GlobULeS – V. UVIT/AstroSat studies of stellar populations in NGC 362: detection of blue lurkers in a globular cluster

Abstract

We report the discovery of four blue lurkers with low- and extremely low mass white dwarf (ELM WD) companions in the Galactic globular cluster NGC 362 using AstroSat’s Ultra Violet Imaging Telescope (UVIT). We analysed the multiwavelength spectral energy distribution (SED) of far-ultraviolet-bright main-sequence stars using data from the UVIT, Ultraviolet Optical Telescope, Gaia EDR3, and 2.2-m ESO/MPI telescopes. Two each of low-mass WDs and ELM WDs are found as companions for the four blue lurkers by the fitting of two-component SED models. The effective temperatures, radii, luminosities, and masses of two low-mass WDs are (35 000, 23 000) K, (0.04, 0.05) R, (1.45, 0.22) L, and (0.2, 0.2) M, while the two ELM WDs are (14 750, 14 750) K, (0.09, 0.10) R, (0.34, 0.40) L, and (0.18, 0.18) M, respectively. The position of blue lurkers within the cluster shows that they originated via the Case A/B mass-transfer mechanism in a low-density environment. This is the first detection of blue lurkers with low-mass WDs and ELM WDs as companions in a globular cluster. The companion’s cooling age is less than 4 Myr, which suggests that they were just recently formed. These binary systems might have originated due to the cluster’s recent core collapse.