C/O: Effects on Habitability of Stellar Exoplanet Systems
Résumé
We assess how differences in the composition of exoplanet host stars might affect the availability of water in their systems, particularly the role of carbon and oxygen abundances. Water, one of the key chemical ingredients for habitability, may be in short supply in carbon-rich, oxygen-poor systems even if planets exist in the ‘habitable zone’. For the solar system, C/O = 0.55 is particularly important in determining the refractory (silicate and metal) to volatile ice ratio expected in material condensed beyond the snow line (Gaidos E. J. Icarus 145, 637, 2000; Wong M. H. et al. in Oxygen in the Solar System, G.J. MacPherson, Ed., 2008). Our analysis of published compositions for a set of exoplanet host stars (Johnson T. V. et al. ApJ. 757(2), 192, 2012) showed that the amount of condensed water ice in those systems might range from as much as 50% by mass for sub-solar C/O = 0.35 to less than a few percent for super-solar C/O = 0.7. A recent analysis using similar techniques (Pekmezci G. S., Dottorato di Ricerca in Astronomia, Università Degli Studi di Roma “Tor Vergata”, 2014) of a much larger stellar composition data set for 974 FGK stars (Petigura E. and Marcy G. Journal of Astrophysics 735, 2011), allows us to assess the possible range of water ice abundance in the circumstellar accretion disks of these ‘solar-type’ stars (of which 72 were known to have one or more planets as of 2011). Stellar C/O in a subset (457 stars) of this stellar database with reported C, O, Ni, and Fe abundances ranges from 0.3 to 1.4. The resulting computed water ice fractions and refractory (silicate + metal) fractions range from ~0 to 0.6 and 0.3 to 0.9 respectively. These results have implications for assessing the habitability of exoplanets since they constrain the amount of water available beyond the snow line for dynamical delivery to inner planets, depending on the host stars’ C/O in the circumstellar nebula. TVJ acknowledges government support at JPL/Caltech, under a contract with NASA. JIL was supported by the JWST Project through NASA. O.M. acknowledges support from CNES.