Masahiro Ikoma

Formation of Giant Planets: Dependences on Core Accretion Rate and Grain Opacity

We have investigated the formation of gaseous envelopes of giant planets with wide ranges of parameters through quasi-static evolutionary simulations. In the nucleated instability model, rapid gas accretion is triggered when the solid core mass exceeds a critical mass. The gas accretion should be regulated essentially by core accretion rate and grain opacity in the outermost envelope. The conventional critical core mass ~5-20 M⊕ (M⊕: Earths mass), however, is based on some nominal values of these quantities. The discovery of extrasolar giant planets requires investigation of the gas accretion processes under various circumstances. Furthermore, the current planetary accretion theory points out that the cores of Jupiter and Saturn would have been isolated from planetesimals and the core accretion would have almost stopped in their later stage of formation before their masses reached the conventional critical core mass. Through numerical simulations of quasi-static evolution of the gaseous envelope, we have investigated the characteristic growth times of the envelope mass for wide ranges of core accretion rate and grain opacity. We also studied the case where core accretion stops before onset of rapid gas accretion. Our main results are (1) the growth time of the envelope mass τg depends strongly on the core mass, moderately on the grain opacity, and weakly on the past core accretion process, and (2) τg is expressed approximately as τg ~ 108(Mcore /M⊕)-2.5(κ gr/1 cm 2 g-1) yr, where Mcore is the core mass and κgr is the grain opacity. Our results combined with the recent planetary accretion theory suggest surface density of solid materials twice as massive as that of the minimum-mass solar nebula model and the longer lifetime of the nebula than the 108 yr needed to form Jupiter and Saturn; otherwise migration of protoplanets may have to be considered. Our extensive parametric study not only confirms the difficulty in the formation of the giant planets quantitatively and rigorously, it also gives essential information in considering the problem of the formation, which is quite useful in applications.

A planetary companion to the hyades giant ε tauri

Wereportthedetection of anextrasolarplanet orbitingTau,oneof thegiantstarsintheHyadesopencluster.This is the first planet ever discovered in an open cluster. Precise Doppler measurements of this star from Okayama Astrophysical Observatory have revealed Keplerian velocity variations with an orbital period of 594:9 � 5:3 days, a semiamplitude of 95:9 � 1: 8ms � 1 , and an eccentricity of 0:151 � 0:023. The minimum mass of the companion is 7:6 � 0:2MJ,andthesemimajoraxisis1:93 � 0:03AUadoptingastellarmassof 2:7 � 0:1M� .Theageof 625Myr for the cluster sets the most secure upper limit ever on the timescale of giant planet formation. The mass of 2.7 Mfor thehoststarisrobustlydeterminedbyisochronefitting,whichmakesthestartheheaviestamongplanet-harboringstars. Puttingtogetherthefactthatnoplanetshavebeenfoundaroundabout100low-massdwarfsinthecluster,thefrequency of massive planets is suggested to be higher around high-mass stars than around low-mass ones. Subject headingg open clusters and associations: individual (Hyades) — planetary systems — stars: individual (� Tauri) — techniques: radial velocities

Constraints on the Mass of a Habitable Planet with Water of Nebular Origin

From an astrobiological point of view, special attention has been paid to the probability of habitable planets in extrasolar systems. The purpose of this study is to constrain a possible range of the mass of a terrestrial planet that can get water. We focus on the process of water production through oxidation of atmospheric hydrogen—the nebular gas having been attracted gravitationally—by oxides available at the planetary surface. For the water production to work well on a planet, a sufficient amount of hydrogen and a temperature high enough to melt the planetary surface are needed. We have simulated the structure of the atmosphere that connects with the protoplanetary nebula for wide ranges of the heat flux, the opacity, and the density of the nebular gas. We have found that both requirements are fulfilled for an Earth-mass planet for wide ranges of the parameters. We have also found that the surface temperature of planets of ≤0.3ME (where ME is Earths mass) is lower than the melting temperature of silicate (~1500 K). On the other hand, a planet of more than several ME becomes a gas giant through runaway accretion of the nebular gas.

Enhanced collisional growth of a protoplanet that has an atmosphere

Once a protoplanet becomes larger than about lunar size, it accumulates a significant atmosphere that surrounds the solid core. When a planetesimal approaches the protoplanet, it interacts with the atmosphere. If enough energy of the planetesimal is lost by gas drag of the atmosphere, it is captured in the atmosphere even if its original trajectory would not lead to a direct collision with the solid core of the protoplanet. This increases the collision rate, resulting in faster growth of the protoplanet. We have derived the analytical calculations for the collision rate, and calculated the structure of the atmosphere and the trajectories of the planetesimals in the atmosphere. As a result of their large gas drag, small planetesimals are easily captured, resulting in a large rate of collision with the protoplanet. A collision rate of a protoplanet of Earth size with a planetesimal of 100 m radius is, for example, enhanced by a factor of ∼10. These effects play an essential role in the study of formation of solid cores of gas giant planets by the core accretion model.

