Katherine A. Kretke

Grain Retention and Formation of Planetesimals near the Snow Line in MRI-driven Turbulent Protoplanetary Disks

The first challenge in the formation of both terrestrial planets and the cores of gas giants is the retention of grains in protoplanetary disks. In most regions of these disks, gas attains sub-Keplerian speeds as a consequence of a negative pressure gradient. Hydrodynamic drag leads to orbital decay and depletion of the solid material in the disk, with characteristic timescales as short as only a few hundred years for meter-sized objects at 1 AU. In this Letter, we suggest a particle retention mechanism that promotes the accumulation of grains and the formation of planetesimals near the water sublimation front or snow line. This model is based on the assumption that, in the regions most interesting for planet formation, the viscous evolution of the disk is due to turbulence driven by the magnetorotational instability (MRI) in the surface layers of the disk. The depth to which MRI effectively generates turbulence is a strong function of grain size and abundance. A sharp increase in the grain-to-gas density ratio across the snow line reduces the column depth of the active layer. As the disk evolves toward a quasi-steady state, this change in the active layer creates a local maximum in radial distribution of the gas surface density and pressure, causing the gas to rotate at super-Keplerian speed and halting the inward migration of grains. This scenario presents a robust process for grain retention that may aid in the formation of proto-gas giant cores preferentially near the snow line.

ASSEMBLING THE BUILDING BLOCKS OF GIANT PLANETS AROUND INTERMEDIATE-MASS STARS

We examine a physical process that leads to the efficient formation of gas giant planets around intermediate-mass stars. In the gaseous protoplanetary disks surrounding rapidly accreting intermediate-mass stars, we show that the midplane temperature (heated primarily by turbulent dissipation) can reach 1000 K out to 1 AU. The thermal ionization of this hot gas couples the disk to the magnetic field, allowing the magnetorotational instability (MRI) to generate turbulence and transport angular momentum. Further from the central star the ionization fraction decreases, decoupling the disk from the magnetic field and reducing the efficiency of angular momentum transport. As the disk evolves toward a quasi-steady state, a local maximum in the surface density and in the midplane pressure both develop at the inner edge of the MRI-dead zone, trapping inwardly migrating solid bodies. Small particles accumulate and coagulate into planetesimals which grow rapidly until they reach isolation mass. In contrast to the situation around solar-type stars, we show that the isolation mass for cores at this critical radius around the more-massive stars is large enough to promote the accretion of significant amounts of gas prior to disk depletion. Through this process, we anticipate a prolific production of gas giants at ~1 AU around intermediate-mass stars.

The N2K consortium. VII. Atmospheric parameters of 1907 metal-rich stars: Finding planet-search targets

We report high-precision atmospheric parameters for 1907 stars in the N2K low-resolution spectroscopic survey, designed to identify metal-rich FGK dwarfs likely to harbor detectable planets. Of these stars, 284 are in the ideal temperature range for planet searches, Teff ≤ 6000 K, and have a 10% or greater probability of hosting planets based on their metallicities. The stars in the low-resolution spectroscopic survey should eventually yield >60 new planets, including 8-9 hot Jupiters. Short-period planets have already been discovered orbiting the survey targets HIP 14810 and HD 149143.

Structure of Magnetorotational Instability Active Protoplanetary Disks

The radial drift of planetary cores poses a challenge to efficient planet formation in standard disk models. However, the rate of this migration is sensitive to both the surface density and temperature profiles of protoplanetary disks. In this paper, we present a new model to self-consistently calculate the structure of a protoplanetary disk in which the magnetorotational instability (MRI) drives angular momentum transport. In this model, we calculate a quasi-steady-state disk model including a schematic representation involving efficient angular momentum transport in the active region with decreased (but non-zero) angular momentum transport in the dead zone. We find that MRI affects not only the surface density distribution but also the temperature profile. In this paper, we present our method and the key novel features evident in our fiducial model. In subsequent papers, we will use this model to study the impact of MRI on the formation and migration of planets.