A Statistical Approach to Planetesimal Condensate Composition beyond the Snowline Based on the Carbon-to-oxygen Ratio

Abstract

The ratio between carbon and oxygen is regarded as an important driver of circumstellar and planetary chemistry, which can be used as a parameter to estimate the fractionation among refractory and volatile portions of a planet. From this motivation, nearly 500 stars including some with planets discovered around them are investigated. The relation between the C/O ratio and fractions of icy and refractory species is traced for planetesimals expected to form in their protostellar disks. It is found that low C/O ratios lead to planets rich in ice, but poor in organic and other refractory materials. With increasing C/O ratios, the ice fraction diminishes, where organics increase and other refractory materials dominate. Apart from that, the carbon portion incorporated in the solid phase and the redox state of the environment are altered to analyze their influence on bulk formation for generated planets. Under zero solid carbon contribution, ice formation decreases and refractory formation increases with increasing C/O ratio. When the carbon contribution is considered to be entirely in the solid phase, refractory materials are not significantly affected by the increasing C/O ratio while ice may even disappear. For reducing conditions, the C/O ratio is not an obstacle for ice formation no matter how high it is. Under oxidizing conditions, however, water is not found where the C/O ratio is greater than 0.8. Bulk densities are also calculated according to alternative scenarios along with compositional distributions, and results are compared to solar system objects. This study, therefore, exemplifies how a simple correlation can be drawn between stellar chemistry, redox state, and planetesimal composition.