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

TL;DR

This study uses VLT/X-shooter spectroscopy to analyze the low-metallicity galaxy PHL 293B, revealing properties of its luminous blue variable star, including wind velocity, spectral evolution, and chemical abundances, contributing to understanding massive star behavior in metal-poor environments.

Contribution

First detailed spectroscopic analysis of the LBV star in PHL 293B across optical and NIR wavelengths, showing temporal evolution and wind acceleration in a low-metallicity galaxy.

Findings

LBV star shows broad emission with P Cygni profiles indicating wind outflows.

Terminal wind velocity is approximately 800 km/s, comparable to LBVs during outbursts.

Spectral evolution over 8 years suggests changes in the LBV star's wind properties.

Abstract

(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.