Abstract
To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks. Based on the parameters used in a pre-computed population of growing planets, their combination allows us to trace the molecular abundances of the gas that is being collected into planetary atmospheres. We include atmospheric pollution of incoming (icy) planetesimals as well as the effect of refractory carbon erosion noted to exist in our own solar system. We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich (by roughly two orders of magnitude) atmospheres than hot Jupiters, whose C/O are primarily determined by gas accretion. Generally we find a “main sequence” between the fraction of planetary mass accreted through solid accretion and the resulting atmospheric C/O; planets of higher solid accretion fraction have lower C/O. Hot Jupiters whose atmospheres have been chemically characterized agree well with our population of planets, and our results suggest that hot-Jupiter formation typically begins near the water ice line. Lower mass hot Neptunes are observed to be much more carbon rich (with 0.33 ≲ C/O ≲ 1) than is found in our models (C/O ~ 10−2), and suggest that some form of chemical processing may affect their observed C/O over the few billion years between formation and observation. Our population reproduces the general mass-metallicity trend of the solar system and qualitatively reproduces the C/O metallicity anti-correlation that has been inferred for the population of characterized exoplanetary atmospheres.
Highlights
The total bulk elemental abundances of the two most abundant atoms have long been studied in the context of star and planet formation
To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks
We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich atmospheres than hot Jupiters, whose C/O are primarily determined by gas accretion
Summary
The total bulk elemental abundances of the two most abundant atoms (after hydrogen and helium) have long been studied in the context of star and planet formation. In this work we wish to expand to a third dimension of comparison: the bulk chemical composition of the planetary atmosphere To build these synthetic populations of planets we randomly select different initial conditions and/or model parameters from underlying distributions (more in 2.3) and compute the resulting planetary growth. By continually resetting the water abundances in each snapshot (as in the passive disk method), we are restricting these reaction pathways Regardless, this resetting has little impact on our previous results, since every modelled planet accreted gas either near or within the water ice line within 2 AU at 1 Myr. We note that in our model the carbon budget in the outer regions of the disk is dominated by CO. Since the oxygen abundance of the ice is already small at these radii, the low abundance of these frozen carbon species result in these high C/O
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