1201.1504 (Jonathan J. Fortney)

On the Carbon-to-Oxygen Ratio Measurement in Nearby Sunlike Stars: Implications for Planet Formation and the Determination of Stellar Abundances    [PDF]

Jonathan J. Fortney

Recent high resolution spectroscopic analysis of nearby FGK stars suggests that a high C/O ratio of greater than 0.8, or even 1.0, is relatively common. Two published catalogs of measurements find C/O>0.8 in 25-30% of systems, and C/O>1.0 in ~6-10% of systems. It has been suggested that in protoplanetary disks with C/O>0.8 that the condensation pathways to refractory planet-making material will differ from what occurred in our solar system, where C/O=0.55. The carbon-rich disks are calculated to make carbon-dominated rocky planets, rather than oxygen-dominated ones, which would be very unlike the Earth. Here we suggest that the derived stellar C/O ratios are overestimated, given the extreme paucity of carbon dwarfs stars (<0.1%) found in large samples of low mass stars, such as from the Sloan Digital Sky Survey. The reason for this overestimation is not immediately apparent, but could be due to the choices of lines used, or limitations of abundance analysis from one-dimensional LTE stellar atmosphere models. Furthermore, from the estimated errors on the measured stellar C/O ratios, we find that the significance of the high C/O tail is weakened, with a true measured fraction of C/O>0.8 in 10-15% of stars, and C/O>1.0 in 1-5%, athough these are still likely overestimates. We suggest that infrared T-dwarf spectra could show how common high C/O is in the stellar neighborhood, as the chemistry and spectra of such objects would differ compared to those with solar-like abundances. We expect that carbon-dominated rocky planets are rarer than others have suggested.

View original: http://arxiv.org/abs/1201.1504