[en] 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's interior suggest that it contains a small core of <10 M₊, while many core-accretion models of Jupiter formation require a large core of >10 M₊ 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 1.7 M₊ 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₊ 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 1500 K 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.

[en] Full text: We present results of the effect of pollution due to planetesimals' disruption in the envelope of protoplanets. We show that taking into account the change of composition due to the addition of elements heavier that H and He in the equation of state and in the opacities, affects dramatically the critical core mass. Furthermore, we compute the timescale for gas accretion onto super critical planets. Comparing this timescale to the one of solid accretion, we discuss the implications on the formation of giant planets. (author)

[en] The present infrared brightness of a planet originates partly from the accretion energy that the planet gained during its formation and hence provides important constraints to the planet formation process. A planet cools down from a hot initial state to the present state by losing energy through radiative emission from its atmosphere. Thus, the atmospheric properties affect the planetary cooling rate. Previous theories of giant planet cooling assume that the atmospheric composition is unchanged throughout the evolution. Planet formation theories, however, suggest that the atmospheres especially of ice giants are rich in heavy elements in the early stages. These heavy elements include condensable species such as H₂O, NH₃, and CH₄, which are expected to have a great impact on atmospheric temperature and thus on radiative emission through latent heat release. In this study we investigate the effect of such condensation on the planetary emission flux and quantify the impact on the cooling timescale. We then demonstrate that the latent heat of these species keeps the atmosphere hot and thus the emission flux high for billions of years, resulting in an acceleration of the cooling of ice giants. This sheds light on the long-standing problem that Uranus is much less bright than theoretically predicted and is different in brightness from Neptune in spite of the similarity in mass and radius. We also find that young ice giants with highly enriched atmospheres are much brighter in the mid-infrared than ice giants with non-enriched atmospheres. This provides important implications for future direct imaging of extrasolar ice giants.

[en] Violent variation of transit depths and an ingress-egress asymmetry of the transit light curve discovered in KIC 12557548 have been interpreted as evidence of a catastrophic evaporation of atmosphere with dust ( M-dot _p∼>1 M_⊕ Gyr^–1) from a close-in small planet. To explore what drives the anomalous atmospheric escape, we perform time-series analysis of the transit depth variation of Kepler archival data for ∼3.5 yr. We find a ∼30% periodic variation of the transit depth with P ₁ = 22.83 ± 0.21 days, which is within the error of the rotation period of the host star estimated using the light curve modulation, P _rot = 22.91 ± 0.24 days. We interpret the results as evidence that the atmospheric escape of KIC 12557548b correlates with stellar activity. We consider possible scenarios that account for both the mass loss rate and the correlation with stellar activity. X-ray and ultraviolet (XUV)-driven evaporation is possible if one accepts a relatively high XUV flux and a high efficiency for converting the input energy to the kinetic energy of the atmosphere. Star-planet magnetic interaction is another possible scenario, though huge uncertainty remains for the mass loss rate

[en] Motivated by recent detection of transiting high-density super-Earths, we explore the detectability of hot rocky super-Earths orbiting very close to their host stars. In an environment hot enough for their rocky surfaces to be molten, they would have an atmosphere composed of gas species from the magma oceans. In this study, we investigate the radiative properties of the atmosphere that is in gas/melt equilibrium with the underlying magma ocean. Our equilibrium calculations yield Na, K, Fe, Si, SiO, O, and O_2 as the major atmospheric species. We compile the radiative absorption line data of those species available in the literature and calculate their absorption opacities in the wavelength region of 0.1–100 μm. Using them, we integrate the thermal structure of the atmosphere. Then, we find that thermal inversion occurs in the atmosphere because of the UV absorption by SiO. In addition, we calculate the ratio of the planetary to stellar emission fluxes during secondary eclipse, and we find prominent emission features induced by SiO at 4 μm detectable by Spitzer, and those at 10 and 100 μm detectable by near-future space telescopes

[en] Accreting planets have been detected through their hydrogen-line emission, specifically Hα. To interpret this, stellar-regime empirical correlations between the Hα luminosity L _Hα and the accretion luminosity L _acc or accretion rate $\stackrel{\dot{}<!--\; ??\; -->}{M}$ have been extrapolated to planetary masses, however without validation. We present a theoretical L _acc–L _Hα relationship applicable to a shock at the surface of a planet. We consider wide ranges of accretion rates and masses and use detailed spectrally resolved, nonequilibrium models of the postshock cooling. The new relationship gives a markedly higher L _acc for a given L _Hα than fits to young stellar objects, because Lyα, which is not observable, carries a large fraction of L _acc. Specifically, an L _Hα measurement needs 10 to 100 times higher L _acc and $\stackrel{\dot{}<!--\; ??\; -->}{M}$ than previously predicted, which may explain the rarity of planetary Hα detections. We also compare the $\stackrel{\dot{}<!--\; ??\; -->}{M}$–L _Hα relationships coming from the planet-surface shock or implied by accretion-funnel emission. Both can contribute simultaneously to an observed Hα signal, but at low (high) $\stackrel{\dot{}<!--\; ??\; -->}{M}$ the planetary-surface shock (heated funnel) dominates. Only the shock produces Gaussian line wings. Finally, we discuss accretion contexts in which different emission scenarios may apply, putting recent literature models in perspective, and also present L _acc–L _line relationships for several other hydrogen lines.

