thus, it has the potential of providing novel insights into the origins of giant planets as well as their atmospheric properties.

DOI： 10.1088/0004-637X/774/1/11

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arXiv

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Language：English   Publisher：Institute of Physics Publishing  

We present five new transit light curves of GJ 1214b taken in the BJHK s bands. Two transits were observed in the B band using the Subaru Prime Focus Camera (Suprime-Cam) and the Faint Object Camera and Spectrograph (FOCAS) instruments on board the Subaru 8.2 m telescope, and one transit was done in the JHK s bands simultaneously with the Simultaneous Infrared Imager for Unbiased Survey (SIRIUS) camera on the Infrared Survey Facility (IRSF) 1.4 m telescope. Markov Chain Monte Carlo analyses show that the planet-to-star radius ratios are R p/R s = 0.11651 ± 0.00065 (B band, Subaru/Suprime-Cam), R p/R s = 0.11601 ± 0.00117 (B band, Subaru/FOCAS), R p/R s = 0.11654 ± 0.00080 (J band, IRSF/SIRIUS), (H band, IRSF/SIRIUS), and R p/R s = 0.11547 ± 0.00127 (K s band, IRSF/SIRIUS). The Subaru Suprime-Cam transit photometry shows a possible spot-crossing feature. Comparisons of the new transit depths and those from previous studies with the theoretical models by Howe &amp
Burrows suggest that the high molecular weight atmosphere (e.g., 1% H2O + 99% N 22) models are most likely, however, the low molecular weight (hydrogen-dominated) atmospheres with extensive clouds are still not excluded. We also report a long-term monitoring of the stellar brightness variability of GJ 1214 observed with the MITSuME 50 cm telescope in the g′, R c, and I c bands simultaneously. The monitoring was conducted for 32 nights spanning 78 nights in 2012, and we find a periodic brightness variation with a period of P s = 44.3 ± 1.2 days and semi-amplitudes of 2.1% ± 0.4% in the g′ band, 0.56% ± 0.08% in the R c band, and 0.32% ± 0.04% in the I c band. © 2013. The American Astronomical Society. All rights reserved.

DOI： 10.1088/0004-637X/773/2/144

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We report the first three-dimensional radiation magnetohydrodynamic (RMHD) simulations of protostellar collapse with and without Ohmic dissipation. We take into account many physical processes required to study star formation processes, including a realistic equation of state. We follow the evolution from molecular cloud cores until protostellar cores are formed with sufficiently high resolutions without introducing a sink particle. The physical processes involved in the simulations and adopted numerical methods are described in detail. We can calculate only about one year after the formation of the protostellar cores with our direct three-dimensional RMHD simulations because of the extremely short timescale in the deep interior of the formed protostellar cores, but successfully describe the early phase of star formation processes. The thermal evolution and the structure of the first and second (protostellar) cores are consistent with previous one-dimensional simulations using full radiation transfer, but differ considerably from preceding multi-dimensional studies with the barotropic approximation. The protostellar cores evolve virtually spherically symmetric in the ideal MHD models because of efficient angular momentum transport by magnetic fields, but Ohmic dissipation enables the formation of the circumstellar disks in the vicinity of the protostellar cores as in previous MHD studies with the barotropic approximation. The formed disks are still small (less than 0.35 AU) because we simulate only the earliest evolution. We also confirm that two different types of outflows are naturally launched by magnetic fields from the first cores and protostellar cores in the resistive MHD models.

DOI： 10.1088/0004-637X/763/1/6

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Language：Japanese   Publisher：The Japanese Society for Planetary Sciences  

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Motivated by recent discoveries of low-density super-Earths with short orbital periods, we have investigated in situ accretion of H-He atmospheres on rocky bodies embedded in dissipating warm disks, by simulating quasi-static evolution of atmospheres that connect to the ambient disk. We have found that the atmospheric evolution has two distinctly different outcomes, depending on the rocky body's mass: while the atmospheres on massive rocky bodies undergo runaway disk-gas accretion, those on light rocky bodies undergo significant erosion during disk dispersal. In the atmospheric erosion, the heat content of the rocky body that was previously neglected plays an important role. We have also realized that the atmospheric mass is rather sensitive to disk temperature in the mass range of interest in this study. Our theory is applied to recently detected super-Earths orbiting Kepler-11 to examine the possibility that the planets are rock-dominated ones with relatively thick H-He atmospheres. The application suggests that the in situ formation of the relatively thick H-He atmospheres inferred by structure modeling is possible only under restricted conditions, namely, relatively slow disk dissipation and/or cool environments. This study demonstrates that low-density super-Earths provide important clues to understanding of planetary accretion and disk evolution.

