Characterization of blue and yellow straggler stars of Berkeley 39 using Swift/UVOT

Abstract

We characterize blue straggler stars (BSS) and yellow straggler stars (YSS) of an open cluster (OC) Berkeley 39 using multi-wavelength observations including Swift/UVOT. Our analysis also makes use of ultraviolet (UV) data from GALEX, optical data from Gaia DR3 and Pan-STARRS, and infrared data from 2MASS, Spitzer/IRAC, and WISE. Berkeley 39 is a ~6 Gyr old Galactic OC located at a distance of ~4200 pc. We identify 729 sources as cluster members utilizing a machine learning algorithm, ML-MOC, on Gaia DR3 data. Of these, 17 sources are classified as BSS candidates and four as YSS candidates. We construct multi-wavelength spectral energy distributions (SEDs) of 16 BSS and 2 YSS candidates, within the Swift/UVOT field, to analyze their properties. Out of these, 8 BSS candidates and both the YSS candidates are successfully fitted with single-component SEDs. Five BSS candidates show marginal excess in the near-UV (fractional residual < 0.3 in all but one UVOT filter), whereas three BSS candidates show moderate to significant excess in the near-UV (fractional residual > 0.3 in at least two UVOT filters). We present the properties of the BSS and YSS candidates, estimated based on the SED fits.