Discovery of a massive variable star with Z=Zo/36 in the galaxy DDO 68

TL;DR

This paper reports the discovery of a massive variable star with extremely low metallicity (Z=Zo/36) in galaxy DDO 68, evidenced by transient spectral features indicating a luminous blue variable star.

Contribution

It presents the first identification of a low-metallicity massive variable star in a dwarf galaxy, providing insights into stellar evolution at very low metallicities.

Findings

Detection of transient emission lines in Knot 3 of DDO 68

Spectral evidence of a luminous blue variable star

Estimated metallicity of the progenitor star as Z=Zo/36

Abstract

The Local Volume dwarf galaxy DDO 68, from the spectroscopy of its two brightest HII regions (Knots 1 and 2) was designated as the second most metal-poor star-forming galaxy [12+log(O/H)=7.14]. In the repeated spectral observations in 2008 January with the 6-m telescope (BTA) of the HII region Knot 3 [having 12+log(O/H)=7.10+-0.06], we find a strong evidence of a transient event related to a massive star evolution. From the follow-up observation with the higher spectral resolution in 2008 February, we confirm this phenomenon, and give parameters of its emission-line spectrum comprising of Balmer HI and HeI lines. The luminosities of the strongest transient lines (Ha, Hb) are of a few 10^36 erg s^-1. We also detected an additional continuum component in the new spectrum of Knot 3, which displays the spectral energy distribution raising to ultraviolet. The estimate of the flux of this continuum leads us to its absolute V-band magnitude of ~-7.1. Based on the spectral properties of this transient component, we suggest that it is related to an evolved massive star of luminous blue variable type with Z=Zo/36. We briefly discuss observational constraints on parameters of this unique (in the aspect of the record low metallicity of the progenitor massive star) event and propose several lines of its study.