Planet formation in the PDS 70 system

Abstract

Understanding of the chemical link between protoplanetary disks and planetary atmospheres is complicated by the fact that the popular targets in the study of disks and planets are widely separated both in space and time. The 5 Myr PDS 70 systems offers a unique opportunity to directly compare the chemistry of a giant planet’s atmosphere to the chemistry of its natal disk. To this end, we derived our current best physical and chemical model for the PDS 70 disk through forward modelling of the 12CO, C18O, and C2H emission radial profiles with the thermochemical code DALI and found a volatile carbon-to-oxygen number ratio (C/O) above unity in the outer disk. Using what we know of the PDS 70 disk today, we analytically estimated the properties of the disk as it was 4 Myr in the past when we assume that the giant planets started their formation, and computed a chemical model of the disk at that time. We computed the formation of PDS 70b and PDS 70c using the standard core-accretion paradigm and accounted for the accretion of volatile and refractory sources of carbon and oxygen to estimate the resulting atmospheric C/O for these planets. Our inferred C/O of the gas in the PDS 70 disk indicates that it is marginally carbon rich relative to the stellar C/O = 0.44, which we derived from an empirical relation between stellar metallicity and C/O. Under the assumption that the disk has been carbon rich for most of its lifetime, we find that the planets acquire a super-stellar C/O in their atmospheres. If the carbon-rich disk is a relatively recent phenomenon (i.e. developed after the formation of the planets at ~1 Myr), then the planets should have close to the stellar C/O in their atmospheres. This work lays the groundwork to better understand the disk in the PDS 70 system as well as the planet formation scenario that produced its planets.