Shedding some light on planet formation: Temperature perturbations caused by stellar illumination near an embedded protoplanet and their effects on planet formation processes.

摘要

I present a method for calculating radiative transfer on a protoplanetary disk perturbed by a protoplanet and examine some of the ramifications of the resulting temperature variations. The gravitational potential of a protoplanet compresses the disk material around it, so that the disk's surface takes on the shape of a well. When such a well is illuminated by stellar irradiation at grazing incidence, it results in cooling in a shadowed region and heating in an exposed region.;I calculate the variation of disk temperatures in an analytic disk model with a perturbing planet including radiative transfer, with viscous heating and stellar illumination as the primary heat sources. Planets at the gap-opening threshold can induce temperature variations of up to +/-30% in the disk photosphere. Shadowing and illumination in the vicinity of a planet affects ice formation in regions close to the sublimation temperature of water ice. In particular, shadowing effects allow ice to form closer to the star than previously expected, effectively moving the "snow line" inward. The presence or absence of ice can enhance or reduce, respectively, the accretion rate of disk material onto a planet.;The rate of Type I migration in the planet-disk system is also affected by temperature perturbations. The vertical thickness of the disk reduces the Type I migration rate by a factor of about two compared to a two-dimensional disk. A planet at the gap-opening threshold at a distance where viscous heating is minimal has a migration rate of up to another factor of two smaller than the unperturbed disk because of temperature perturbations resulting from shadowing and illumination at the disk's surface. This may be a mechanism for saving planets from falling into their parents stars, resulting in the high frequency of extrasolar planets we observe.;Even relatively small protoplanets can induce significant temperature variations in a passive disk which can change planetary accretion rates and migration rates. Therefore, many of the processes involved in planet formation should not be modeled with a locally isothermal equation of state.