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