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
fl
ux 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
fi
tted model SEDs to seven WDs and
fi
nd that four of them have mass
>
0.5
M
e
and cooling age of less than
200 Myr, thus demanding BSS progenitors. SED
fi
ts 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
con
fi
rmation. Nine sources have X-ray and excess UV
fl
ux, 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.
