Chemical Abundances in the Secondary Star of the Black Hole Binary V4641 Sagittarii (SAX J1819.3−2525)

We report on detailed spectroscopic studies performed for the secondary starin the black hole binary (micro-quasar) V4641 Sgr in order to examine itssurface chemical composition and to see if its surface shows any signature ofpollution by ejecta from a supernova explosion. High-resolution spectra ofV4641 Sgr observed in the quiescent state in the blue-visual region arecompared with those of the two bright well-studied B9 stars (14 Cyg and $\nu$Cap) observed with the same instrument. The effective temperature of V4641 Sgr(10500 $\pm$ 200 K) is estimated from the strengths of He~{\sc i} lines, whileits rotational velocity, $\it v$ sin $\it i$ (95 $\pm$ 10 km s${}^{-1}$), isestimated from the profile of the Mg~{\sc ii} line at 4481 \AA. We obtainabundances of 10 elements and find definite over-abundances of N (by 0.8 dex ormore) and Na (by 0.8 dex) in V4641 Sgr. From line-by-line comparisons of eightother elements (C, O, Mg, Al, Si, Ti, Cr, and Fe) between V4641 Sgr and the tworeference stars, we conclude that there is no apparent difference in theabundances of these elements between V4641 Sgr and the two normal late B-typestars, which have been reported to have solar abundances. An evolutionary modelof a massive close binary system has been constructed to explain the abundancesobserved in V4641 Sgr. The model suggests that the progenitor of the black holeforming supernova was as massive as ~ 35 Msun on the main-sequence and, afterbecoming a ~ 10 Msun He star, underwent "dark" explosion which ejected only Nand Na-rich outer layer of the He star without radioactive $^{56}$Ni.Comment: 13 pages, 14 figures. Accepted for publication in the PASJ: Publications of the Astronomical Society of Japa