The carbon-to-oxygen ratio in stars with planets

摘要

Context. In some recent works, the C/O abundance ratio in high-metallicity stars with planets is found to vary by more than a factor of two, i.e. from ~0.4 to C/O ? 1. This has led to discussions about the existence of terrestrial planets with a carbon-dominated composition that is very different from the composition of the Earth. Aims. The mentioned C/O values were obtained by determining carbon abundances from high-excitation C?i lines and oxygen abundances from the forbidden [O?i] line at 6300??. This weak line is, however, strongly affected by a nickel blend at high metallicities. Aiming for more precise C/O ratios, oxygen abundances in this paper are derived from the high-excitation O?i triplet at 7774??. Methods. The C?i lines at 5052 and 5380?? in HARPS spectra were applied to determine carbon abundances of 33 solar-type stars for which FEROS spectra are available for determining oxygen abundances from the O?iλ7774 triplet. Differential abundances with respect to the Sun were derived from equivalent widths using MARCS model atmospheres. Non-LTE corrections were included, and the analysis was carried out with both spectroscopic and photometric estimates of stellar effective temperatures and surface gravities. Results. The results do not confirm the high C/O ratios previously found. C/O shows a tight, slightly increasing dependence on metallicity, i.e. from C/O ? 0.58 at [Fe/H]??=?0.0 to C/O ? 0.70 at [Fe/H]??=?0.4 with an rms scatter of only 0.06. Conclusions. Recent findings of C/O ratios higher than 0.8 in high-metallicity stars seem to be spurious due to statistical errors in estimating the strength of the weak [O?i] line in the Ni?i blended λ6300 feature. Assuming that the composition of a proto-planetary disk is the same as that of the host star, the C/O values found in this paper lend no support to the existence of carbon-rich planets. The small scatter of C/O among thin-disk stars suggests that the nucleosynthesis products of Type II supernovae and low- to intermediate-mass stars are well mixed in the interstellar medium.