On variability of DDO68-V1, a unique extremely metal-poor LBV

Abstract

DDO68-V1 is a Luminous Blue Variable (LBV) star in the eXtremely Metal-Poor (XMP) galaxy DDO68. It resides in the HII region with 12+log(O/H)~7.1 dex, or Z ~ Zo/40. Since DDO68-V1 is the only known LBV with a so low initial metallicity, its in-deep study can give the hints for understanding the LBV evolutionary stage and the nature of their powerful and highly variable mass loss in the very low-metallicity regime. Our goal is to study the optical variability of DDO68-V1 during the last 36 years, with the emphasis on the period of the last 8 years, after the LBV giant eruption. We use our published results of monitoring in B, V, R bands of the total flux of HII region 'Knot 3', containing the LBV, along with photometry of the archive Hubble Space Telescope (HST) images, obtained in May 2010 and December 2017. This data allow us to disentangle the variable light of DDO68-V1 and that of the underlying HII region. From all available photometry of Knot 3, we derive the V-band lightcurve of DDO68-V1 since 1988, with a higher cadence during the years 2015-2023, when the lightcurve resembles that of S Doradus. The new data reveal the full range of DDO68-V1 absolute magnitudes M_V of [-5.9, --10.8] mag. The LBV variations after the fading of the 'giant eruption' show the unusually large amplitude of delta_V > 3.0-3.5mag on the time-scale of ~1-1.5 year. The apparent changes of the integrated B-V colour of Knot 3 are consistent with the expected colour variations of the LBV in course of the S Doradus 'normal eruptions'. These data, along with spectra of DDO68-V1, demonstrate the need for a higher-cadence photometry of DDO68-V1, in order to probe the possible periodicity in its lightcurve and binarity of the object.