Scientific achievements. Galaxy

2025

Discovery of unusually strong variability in DDO68-V1, a bright blue variable (LBV) with extremely low metallicity

LBVs comprise a small group of young massive stars in advanced evolutionary stages, during which they lose a significant portion of their mass in winds and during “giant eruptions”. In the LBV phase they spend only a brief part of their lives ending it with a supernova explosion or transitioning to Wolf–Rayet stars. Since most LBVs are found in the Milky Way and nearby massive galaxies, their metallicities are around solar or much lower. A characteristic feature of LBVs is optical variability (of the S Dor-type) on timescales of years with amplitudes up to 1–2 magnitudes, while their bolometric luminosity remains nearly constant. Understanding the influence of massive stars on the evolution of the first galaxies is particularly timely in the context of discoveries made with the JWST space telescope. To understand processes in galaxies of the early Universe, models of massive stars with very low metallicities, characteristic of that epoch, are needed. Validation of such models is possible only on real stars visible in galaxies with extremely low metallicity, which are exceedingly rare in the local Universe.

DDO68-V1, an LBV with record-low metallicity, Z ~ 0.025 Z_⊙, is a unique object of the Local Universe. Based on the results of our eight-year photometric monitoring of DDO68-V1 with the SAO RAS and SAI MSU: the BTA, Zeiss-1000, and 2.5-m of CMO, conducted after the giant eruption phase, we first detected a phenomenon of strong brightness variability with an amplitude of 3–3.5 magnitudes on a timescale of about a year. This could be an S Dor-type variability with an amplitude much larger than that of known LBVs or a new type of variability. Its nature is unknown and may be related to the star's extremely low metallicity and/or its state after the giant eruption. This discovery provides insights into the evolution of very low-metallicity massive stars and offers a new impetus for their modeling and interpretation of data on early-Universe galaxies.

The work was carried out within the framework of the state assignment of SAO RAS and partially supported by the Ministry of Education and Science of Russia grant.

Authors:
S.A. Pustilnik, Yu.A. Perepelitsyna, A.S. Vinokurov, A.S. Moskvitin, A.N. Burenkov, O.A. Maslennikova, O.I. Spiridonova (SAO RAS) in cooperation with E.S. Egorova (University of Heidelberg), V.P. Goranski (SAI MSU).

Published:
1. Pustilnik S.A., Perepelitsyna Y.A., Vinokurov A.S., Egorova E.S, Moskvitin A.S., Goransky V.P., Burenkov A.N., Maslennikova O.A., Spiridonova O.I. Monitoring of DDO68 “Northern Ring” SF Regions in 2016-2023, 2024, Astroph. Bulletin, v. 79, 593-612.
2. Pustilnik S.A., Perepelitsyna Y.A. The variability of DDO68-V1, a unique, extremely metal-poor luminous blue variable, 2025, Astronomy & Astrophysics Letters, 695, L7.

V694 Mon: unique observations of dynamical mass transfer between components

V694 Mon is a well-known symbiotic system in which high-velocity gas jets reaching 6500 km/s were previously observed. The earliest images of this system, dating to 1899, were found in the Moscow Astronomical Plate Archives of SAI MSU. Over the next 125 years, its brightness increased by about 100 times. In 2018, the authors of this work detected abrupt changes in its behavior. Follow-up observations over six years showed that the Roche lobe of the compact component—the jet source—became completely filled with the matter flowing from the cool giant. A new photosphere formed in this region corresponding to an A4 I spectral-type star with normal stellar radiation characteristics. A particularly important new result was a February 2022 flare of the s-process element emission observed with the SAO RAS 6-meter telescope (BTA). The dynamic mass-transfer mode in such systems had been theoretically predicted, but the rate of its development in V694 Mon proved to be unexpectedly high. For the first time in astrophysics, this process has been traced in such detail. The study yields unique data on the operation of the natural layered thermonuclear source inside the red giant and how its activity products interact with the surrounding environment. Short-lived radioactive isotopes impossible to obtain in laboratory conditions were identified, which is of interest to nuclear physics specialists as well. This study sheds light on a critical phase of evolution for binary systems with dynamic mass transfer and the formation of a common envelope.

The work was carried out within the framework of the state assignment of SAO RAS.

Authors:
Goranskij V.P. (SAI MSU, SAO RAS); Barsukova E.A., Burenkov A.N., Yakunin I.A. (SAO RAS), Metlova N.V., Zharova A.V. (SAI MSU).

Published:
Goranskij V.P., Barsukova E.A., Burenkov A.N., Metlova N.V., Zharova A.V. and Yakunin I.A. V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode Galaxies V. 13, art. 59. P. 1-13
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