Hunting for UVdim stars in Galactic open clusters. Clues from ultraviolet photometry

ABSTRACT Split main-sequences and extended main-sequence turn-offs (eMSTOs) have been observed in nearly all Magellanic Clouds clusters younger than 2 Gyr. More recently, Hubble Space Telescope (HST) ultraviolet photometry uncovered a puzzling new population of UV-absorbed stars, dubbed ‘UVdim’, in five Magellanic Clouds clusters aged between 40 and 200 Myr, as well as in one 1.5 Gyr-old cluster. These UVdim stars predominantly lie on the blue MS, which is composed of slow rotators, and their distinct UV properties are believed to stem from dusty circumstellar discs. Although eMSTOs are common in both Magellanic Clouds and Galactic open clusters (OCs) of comparable ages, UVdim stars have not yet been investigated in Galactic OCs. In this work, we fill that gap by combining Swift/Ultraviolet-Optical Telescope, SkyMapper, and Gaia photometry to extend the search for UVdim stars to 35 Galactic OCs younger than 2 Gyr. By constructing colour–colour diagrams analogous to those employed with HST WFC3/UVIS, we find no evidence of UVdim-like stars in most Galactic open clusters and identify possible UVdim candidates in only five systems.The rarity of UVdim stars in young OCs suggests a potential difference between Magellanic Cloud clusters and their Milky Way counterparts, although the underlying reason remains unclear.