Ultraviolet Variability in Solar-type Members of the M67 Open Cluster

Abstract Solar-type members of the rich, nearly solar-age and solar-metallicity M67 open cluster are systematically investigated for ultraviolet variability. We utilize archival Galaxy Evolution Explorer (GALEX) data, which feature several imaging observation epochs spanning 5 yr. Stars in or suspected of being in binary systems are avoided as well as stars that are blended in GALEX data, leading to a sample of 66 solar-type stars. We assess variability over a variety of timescales that probe flares and longer-term trends that could be due to rotation and activity cycles. We do not find conclusive evidence for variability and determine that solar-type members of M67 do not display >30% near-ultraviolet (NUV) variability over timescales ranging from days to years. Furthermore, within 50 s cadence lightcurves generated for each of the imaging epochs we find no NUV flares that are ≳2× the quiescent stellar NUV emission level; the implied ultraviolet flare rate derived from this study is in mild tension with that derived for stars observed by GALEX in the primary Kepler field. This M67 GALEX study presents one of the most comprehensive ultraviolet data sets currently available for probing continuum emission variability for old Sun-like stars; the planned NASA UVEX mission has the potential to dramatically expand upon this work.