C/O classification of gas giants

C/O ratio as a Dimension for Characterizing Exoplanetary Atmospheres

Proposes a 2-dimensional classification scheme: oxygen-rich (O) vs carbon-rich (C), low irradiation (1) vs high irradiation (2). The irradiation boundary for O-type planets is at the critical flux for thermal inversion, estimated to be in the range 1–5×10⁹ erg/s/cm² (gotta love astronomers and their CGS units). O1 and O2 roughly correspond to the pL and pM classes that have been previously mentioned. For C-type planets the division is set at T=1200 K, motivated by the depletion in abundance of H₂O and enrichment of CH₄ with respect to O-type planets.

Six systems are considered:

XO-1b: C2
CoRoT-2b: tentative C2
WASP-14b: either O1 or C2, the oxygen-rich classification is a marginally better fit to the data
WASP-19b: tentative C2
WASP-33b: either O2 or C2, the carbon-rich classification is a marginally better fit to the data
WASP-12b: C2

A Photometric Study of the Hot Exoplanet WASP-19b
http://arxiv.org/abs/1212.3553

Demonstration of the feasibility of obtaining such measurements with 1m-class telescopes.

Atmosphere seems to be better modelled as inversion-free but oxygen-rich, which is somewhat unexpected. Perhaps the TiO and VO are being destroyed by the stronger UV output of the active host star.

WASP-12b update.

Re-evaluating Hot Jupiter WASP-12b: An Update
http://arxiv.org/abs/1301.0968

Data insufficient to constrain the composition.