A Blue Straggler Binary System with at least Three Progenitors in the Core of a Globular Cluster?

Abstract

We show that the X-ray source W31 in the core of the globular cluster 47 Tucanae is physically associated with the bright blue straggler BSS-7. The two sources are astrometrically matched to 0.061\arcsec, with a chance coincidence probability of less than 1%. We then analyse optical time-series photometry obtained with the {\em Hubble Space Telescope} (HST) and find that BSS-7 displays a 1.56 day periodic signal in the I band. We also construct a broad-band (far-ultraviolet through far-red) spectral energy distribution for BSS-7 and fit this with single and binary models. The binary model is a better fit to the data, and we derive the corresponding stellar parameters. All of our findings are consistent with BSS-7 being a detached binary consisting of a blue straggler primary with an X-ray-active, upper-main-sequence companion. The formation of such a system would necessarily involve at least three stars, which is consistent with recent N-body models in which blue stragglers often form via multiple encounters that can involve both single and binary stars. However, we cannot yet entirely rule out the possibility that BSS-7 descended directly from a binary system via mass transfer. The system parameters needed to distinguish definitively between these scenarios may be obtainable from time-resolved spectroscopy.

19 pages, 3 figures, accepted for publication in ApJ; revised version includes a discussion of an alternative 2-progenitor binary evolution scenario, and an appendix discussing other probable/possible BSS/X-ray matches in globular clusters and related sources

Cited by in corpus (10)

• A Binary Origin for Blue Stragglers in Globular Clusters
• Where the Blue Stragglers Roam: Searching for a Link Between Formation and Environment
• Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188
• Violent Stellar Merger Model for Transient Events
• Stellar Exotica in 47 Tucanae
• Chandra studies of the globular cluster 47 Tucanae: A deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars, and new constraints to r-mode instability window
• HST/ACS Imaging of Omega Centauri: Optical Counterparts of Chandra X-Ray Sources
• Combined Chandra, XMM-Newton and Hubble Space Telescope observations of the Galactic globular cluster NGC 2808
• Spectral Energy Distribution of Blue Stragglers in the core of 47 Tucanae
• Formation and Evolution of Blue Stragglers in 47 Tucanae