Letter to the Editor

The variability of DDO68-V1, a unique, extremely metal-poor luminous blue variable

Special Astrophysical Observatory of RAS, Nizhnij Arkhyz, Karachai-Circassia, 369167, Russia

^⋆ Corresponding author; sap@sao.ru

Received: 1 December 2024
Accepted: 21 February 2025

Abstract

Context. 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 ∼ Z_⊙/40. Since DDO68-V1 is the only known LBV with such a low initial metallicity, its in-deep study can give hints for understanding the LBV evolutionary stage and the nature of their powerful and highly variable mass loss in the very low-metallicity regime.

Aims. Our goal is to study the optical variability of DDO68-V1 during the last 36 years, with an emphasis on the period of the last 8 years, after the LBV giant eruption.

Methods. We used our published monitoring results in the B, V, and R bands of the total flux of HII region ‘Knot 3’, containing the LBV, along with photometry of the archive Hubble Space Telescope images, obtained in May 2010 and December 2017. This data allowed us to disentangle the variable light of DDO68-V1 and that of the underlying HII region.

Results. From all available photometry of Knot 3, we derived the V-band light curve of DDO68-V1 since 1988, with a higher cadence during the years 2015–2023, when the light curve resembles that of S Doradus.

Conclusions. 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 δ V ≳ 3.0–3.5 mag on the timescale 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 over the 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 light curve and binarity of the object. Such data, in turn, will stimulate the elaboration of the advanced models of the very metal-poor massive stars and their application to the study of the early Universe.