VLT/X-shooter observations of the low-metallicity blue compact dwarf
  galaxy PHL 293B including a luminous blue variable star
  (abridged) We present VLT/X-shooter spectroscopic observations in the
wavelength range 3000-23000A of the extremely metal-deficient blue compact
dwarf (BCD) galaxy PHL 293B containing a luminous blue variable (LBV) star. We
determine abundances of N, O, Ne, S, Ar, and Fe and study the properties of the
LBV from the fluxes and widths of broad emission lines. We derive an
interstellar oxygen abundance of 12+log O/H = 7.71+/-0.02, which is in
agreement with previous determinations. The observed fluxes of narrow Balmer,
Paschen and Brackett hydrogen lines correspond to the theoretical recombination
values after correction for extinction with a single value C(Hbeta) = 0.225.
This implies that the star-forming region observed in the optical range is the
only source of ionisation and there is no additional source of ionisation that
is seen in the NIR range but is hidden in the optical range. For the LBV star
in PHL 293B we find broad emission with P Cygni profiles in several Balmer
hydrogen emission lines and for the first time in several Paschen hydrogen
lines and in several HeI emission lines, implying temporal evolution of the LBV
on a time scale of 8 years. The Halpha luminosity of the LBV star is by one
order of magnitude higher than the one obtained for the LBV star in NGC
2363=Mrk 71 which has a slightly higher metallicity 12+logO/H = 7.87. The
terminal velocity of the stellar wind in the low-metallicity LBV of PHL293B is
high, ~800 km/s, and is comparable to that seen in spectra of some
extragalactic LBVs during outbursts. We find that the averaged terminal
velocities derived from the Paschen and HeI emission lines are by some ~40-60
km/s lower than those derived from the Balmer emission lines. This probably
indicates the presence of the wind accelerating outward.
