UOCS. IV. Discovery of diverse hot companions to blue stragglers in the old open cluster King 2

TL;DR

This study used UVIT on extit{ASTROSAT} and Gaia data to identify diverse hot companions to blue straggler stars in the old open cluster King 2, revealing a significant fraction formed via mass transfer and diverse companion properties.

Contribution

It reports the discovery of hot companions to blue stragglers in King 2, showing their diversity and suggesting formation via mass transfer in a 6 Gyr old open cluster.

Findings

Six bright BSSs have hotter companions similar to EHB/sdB stars.

At least 15% of BSSs formed via mass transfer.

Hot companions can form in open clusters as young as 6 Gyr.

Abstract

King 2, one of the oldest clusters in the Milky Way, with an age of $\sim$ 6 Gyr and distance of $\sim 5700$ pc, has been observed with UVIT payload on the \textit{ASTROSAT}. With membership information derived from {\it Gaia} EDR3, the cluster is found to have 39 blue straggler stars (BSSs). We created multi-wavelength spectra-energy distributions (SED) of all the BSSs. Out of 10 UV detected BSSs, 6 bright ones fitted with double component SEDs and were found to have hotter companions with properties similar to extreme horizontal branch (EHB)/subdwarf B (sdB) stars, with a range in luminosity and temperature, suggesting a diversity among the hot companions. We suggest that at least 15\% of BSSs in this cluster are formed via mass-transfer pathway. When we compared their properties to EHBs and hotter companions to BSS in open and globular clusters, we suggest that EHB/sdBs like companions can form in binaries of open clusters as young as 6 Gyr.