On the chemical abundance differences between the solar twin visual binary system 16 Cygni A and B

Abstract

The visual binary system 16~Cyg~A+B consists of similar solar twins, but a planetary companion is detected only in B. An intensive spectroscopic differential analysis is carried out to the Sun, 16~Cyg~A, and 16~Cyg~B, with particular attentions being paid to (i) precisely establishing the differential atmospheric parameters/metallicity between A and B, and (ii) determining the important CNO abundances based on the lines of CH, NH, and OH molecules. The following results are obtained. (1) The Fe abundances (relative to the Sun) are [Fe/H]^A=+0.09 and [Fe/H]^B=+0.06 (i.e., A is slightly metal-rich than B by +0.03~dex). This lends support to the consequences of recently published papers, while the conclusion once derived by the author (almost the same metallicity for A and B) is acknowledged to be incorrect. (2) The differential abundances (Delta[X/H]) of volatile CNO with low T_c (condensation temperature) are apparently lower than those of refractory Fe group elements of higher T_c, leading to a positive gradient in the Delta[X/H] vs. T_c relation being more conspicuous for A than B. This is qualitatively consistent with previous studies, though the derived slope is quantitatively somewhat steeper than that reported by other authors.