Monitoring of DDO 68 ‘‘Northern Ring’’ SF Regions in 2016–2023

Abstract

DDO 68 is a star-forming (SF) dwarf galaxy residing in a nearby void. Its gas metallicity is among the lowest known in the local Universe, with the \(12+\log(\textrm{O/H})\) parameter in the range of 6.96–7.3 dex. Six of its SF regions are located in or near the so-called ‘‘Northern Ring’’, in which the Hubble Space Telescope (HST) images reveal many luminous young stars. We present for these SF regions (Knots) the results of optical monitoring in 35 epochs during the years 2016–2023. The data was acquired with the 6-m (BTA) and 1-m telescopes of the Special Astrophysical Observatory, and the 2.5-m telescope of the MSU Caucasian Mountain Observatory. We complement the above results with the archival data from 10 other telescopes for 11 epochs during the years 1988–2013 and with three our BTA observations between 2005 and 2015. Our goal is to search for variability of these Knots and to relate it to the probable light variations of their brightest stars. One of them, DDO 68-V1 (in Knot 3), was identified in 2008 with a luminous blue variable (LBV) star born in the lowest metallicity environments. For Knot 3, variations of its integrated light in the previous epochs reached about \(0\overset{\textrm{m}}{.}8\). In the period since 2016, the amplitude of the variations in Knot 3 has reached about \(0\overset{\textrm{m}}{.}3\). For the rest of the Knots, due to the lower amplitudes, the manifestation of variability is less pronounced. We examine the presence of variability via the \(\chi^{2}\) criterion and the Robust Median Statistics and discuss the robustness of the detected variations. The variability is detected according to both criteria in the light curves of all Knots with the \(\chi^{2}\) confidence level at \(\alpha=0.0005\). The peak-to-peak amplitudes of the variations are approximately \(0\overset{\textrm{m}}{.}09\), \(0\overset{\textrm{m}}{.}13\), \(0\overset{\textrm{m}}{.}11\), \(0\overset{\textrm{m}}{.}08\), and \(0\overset{\textrm{m}}{.}16\) for Knots 1, 2, 4, 5, and 6, respectively. The amplitudes of the related variations of the brightest supergiants in these regions can reach about \(3\overset{\textrm{m}}{.}0\).