UVIT open cluster study. I. Detection of a white dwarf companion to a blue straggler in M67: Evidence of formation through mass transfer

Abstract

The old open cluster M67, populated with blue straggler stars (BSSs), is a well known test bed to study the BSS formation pathways. Here, we report the first direct detection of a white dwarf (WD) companion to a BSS in M67, using far-UV images from the Ultra Violet Imaging telescope UVIT on ASTROSAT. Near-simultaneous observations in three far-UV bands combined with GALEX, IUE, ground and space based photometric data covering 0.14 -11.5 µm range for WOCS1007 were found to require a binary fit to its spectral energy distribution (SED), consisting of a BSS and a hot companion. On the other hand, a single spectral fit was found to be satisfactory for the SEDs of two other BSSs, WOCS1006 and WOCS2011, with the latter showing a deficient far-UV flux. The hot companion of WOCS1007 is found to have a T_eff ~ 13250-13750K and a radius of 0.09±0.01 R_☉. A comparison with WD models suggests it to be a low mass WD (~ 0.18 M_☉), in agreement with the kinematic mass from the literature. As a low mass WD (< 0.4 M_☉) necessitates formation through mass transfer (MT) in close binaries, WOCS1007 with a known period of 4.2 days along with its fast rotation, is likely to be formed by a case A or case B binary evolution.

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