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

TL;DR

This study uses UV photometry from GALEX to analyze the stellar populations of the old open cluster M67, identifying various stellar types, activity levels, and formation histories through UV CMDs, spectral analysis, and SED modeling.

Contribution

It provides the first comprehensive UV catalog of M67, demonstrating the effectiveness of UV CMDs and isochrones in identifying unusual stars and stellar activity in this well-studied cluster.

Findings

Detection of 18 BSSs, 3 WDs, and other stellar types in UV.

Identification of chromospheric activity in red giants.

Evidence for recent and ongoing BSS formation over the last 2.5 Gyr.

Abstract

We present the results of ultraviolet (UV) photometry of the old open cluster M67 obtained using Galaxy Evolution Explorer (GALEX) 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 catalog). 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 II $\it h$ +$\it k$ emission lines from the IUE archival spectra for 2 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$_{\odot}$, R/R$_{\odot}$ and T$_{eff}$ of the BSSs and 15 FUV bright stars by constructing the spectral energy distribution (SED) using multi-wavelength 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.