DOI： 10.1088/0004-637X/753/1/66

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Language：English   Publisher：WILEY-BLACKWELL  

We have investigated how envelope pollution by icy planetesimals affects the critical core mass for gas giant formation and the gas accretion time-scales. In the core-accretion model, runaway gas accretion is triggered after a core reaches a critical core mass. All the previous studies on the core-accretion model assumed that the envelope has the solar composition uniformly. In fact, the envelope is likely polluted by evaporated materials of icy planetesimals because icy planetesimals going through the envelope experience mass-loss via strong ablation and most of their masses are deposited in the deep envelope. In this paper, we have demonstrated that envelope pollution in general lowers the critical core masses and hastens gas accretion on to the protoplanet because of the increase in the molecular weight and reduction in the adiabatic temperature gradient. Widely and highly polluted envelopes allow smaller cores to form massive envelopes before disc dissipation. Our results suggest that envelope pollution in the course of planetary accretion has the potential to trigger gas giant formation with small cores. We propose that it is necessary to take into account envelope pollution by icy planetesimals when we discuss gas giant formation based on the core accretion model.

DOI： 10.1111/j.1365-2966.2011.19140.x

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arXiv

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DOI： 10.1088/1742-6596/244/4/042018

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We investigate the critical core mass and the envelope growth timescale, assuming grain-free envelopes, to examine how small cores are allowed to form gas giants in the framework of the core-accretion model. This is motivated by a theoretical dilemma concerning Jupiter formation: modelings of Jupiter&apos;s interior suggest that it contains a small core of &lt;10 M(circle plus), while many core-accretion models of Jupiter formation require a large core of &gt;10M(circle plus) to finish its formation by the time of disk dissipation. Reduction of opacity in the accreting envelope is known to hasten gas giant formation. Almost all the previous studies assumed grain-dominated opacity in the envelope. Instead, we examine cases of grain-free envelopes in this study. Our numerical simulations show that an isolated core of as small as 1.7M(circle plus) is able to capture disk gas to form a gas giant on a timescale of million years if the accreting envelope is grain free; that value decreases to 0.75 M(circle plus) if the envelope is metal free, namely, composed purely of hydrogen and helium. It is also shown that alkali atoms, which are known to be one of the dominant opacity sources near 1500K in the atmospheres of hot Jupiters, have little contribution to determine the critical core mass. Our results confirm that sedimentation and coagulation of grains in the accreting envelope is a key to resolve the dilemma about Jupiter formation.

DOI： 10.1088/0004-637X/714/2/1343

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We report the detection of a large mass planet orbiting around the K0 metal-rich subgiant HD38801 (V = 8.26) by precise radial velocity (RV) measurements from the Subaru Telescope and the Keck Telescope. The star has a mass of 1.36 M(circle dot) and a metallicity of [Fe/H] = +0.26. The RV variations are consistent with a circular orbit with a period of 696.0 days and a velocity semiamplitude of 200.0 m s(-1), which yield a minimum mass for the companion of 10.7 M(JUP) and a semimajor axis of 1.71 AU. Such super-massive objects with very low eccentricities and periods of hundreds of days are uncommon among the ensemble of known exoplanets.

DOI： 10.1088/0004-637X/715/1/550

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We report the detection of a large mass planet orbiting around the K0 metal-rich subgiant HD38801 (V = 8.26) by precise radial velocity (RV) measurements from the Subaru Telescope and the Keck Telescope. The star has a mass of 1.36 M(circle dot) and a metallicity of [Fe/H] = +0.26. The RV variations are consistent with a circular orbit with a period of 696.0 days and a velocity semiamplitude of 200.0 m s(-1), which yield a minimum mass for the companion of 10.7 M(JUP) and a semimajor axis of 1.71 AU. Such super-massive objects with very low eccentricities and periods of hundreds of days are uncommon among the ensemble of known exoplanets.

DOI： 10.1088/0004-637X/715/1/550

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We investigate the critical core mass and the envelope growth timescale,<br />
assuming grain-free envelopes, to examine how small cores are allowed to form<br />
gas giants in the framework of the core accretion model. This is motivated by a<br />
theoretical dilemma concerning Jupiter formation: Modelings of Jupiter&#039;s<br />
interior suggest that it contains a small core of &lt; 10 Earth mass, while many<br />
core accretion models of Jupiter formation require a large core of &gt; 10 Earth<br />
mass to finish its formation by the time of disk dissipation. Reduction of<br />
opacity in the accreting envelope is known to hasten gas giant formation.<br />
Almost all the previous studies assumed grain-dominated opacity in the<br />
envelope. Instead, we examine cases of grain-free envelopes in this study. Our<br />
numerical simulations show that an isolated core of as small as 1.7 Earth mass<br />
is able to capture disk gas to form a gas giant on a timescale of million<br />
years, if the accreting envelope is grain-free; that value decreases to 0.75<br />
Earth mass, if the envelope is metal-free, namely, composed purely of hydrogen<br />
and helium. It is also shown that alkali atoms, which are known to be one of<br />
the dominant opacity sources near 1500 K in the atmospheres of hot Jupiters,<br />
have little contribution to determine the critical core mass. Our results<br />
confirm that sedimentation and coagulation of grains in the accreting envelope<br />
is a key to resolve the dilemma about Jupiter formation.