[en] The Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT) is an optical three-band ($g_2^{\mathrm{\prime <!--\; ???\; -->}}$-, $r_2^{\mathrm{\prime <!--\; ???\; -->}}$- and $z_{\mathrm{s},2}$-band) imager that was recently developed for the 188 cm telescope at Okayama Astrophysical Observatory with the aim of validating and characterizing transiting planets. In a pilot observation with MuSCAT we observed a primary transit of HAT-P-14b, a high-surface gravity (g_p = 38 ms⁻²) hot Jupiter around a bright (V = 10) F-type star. From a 2.9 hr observation we achieved the five-minute binned photometric precisions of 0.028%, 0.022%, and 0.024% in the $g_2^{\mathrm{\prime <!--\; ???\; -->}}$, $r_2^{\mathrm{\prime <!--\; ???\; -->}}$, and $z_{\mathrm{s},2}$ bands, respectively, which provided the highest-quality photometric data for this planet. Combining these results with those of previous observations, we search for variations of transit timing and duration over five years as well as variations of planet-star radius ratio ($R_{\mathrm{p}}/R_{\mathrm{s}}$) with wavelengths, but can find no considerable variation in any parameters. On the other hand, using the transit-subtracted light curves we simulate the achievable measurement error of $R_{\mathrm{p}}/R_{\mathrm{s}}$ with MuSCAT for various planetary sizes, assuming three types of host stars: HAT-P-14, the nearby K-dwarf HAT-P-11, and the nearby M-dwarf GJ1214. Comparing our results with the expected atmospheric scale heights, we find that MuSCAT is capable of probing the atmospheres of planets as small as a sub-Jupiter ($R_{\mathrm{p}}\sim <!--\; ???\; -->6R_{\oplus <!--\; ???\; -->}$) around HAT-P-14 in all bands, a Neptune ($\sim <!--\; ???\; -->4R_{\oplus <!--\; ???\; -->}$) around HAT-P-11 in all bands, and a super-Earth ($\sim <!--\; ???\; -->2.5R_{\oplus <!--\; ???\; -->}$) around GJ1214 in $r_2^{\mathrm{\prime <!--\; ???\; -->}}$ and $z_{\mathrm{s},2}$ bands. These results promise that MuSCAT will produce fruitful scientific outcomes in the K2 and TESS era.

[en] We report a new evaluation of the accretion properties of PDS 70b obtained with the Very Large Telescope/Multi Unit Spectroscopic Explorer. The main difference from the previous studies of Haffert et al. and Aoyama & Ikoma is in the mass accretion rate. Simultaneous multiple line observations, such as Hα and Hβ, can better constrain the physical properties of an accreting planet. While we clearly detected Hα emissions from PDS 70b, no Hβ emissions were detected. We estimate the line flux of Hβ with a 3σ upper limit to be 2.3 × 10⁻¹⁶ erg s⁻¹ cm⁻². The flux ratio F _Hβ/F _Hα for PDS 70b is <0.28. Numerical investigations by Aoyama et al. suggest that F _Hβ/F _Hα should be close to unity if the extinction is negligible. We attribute the reduction of the flux ratio to the extinction, and estimate the extinction of Hα (A _Hα) for PDS 70b to be >2.0 mag using the interstellar extinction value. By combining with the Hα linewidth and the dereddening line luminosity of Hα, we derive the PDS 70b mass accretion rate to be ≳5 × 10⁻⁷ M _Jup yr⁻¹. The PDS 70b mass accretion rate is an order of magnitude larger than that of PDS 70. We found that the filling factor f _f (the fractional area of the planetary surface emitting Hα) is ≳0.01, which is similar to the typical stellar value. The small value of f _f indicates that the Hα emitting areas are localized at the surface of PDS 70b.

[en] 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), R_p/R_s= 0.11550^+0.00142_-0.00153 (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 and Burrows suggest that the high molecular weight atmosphere (e.g., 1% H₂O + 99% N₂) 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

[en] We present optical (g', R_c, and I_c) to near-infrared (J) simultaneous photometric observations for a primary transit of GJ3470b, a Uranus-mass transiting planet around a nearby M dwarf, by using the 50 cm MITSuME telescope and the 188 cm telescope, both at the Okayama Astrophysical Observatory. From these data, we derive the planetary mass, radius, and density as 14.1 ± 1.3 M_⊕, 4.32^+0.21_-0.10 R_⊕, and 0.94 ± 0.12 g cm^–3, respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5 μm photometry (0.72^+0.13_-0.12 g cm^–3). Although the planetary radius is about 10% smaller than that reported by Demory et al., this difference does not alter their conclusion that the planet possesses a hydrogen-rich envelope whose mass is approximately 10% of the planetary total mass. On the other hand, we find that the planet-to-star radius ratio (R_p /R_s ) in the J band (0.07577^+0.00072_-0.00075) is smaller than that in the I_c (0.0802 ± 0.0013) and 4.5 μm (0.07806^+0.00052_-0.00054) bands by 5.8% ± 2.0% and 2.9% ± 1.1%, respectively. A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorption and/or scattering by atmospheric molecules. Although the significance of the observed R_p /R_s variations is low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission-spectroscopic observations of this planet to search for certain molecular features, such as H₂O, CH₄, and CO, without being prevented by clouds.