ON THE CARBON-TO-OXYGEN RATIO MEASUREMENT IN NEARBY SUN-LIKE STARS: IMPLICATIONS FOR PLANET FORMATION AND THE DETERMINATION OF STELLAR ABUNDANCES

摘要

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 find C/O>0.8 in 25%–30% of systems, and C/O>1.0 in ～6%–10%. It has been suggested that in protoplanetary disks with C/O>0.8 that the condensation pathways to refractory solids will differ from what occurred in our solar system, where C/O=0.55. The carbonrich disks are calculated to make carbon-dominated rocky planets, rather than oxygen-dominated ones. Here we suggest that the derived stellar C/O ratios are overestimated. One constraint on the frequency of high C/O is the relative paucity of carbon dwarf stars (10~(-3)-10~(-5)) found in large samples of low-mass stars. We suggest reasons for this overestimation, including a high C/O ratio for the solar atmosphere model used for differential abundance analysis, the treatment of a Ni blend that affects the O abundance, and limitations of 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%, although these are still likely overestimates. We suggest that infrared T-dwarf spectra could showhowcommon high C/Ois in the stellar neighborhood, as the chemistry and spectra of such bjectswould differ compared to those with solar-like abundances.While possible at C/O>0.8, we expect that carbon-dominated rocky planets are rarer than others have suggested.