Discovery of a barium blue straggler star in M67 and sighting of its WD companion*

TL;DR

This paper reports the discovery of a barium-enhanced blue straggler star in M67 with a white dwarf companion, providing evidence of recent mass transfer and chemical enrichment in the cluster.

Contribution

First detection of a white dwarf companion to a barium blue straggler in M67, linking chemical enrichment to recent mass transfer events.

Findings

Barium blue straggler WOCS 9005 shows s-process element enhancement.

White dwarf companion identified through UV spectral energy distribution.

Estimated WD cooling age is approximately 60 million years.

Abstract

We report the discovery of a barium blue straggler star (BSS) in M67, exhibiting enhancements in slow neutron-capture ($s$-) process elements. Spectroscopic analysis of two BSSs (WOCS\,9005 \& WOCS\,1020) and 4 stars located near the main-sequence turn-off using GALAH spectra, showed that WOCS\,9005 has a significantly high abundance of the s-process elements ([Ba/Fe] = 0.75$\pm$0.08, [Y/Fe] = 1.09$\pm$0.07, [La/Fe] = 0.65$\pm$0.06). The BSS (WOCS\,9005) is a spectroscopic binary with a known period, eccentricity and a suspected white dwarf (WD) companion with a kinematic mass of 0.5 M$_\odot$. The first `sighting' of the WD in this barium BSS is achieved through multi-wavelength spectral energy distribution (SED) with the crucial far-UV data from the UVIT/\textit{AstroSat}. The parameters of the hot and cool companions are derived using binary fits of the SED using two combinations of models, yielding a WD with T$_{eff}$ in the range 9750--15250 K. Considering the kinematic mass limit, the cooling age of the WD is estimated as $\sim$ 60 Myr. The observed enhancements are attributed to a mass transfer (MT) from a companion asymptotic giant branch star, now a WD. We estimate the accreted mass to be 0.15 M$_{\odot}$, through wind accretion, which increased the envelope mass from 0.45 M$_{\odot}$. The detection of chemical enhancement, as well as the sighting of WD in this system have been possible due to the recent MT in this binary, as suggested by the young WD.