UVIT Open Cluster Study. II. Detection of extremely low mass white
  dwarfs and post-mass transfer binaries in M67
  A detailed study of the ultraviolet (UV) bright stars in the old open star
cluster, M67 is presented based on the far-UV observations using the {\it Ultra
Violet Imaging Telescope} ({\it UVIT}) on {\it ASTROSAT}. The UV and UV-optical
colour-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 (SB). The CMDs suggest
the presence of excess UV flux in many members, which could be extrinsic or
intrinsic to them. We construct multi-wavelength spectral energy distribution
(SED) using photometric data from the {\it UVIT, Gaia} DR2, 2MASS and WISE
surveys along with optical photometry. We fitted model SEDs to 7 WDs and find 4
of them have mass $>$ 0.5 \(M_\odot\) 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. 12 sources
with possible WD companion need further confirmation. 9 sources have X-ray and
excess UV flux, possibly arising out of stellar activity. This study
demonstrates that UV observations are key to detect and characterise the ELM
WDs in non-degenerate 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.
