A search for dormant binaries with degenerate components in omega Centauri and NGC 6397

TL;DR

This study conducts the first spectroscopic search for quiescent degenerate binaries in globular clusters, identifying potential systems with massive dim components and proposing subpopulations of blue stragglers with different evolutionary histories.

Contribution

It introduces a novel spectroscopic method to identify dormant degenerate binaries in globular clusters and reports potential candidates with massive components.

Findings

Identified candidate systems with dim components over one solar mass.

Detected sinusoidal light curves consistent with ellipsoidal variability.

Suggested existence of two blue straggler subpopulations in omega Centauri.

Abstract

We report on the first spectroscopic search for quiescent degenerate binaries in globular clusters. Our survey is based on a sample of short-period optical variables which are likely optical counterparts of quiescent X-ray sources in omega Centauri (NGC 5139) and NGC 6397. All of the studied candidates have nearly sinusoidal light curves with amplitudes of 0.1--0.35~mag in $V$ and periods of 0.1--1.3~days. This type of variability, most probably originating from the ellipsoidal effect, has been observed in several X-ray novae when they settled into quiescence after an outburst. We find that two of the surveyed systems harbor dim components with masses in excess of one solar masss, making them attractive targets for future investigations. We also suggest that there are two subpopulations of blue stragglers in omega Centauri, differing in mass-transfer history and/or helium content.