Ultraviolet stellar population of the old open cluster M67 (NGC 2682)

Abstract

We present the results of ultraviolet (UV) photometry of the old open cluster M67 obtained using Galaxy Evolution Explorer in far-ultraviolet (FUV) and near-ultraviolet (NUV) bands. UV detections of 18 blue straggler stars (BSSs), 3 white dwarfs (WDs), 4 yellow straggler stars, 2 sub–subgiants, and 25 X-ray sources are presented (along with an online catalogue). We demonstrate the capability of UV colour–magnitude diagrams (CMDs) along with the UV isochrones to identify potential stars which defy standard stellar evolution in this well-studied cluster. We also detect a few main-sequence turn-off and subgiant branch stars with excess flux in the FUV and/or NUV. UV continuum excess as well as Mg IIh + k emission lines from the IUE archival spectra for two red giants are detected, suggestive of their chromospheric activity. We suggest that a large number of stars in this cluster are chromospherically active, whereas the bright BSS are unlikely to be active. We also estimate the fundamental parameters L/L⊙, R/R⊙, and Teff of the BSSs and 15 FUV bright stars by constructing the spectral energy distribution (SED) using multiwavelength data. We identify three groups among the BSSs, based on their properties. The H–R diagram of BSSs with isochrones suggests that the BSSs in M67 are formed in the last 2.5 Gyr–400 Myr, more or less continuously. We identify 7 potential MS + WD candidates based on large UV excess from a probable 11, based on SEDs.