DOI： 10.1088/0004-637X/714/2/1343

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We report the detection of a substellar companion orbiting the G5 dwarf HD 16760 from the N2K sample. Precise Doppler measurements of the star from Subaru and Keck revealed a Keplerian velocity variation with a period of 466.47 +/- 0.35 d, a semiamplitude of 407.71 +/- 0.84 m s(-1), and an eccentricity of 0.084 +/- 0.003. Adopting a stellar mass of 0.78 +/- 0.05 M(circle dot), we obtain a minimum mass for the companion of 13.13 +/- 0.56 M(JUP), which is close to the planet/brown-dwarf transition, and the semimajor axis of 1.084 +/- 0.023 AU. The nearly circular orbit despite the large mass and intermediate orbital period makes this companion unique among known substellar companions.

DOI： 10.1088/0004-637X/703/1/671

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Language：English   Publisher：IOP PUBLISHING LTD  

We report the detection of a substellar companion orbiting the G5 dwarf HD 16760 from the N2K sample. Precise Doppler measurements of the star from Subaru and Keck revealed a Keplerian velocity variation with a period of 466.47 +/- 0.35 d, a semiamplitude of 407.71 +/- 0.84 m s(-1), and an eccentricity of 0.084 +/- 0.003. Adopting a stellar mass of 0.78 +/- 0.05 M(circle dot), we obtain a minimum mass for the companion of 13.13 +/- 0.56 M(JUP), which is close to the planet/brown-dwarf transition, and the semimajor axis of 1.084 +/- 0.023 AU. The nearly circular orbit despite the large mass and intermediate orbital period makes this companion unique among known substellar companions.

DOI： 10.1088/0004-637X/703/1/671

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Language：English   Publisher：AMER INST PHYSICS  

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the alpha-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

DOI： 10.1063/1.3152287

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：English   Publisher：AMER INST PHYSICS  

We report the detection of a new large-mass planet orbiting around a K0 IV(V = 8.26) star which has a minimum mass M-p sin i = 10.70 +/- 0.50M(Jup) in a 696.0 +/- 2.6-day orbit. It was detected in precise radial velocity (RV) measurements from Subaru and Keck. The derived orbital parameters, based on a chi(2) which minimized by Downhill Simplex algorithm, suggests that these radial velocity variations are consistent with an almost circular planetary orbit and a Mars-like semimajor axis(e similar to 0.0, a = 1.70 +/- 0.03AU). Extra-solar planets that have several times the mass of Jupiter orbiting in periods of hundreds or thousands of days, with very low eccentricities(e &lt; 0.1), are rare discoveries. Our detection presents a new sample of these circular orbit massive planets.

DOI： 10.1063/1.3215854

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Language：English   Publisher：AMER INST PHYSICS  

Interior models suggest that transiting giant extrasolar planets have a significant difference in the amount of heavy elements, including their solid cores. This divergence may have originated from planetesimals accreted onto a protoplanet during the stage of runaway gas accretion. Recent work suggests the rate of planetesimals accreted onto a protoplanet depends strongly on the rate of gas accretion. We have calculated the evolution of a protoplanet by using a one-dimensional quasi-static model and investigated gas accretion rates onto a protoplanet during runaway gas accretion. We have also examined the effects of changing the equation of state for gas, grain opacity, and an initial core mass on the results. After the onset of runaway gas accretion, the magnitude of gas accretion rates was strongly dependent on the three factors, but the dependence of gas accretion rates on planetary mass was not affected by them.

DOI： 10.1063/1.3215856

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Language：Japanese   Publisher：日本惑星科学会  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：English   Publisher：Proceedings of the International Astronomical Union  

The origins of extrasolar gas giant planets have been discussed, based on our understanding of the gas giant planets in the solar system, Jupiter and Saturn. However, how Jupiter and Saturn formed is still uncertain because of the uncertainty in their interiors, especially the core mass (Mc). The uncertainty in Mc is partly due to those in observational data such as gravitational moments (J2n), equatorial radius (Req) and 1-bar temperatures (T1bar). New frontiers mission to Jupiter by NASA (JUNO) launched in 2011 is expected to reduce the observational errors. However, it is not necessarily clear yet which observational uncertainty dominates and how accurate observation is needed to constrain Mc enough to know the origin of Jupiter. Thus, modeling the interior of Jupiter, we evaluate each effect on Mc and required precision. We have found that the observational error of 5% in T1bar yields an error of several M in Mc. We have also found that the values of J6 of our successful models are confined in a narrow range compared to its observational error. This implies that comparison between the values of J6 of our successful models and the J6 value obtained from JUNO mission helps us to know whether the present theoretical model is valid. © 2008 International Astronomical Union.

DOI： 10.1017/S1743921308016554

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Publisher：THE JAPANESE SOCIETY FOR PLANETARY SCIENCES  

Equation of state (EOS) for hydrogen under high pressure is a key quantity in un-derstanding the internal structures of gas giant planets like Jupiter. We just started laser experiments using the Gekko XII system in ILE, Osaka University to determine the EOS for hydrogen under high pressure around 2 Mbar. Our goal is to reveal the transition phase from the molecular form to metallic hydrogen. We will show the current status of our experiment, which will be able to give a constraint on the formation scenario of Jovian planets.

DOI： 10.14909/jsps.2006f.0.56.0

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Presentation type：Poster presentation  

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Presentation type：Poster presentation  

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Presentation type：Poster presentation  

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