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Register a new userToward a Deterministic Model of Planetary Formation V. Accumulation Near the Ice Line
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We address two outstanding issues in the sequential accretion scenario for gas giant planet formation, the retention of dust grains in the presence of gas drag and that of cores despite type I migration. The efficiency of these processes is determined by the disk structure. Theoretical models suggest that planets form in protostellar disk regions with an inactive neutral ``dead zone near the mid plane, sandwiched together by partially ionized surface layers where magnetorotational instability is active. Due to a transition in the abundance of dust grains, the active layers thickness decreases abruptly near the ice line. Over a range of modest accretion rates ($sim 10^{-9}-10^{-8} M_odot$ yr$^{-1}$), the change in the angular momentum transfer rate leads to local surface density and pressure distribution maxima near the ice line. The azimuthal velocity becomes super-Keplerian and the grains accumulate in this transition zone. This barrier locally retains protoplanetary cores and enhances the heavy element surface density to the critical value needed to initiate efficient gas accretion. It leads to a preferred location and epoch of gas giant formation. We simulate and reproduce the observed frequency and mass-period distribution of gas giants around solar type stars without having to greatly reduce the type I migration strength. The mass function of the short-period planets can be utilized to calibrate the efficiency of type I migration and to extrapolate the fraction of stars with habitable terrestrial planets.
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In a further development of a deterministic planet-formation model (Ida & Lin 2004), we consider the effect of type-I migration of protoplanetary embryos due to their tidal interaction with their nascent disks. During the early embedded phase of protostellar disks, although embryos rapidly emerge in regions interior to the ice line, uninhibited type-I migration leads to their efficient self-clearing. But, embryos continue to form from residual planetesimals at increasingly large radii, repeatedly migrate inward, and provide a main channel of heavy element accretion onto their host stars. During the advanced stages of disk evolution (a few Myr), the gas surface density declines to values comparable to or smaller than that of the minimum mass nebula model and type-I migration is no longer an effective disruption mechanism for mars-mass embryos. Over wide ranges of initial disk surface densities and type-I migration efficiency, the surviving population of embryos interior to the ice line has a total mass several times that of the Earth. With this reservoir, there is an adequate inventory of residual embryos to subsequently assemble into rocky planets similar to those around the Sun. But, the onset of efficient gas accretion requires the emergence and retention of cores, more massive than a few M_earth, prior to the severe depletion of the disk gas. The formation probability of gas giant planets and hence the predicted mass and semimajor axis distributions of extrasolar gas giants are sensitively determined by the strength of type-I migration. We suggest that the observed fraction of solar-type stars with gas giant planets can be reproduced only if the actual type-I migration time scale is an order of magnitude longer than that deduced from linear theories.
The ubiquity of planets and diversity of planetary systems reveal planet formation encompass many complex and competing processes. In this series of papers, we develop and upgrade a population synthesis model as a tool to identify the dominant physical effects and to calibrate the range of physical conditions. Recent planet searches leads to the discovery of many multiple-planet systems. Any theoretical models of their origins must take into account dynamical interaction between emerging protoplanets. Here, we introduce a prescription to approximate the close encounters between multiple planets. We apply this method to simulate the growth, migration, and dynamical interaction of planetary systems. Our models show that in relatively massive disks, several gas giants and rocky/icy planets emerge, migrate, and undergo dynamical instability. Secular perturbation between planets leads to orbital crossings, eccentricity excitation, and planetary ejection. In disks with modest masses, two or less gas giants form with multiple super-Earths. Orbital stability in these systems is generally maintained and they retain the kinematic structure after gas in their natal disks is depleted. These results reproduce the observed planetary mass-eccentricity and semimajor axis-eccentricity correlations. They also suggest that emerging gas giants can scatter residual cores to the outer disk regions. Subsequent in situ gas accretion onto these cores can lead to the formation of distant (> 30AU) gas giants with nearly circular orbits.
I examine a variety of snow crystal growth measurements taken at a temperature of -5 C, as a function of supersaturation, background gas pressure, and crystal morphology. Both plate-like and columnar prismatic forms are observed under different conditions at this temperature, along with a diverse collection of complex dendritic structures. The observations can all be reasonably understood using a single comprehensive physical model for the basal and prism attachment kinetics, together with particle diffusion of water vapor through the surrounding medium and other well-understood physical processes. A critical model feature is structure-dependent attachment kinetics (SDAK), for which the molecular attachment kinetics on a faceted surface depend strongly on the nearby mesoscopic structure of the crystal.
I examine a variety snow crystal growth experiments performed at temperatures near -2 C, as a function of supersaturation, background gas pressure, and crystal morphology. Although the different experimental data were obtained using quite diverse experimental techniques, the resulting measurements can all be reasonably understood using a single comprehensive physical model for the basal and prism attachment kinetics, together with particle diffusion of water vapor through the surrounding medium and other well-understood physical processes. As with the previous paper in this series, comparing and reconciling different data sets at a single temperature yields significant insights into the underlying physical processes that govern snow crystal growth dynamics.
Context. HD72946 is a bright and nearby solar-type star hosting a low-mass companion at long period (P~16 yr) detected with the radial velocities (RV) method. The companion has a minimum mass of 60.4+/-2.2 MJ and might be a brown dwarf. Its expected semi-major axis of ~243 mas makes it a suitable target for further characterization with high-contrast imaging, in particular to measure its inclination, mass, and spectrum and thus definitely establish its substellar nature. Aims. We aim to further characterize the orbit, atmosphere, and physical nature of HD72946B. Methods. We present high-contrast imaging data in the near-infrared with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. We also use proper motion measurements of the star from Hipparcos and Gaia. Results. The SPHERE data reveal a point source with a contrast of ~9 mag at a projected separation of ~235 mas. No other point sources are detected in the field of view. By jointly fitting the RV, imaging, and proper motion data, we constrain all the orbital parameters of HD72946B and assess a dynamical mass of 72.4+/-1.6 MJ and a semi-major axis of 6.45$^{+0.08}_{-0.07}$ au. Empirical comparison of its IFS spectrum to template dwarfs indicates a spectral type of L5.0+/-1.5. The J-H3 color is close to the expectations of the DUSTY models and suggests a cloudy atmosphere. Comparison with atmospheric models of the spectrophotometry suggests an effective temperature of ~1700 K. The bolometric luminosity (log(L/LS)=-4.11+/-0.10 dex) and dynamical mass of HD72946B are more compatible with evolutionary models for an age range of ~0.9-3 Gyr. The formation mechanism of the companion is currently unclear as it appears slightly away from the bulk of model predictions. HD72946B is currently the closest benchmark brown dwarf companion to a solar-type star with imaging, RV, and proper motion measurements.
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