Connecting the Chemical Composition of Planetary Atmospheres with Planet Formation

Abstract

What sets the observable chemical composition of exoplanetary atmospheres? The available chemical abundance of the planet's natal protoplanetary disk gas will have a deciding role in the bulk abundance of the atmosphere very early in the planet's life. While late accretion of ices and inter-atmosphere physical processing can change the observable chemical abundances. We have developed a theoretical model which connects the chemical and physical evolution of an accretion disk with the growth of a young planet to predict the bulk chemical abundance of the planetary atmosphere that is inherited from the disk.

We assess what variation in atmospheric chemical abundances are attributed to different planet formation histories. We find differences in the relative abundances of primary nitrogen carriers NH$_3$ and N$_2$ depending on {\it when} the planet accreted its gas. Early ($t<1$ Myr) accreters predominately accreted warmer gas which tend to have its nitrogen in NH$_3$, while later protoplanets accrete colder, more N$_2$ dominated gas.

Furthermore we compute the carbon-to-oxygen ratio (C/O) for each planets, which is used to infer {\it where} a planet forms in its accretion disk. We find that each of our planets accrete their gas very close to the water ice line, thereby accreting `pristine' gas with C/O$_{planet}$ exactly matching its host star. We extend our results by tuning our initial disk parameters to reproduce the properties of the HL Tau disk. We produce three models that span the range of measured gas masses, and one model which studies a UV quiet system. We generally find that planet formation is efficient enough to produce a Jupiter-massed planet within the predicted 1 Myr age of the disk. We find a correspondence between the radial locations of ice lines within our astrochemical model and the set of observed dust gaps in the HL Tau system.