UVIT Open Cluster Study. II. Detection of Extremely Low Mass White Dwarfs and Post–Mass Transfer Binaries in M67

Abstract

A detailed study of the UV-bright stars in the old open star cluster M67 is presented based on the far-UV observations using the Ultra Violet Imaging Telescope (UVIT) on ASTROSAT . The UV and UV–optical color–magnitude diagrams (CMDs), along with overlaid isochrones, are presented for the member stars, which include blue straggler stars (BSSs), triple systems, white dwarfs (WDs), and spectroscopic binaries. The CMDs suggest the presence of excess UV flux in many members, which could be extrinsic or intrinsic to them. We construct multiwavelength spectral energy distributions (SEDs) using photometric data from the UVIT , Gaia DR2, Two Micron All Sky Survey, and Wide-Field Infrared Survey Explorer surveys, along with optical photometry. We fitted model SEDs to seven WDs and find that four of them have mass >0.5 M ⊙ and cooling age of less than 200 Myr, thus demanding BSS progenitors. SED fits to 23 stars detect extremely low mass (ELM) WD companions to WOCS2007, WOCS6006, and WOCS2002 and a low-mass WD to WOCS3001, which suggest these to be post–mass transfer (MT) systems. Twelve sources with possible WD companions need further confirmation. Nine sources have X-ray and excess UV flux, possibly arising out of stellar activity. This study demonstrates that UV observations are key to detecting and characterizing the ELM WDs in nondegenerate systems, which are ideal test beds to explore the formation pathways of these peculiar WDs. The increasing detection of post-MT systems among BSSs and main-sequence stars suggests a strong MT pathway and stellar interactions in M67.