Discovery of white dwarf companions of blue metal-poor stars using UVIT/AstroSat

Abstract

Blue metal-poor (BMP) stars are main-sequence stars that appear bluer and more luminous than normal turn-off stars of metal-poor globular clusters. These stars are believed to be either field blue straggler stars (FBSS) formed via post mass transfer mechanism or accreted from dwarf satellite galaxies of the Milky Way. A significant fraction of BMP stars are discovered to be binaries, typically with long periods and circular orbits, likely to have a compact companion. UVIT/AstroSat has observed 27 BMP stars in two FUV filters, F148W and F169M. We report the first ever discovery of white dwarfs companions in 14 stars. Out of these 14 stars, 10 are known to be either single-lined spectroscopic binaries or binary candidates with the periods of 0.9 to 840 days and eccentricity 0 to 0.5. 3 BMP stars are known to be radial velocity constant stars. We suspect that orbits of these stars may be either inclined or distance between binaries may be too large to observe the variation in radial velocities. For the remaining 1 star, no previous information on binarity is available. The hot companions with estimated temperatures, Teff ∼ 10500 − 40000 K, are white dwarfs (WDs) of extremely low-mass (∼ 0.20 M_sun), low-mass (∼ 0.20 – 0.40 M_sun), normal mass (∼ 0.40 – 0.60 M_sun), and high-mass (∼ 0.8M_sun). Our analysis suggests that both mass transfer and merger in hierarchial triple system channels play a major role in the formation of BMP stars and thus atleast 14 BMP stars in our sample are confirmed FBSS.