THE EFFECTS OF SNOWLINES ON C/O IN PLANETARY ATMOSPHERES

Abstract

The C/O ratio is predicted to regulate the atmospheric chemistry in hot\nJupiters. Recent observations suggest that some exo-planets, e.g. Wasp 12- b,\nhave atmospheric C/O ratios substantially different from the solar value of\n0.54. In this paper we present a mechanism that can produce such atmospheric\ndeviations from the stellar C/O ratio. In protoplanetary disks, different\nsnowlines of oxygen- and carbon-rich ices, especially water and carbon\nmonoxide, will result in systematic variations in the C/O ratio both in the gas\nand in the condensed phase. In particular, between the H2O and CO snowlines\nmost oxygen is present in icy grains - the building blocks of planetary cores\nin the core accretion model - while most carbon remains in the gas-phase. This\nregion is coincidental with the giant-planet forming zone for a range of\nobserved protoplanetary disks. Based on standard core accretion models of\nplanet formation, gas giants that sweep up most of their atmospheres from disk\ngas outside of the water snowline will have C/O?1, while atmospheres\nsignificantly contaminated by evaporating planetesimals will have stellar or\nsub-stellar C/O when formed at the same disk radius. The overall metallicity\nwill also depend on the atmosphere formation mechanism, and exoplanetary\natmospheric compositions may therefore provide constraints on where and how a\nspecific planet formed.\n