Chemical Abundances of the Secondary Star in the Black Hole X-Ray Binary
  V404 Cygni
  We present a chemical abundance analysis of the secondary star in the black
hole binary V404 Cygni, using Keck I/HIRES spectra. We adopt a
$\chi^2$-minimization procedure to derive the stellar parameters, taking into
account any possible veiling from the accretion disk. With these parameters we
determine the atmospheric abundances of O, Na, Mg, Al, Si, Ca, Ti, Fe, and Ni.
The abundances of Al, Si, and Ti appear to be slightly enhanced when comparing
with average values in thin-disk solar-type stars. The O abundance, derived
from optical lines, is particularly enhanced in the atmosphere of the secondary
star in V404 Cygni. This, together with the peculiar velocity of this system as
compared with the Galactic velocity dispersion of thin-disk stars, suggests
that the black hole formed in a supernova or hypernova explosion. We explore
different supernova/hypernova models having various geometries to study
possible contamination of nucleosynthetic products in the chemical abundance
pattern of the secondary star. We find reasonable agreement between the
observed abundances and the model predictions. However, the O abundance seems
to be too high regardless of the choice of explosion energy or mass cut, when
trying to fit other element abundances. Moreover, Mg appears to be
underabundant for all explosion models, which produces Mg abundances roughly 2
times higher than the observed value.
