| Abstract: We present a homogeneous, multi-wavelength investigation of hot stellar populations in star clusters, aimed at constraining non-canonical evolutionary pathways driven by binary interaction, stellar collisions, and cluster dynamics. Our analysis focuses on blue straggler stars (BSSs), blue lurkers (BLs), horizontal branch (HB) stars, and white dwarfs (WDs) in the core-collapsed globular cluster NGC 362 and the open cluster NGC 2420, using UV–optical data from AstroSat/UVIT, UVOT, Gaia EDR3, HST, and the ESO/MPI 2.2-m telescope.
In NGC 362, we analyze 26 BSSs and identify 12 binary systems hosting extremely low-mass white dwarf (ELM WD) companions, providing strong evidence for formation via Case A/B mass-transfer channels. Cooling ages of nine ELM WDs (< 500 Myr) indicate recent binary evolution. The radial distribution of BSSs relative to a reference population exhibits strong central segregation, classifying NGC 362 as dynamically old and consistent with an advanced core-collapse phase that enhances binary-mediated and collisional interactions. We report the first detection of blue lurkers in a globular cluster, identifying four BL systems with low-mass or ELM WD companions through two-component SED fitting; their extremely young cooling ages (< 4 Myr) imply recent formation linked to late-stage dynamical evolution.
Core-collapsed globular clusters are ideal environments for stellar collision products. We identify 17 far-UV bright WD members in NGC 362 using UVIT and HST data. Fourteen WDs are well described by single-component SEDs, while three require two-component WD–main-sequence (MS) models. The WDs span effective temperatures of 22,000–70,000 K and masses of 0.30–1.13 M⊙, while the MS companions are low-mass (0.14–0.24 M⊙). The detection of massive WDs and WD–MS binaries provides the first evidence for massive WD formation in a core-collapsed cluster, a missing link for fast radio burst progenitors in globular clusters, and suggests potential evolutionary pathways toward Type Ia supernovae and FRBs. |
