2006PASJ...58..595S

We report on detailed spectroscopic studies performed for the secondary star in the black hole binary (micro-quasar) V4641 Sgr in order to examine its surface chemical composition and to see if its surface shows any signature of pollution by ejecta from a supernova explosion. High-resolution spectra of V4641 Sgr observed in the quiescent state in the blue-visual region are compared with those of the two bright well-studied B9 stars (14 Cyg and n Cap) observed with the same instrument. The effective temperature of V4641 Sgr (10500±200K) is estimated from the strengths of He <FONT SIZE="-2">I</FONT> lines, while its rotational velocity, v sin i (95 ± 10km/s), is estimated from the profile of the Mg <FONT SIZE="-2">II</FONT> line at 4481Å. We obtain abundances of 10 elements and find definite over-abundances of N (by 0.8dex or more) and Na (by 0.8dex) in V4641 Sgr. From line-by-line comparisons of eight other elements (C, O, Mg, Al, Si, Ti, Cr, and Fe) between V4641 Sgr and the two reference stars, we conclude that there is no apparent difference in the abundances of these elements between V4641 Sgr and the two normal late B-type stars, which have been reported to have solar abundances. An evolutionary model of a massive close binary system has been constructed to explain the abundances observed in V4641 Sgr. The model suggests that the progenitor of the black hole forming supernova was as massive as ∼35M_☉ on the main-sequence and, after becoming a ∼10M_☉ He star, underwent a ``dark'' explosion which ejected only N- and Na-rich outer layer of the He star without radioactive ⁵⁶Ni.