VLT/X-shooter observations of the low-metallicity blue compact dwarf galaxy PHL 293B including a luminous blue variable star^⋆,⋆⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: izotov@mao.kiev.ua
² Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
³ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen 92190 Meudon, France
⁴ Institut für Astrophysik, Göttingen Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Received: 10 December 2010
Accepted: 20 July 2011

Abstract

Context. We present VLT/X-shooter spectroscopic observations in the wavelength range λλ3000–23 000 Å of the extremely metal-deficient blue compact dwarf (BCD) galaxy PHL 293B containing a luminous blue variable (LBV) star and compare them with previous data.

Aims. This BCD is one of the two lowest-metallicity galaxies where LBV stars were detected, allowing us to study the LBV phenomenon in the extremely low metallicity regime.

Methods. We determine abundances of nitrogen, oxygen, neon, sulfur, argon, and iron by analyzing the fluxes of narrow components of the emission lines using empirical methods and study the properties of the LBV from the fluxes and widths of broad emission lines.

Results. 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(Hβ) = 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. We detect three v = 1–0 vibrational lines of molecular hydrogen. Their flux ratios and non-detection of v = 2–1 and 3–1 emission lines suggest that collisional excitation is the main source producing H₂ lines. 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 He i emission lines, implying temporal evolution of the LBV on a time scale of 8 years. The Hα 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^-1, 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 He i emission lines are by some  ~40–60 km s^-1 lower than those derived from the Balmer emission lines. This probably indicates the presence of the wind accelerating outward.