PLANET ENGULFMENT BY ∼1.5-3 M ☉ RED GIANTS

Recent radial-velocity surveys for GK clump giants have revealed that planets also exist around ~1.5-3 Msun stars. However, no planets have been found inside 0.6 AU around clump giants, in contrast to solar-type main-sequence stars, many of which harbor short-period planets such as hot Jupiters. In this study we examine the possibility that planets were engulfed by host stars evolving on the red-giant branch (RGB). We integrate the orbital evolution of planets in the RGB and helium burning (HeB) phases of host stars, including the effects of stellar tide and stellar mass loss. Then we derive the critical semimajor axis (or the survival limit) inside which planets are eventually engulfed by their host stars after tidal decay of their orbits. Especially, we investigate the impact of stellar mass and other stellar parameters on the survival limit in more detail than previous studies. In addition, we make detailed comparison with measured semimajor axes of planets detected so far, which no previous study did. We find that the critical semimajor axis is quite sensitive to stellar mass in the range between 1.7 and 2.1 Msun, which suggests a need for careful comparison between theoretical and observational limits of existence of planets. Our comparison demonstrates that all those planets are beyond the survival limit, which is consistent with the planet-engulfment hypothesis. However, on the high-mass side (> 2.1 Msun), the detected planets are orbiting significantly far from the survival limit, which suggests that engulfment by host stars may not be the main reason for the observed lack of short-period giant planets. To confirm our conclusion, the detection of more planets around clump giants, especially with masses > 2.5 Msun, is required.

In Situ Accretion of Hydrogen-rich Atmospheres on Short-period Super-Earths: Implications for the Kepler-11 Planets

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 bodys 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.

SELF-CONSISTENT MODEL ATMOSPHERES AND THE COOLING OF THE SOLAR SYSTEM'S GIANT PLANETS

We compute grids of radiative-convective model atmospheres for Jupiter, Saturn, Uranus, and Neptune over a range of intrinsic fluxes and surface gravities. The atmosphere grids serve as an upper boundary condition for models of the thermal evolution of the planets. Unlike previous work, we customize these grids for the specific properties of each planet, including the appropriate chemical abundances and incident fluxes as a function of solar system age. Using these grids, we compute new models of the thermal evolution of the major planets in an attempt to match their measured luminosities at their known ages. Compared to previous work, we find longer cooling times, predominantly due to higher atmospheric opacity at young ages. For all planets, we employ simple standard cooling models that feature adiabatic temperature gradients in the interior H/He and water layers, and an initially hot starting point for the calculation of subsequent cooling. For Jupiter, we find a model cooling age ~10% longer than previous work, a modest quantitative difference. This may indicate that the hydrogen equation of state used here overestimates the temperatures in the deep interior of the planet. For Saturn, we find a model cooling age ~20% longer than previous work. However, an additional energy source, such as that due to helium phase separation, is still clearly needed. For Neptune, unlike in work from the 1980s and 1990s, we match the measured T eff of the planet with a model that also matches the planets current gravity field constraints. This is predominantly due to advances in the high-pressure equation of state of water. This may indicate that the planet possesses no barriers to efficient convection in its deep interior. However, for Uranus, our models exacerbate the well-known problem that Uranus is far cooler than calculations predict, which could imply strong barriers to interior convective cooling. The atmosphere grids are published here as tables, so that they may be used by the wider community.

Formation of Giant Planets in Dense Nebulae: Critical Core Mass Revisited

The formation of giant planets is explained by the nucleated instability model, in which a solid core captures a large amount of nebular gas when it grows to critical core mass. It is well known that critical core mass scarcely depends on the boundary conditions of the envelope, i.e., its distance from the central star and the density and temperature of the nebular gas. However, this is not the case when the envelope is wholly convective. Such a situation is realized if we consider the formation of giant planets close to central stars and/or in dense cool nebulae. In the present study, we extensively investigate the dependence of the critical core mass on the distance from the central star and on the density and temperature of the nebular gas; we found that the critical core mass reduces to 2-3 M⊕ at 0.1 AU in dense nebulae with a surface density about 20 times larger than that in the minimum-mass solar nebula model. This result suggests a possibility of in situ formation of the detected extrasolar giant planets close to the central stars.

Origin of the Ocean on the Earth: Early Evolution of Water D/H in a Hydrogen-rich Atmosphere

Abstract The origin of the Earths ocean has been discussed on the basis of deuterium/hydrogen ratios (D/H) of several sources of water in the Solar System. The average D/H of carbonaceous chondrites (CCs) is known to be close to the current D/H of the Earths ocean, while those of comets and the solar nebula are larger by about a factor of two and smaller by about a factor of seven, respectively, than that of the Earths ocean. Thus, the main source of the Earths ocean has been thought to be CCs or adequate mixing of comets and the solar nebula. However, those conclusions are correct only if D/H of water on the Earth has remained unchanged for the past 4.5 Gyr. In this paper, we investigate evolution of D/H in the ocean in the case that the early Earth had a hydrogen-rich atmosphere, the existence of which is predicted by recent theories of planet formation no matter whether the nebula remains or not. Then we show that D/H in the ocean increases by a factor of 2–9, which is caused by the mass fractionation during atmospheric hydrogen loss, followed by deuterium exchange between hydrogen gas and water vapor during ocean formation. This result suggests that the apparent similarity in D/H of water between CCs and the current Earths ocean does not necessarily support the CCs origin of water and that the apparent discrepancy in D/H is not a good reason for excluding the nebular origin of water.

Gas giant formation with small cores triggered by envelope pollution by icy planetesimals

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.