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 arcseconds, with a chance coincidence probability of less than 1 per cent. We then analyse optical time-series photometry obtained with the 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 suggest that the BSS-7 system consists of a blue straggler primary with an X-ray-active, upper-main-sequence companion. The formation of such a system must have involved at least three stars, so the simplest blue straggler formation scenarios -- primordial binary coalescence or a single 2-body collision -- cannot account for it. However, our results are consistent with recent N-body models, in which blue stragglers often form via multiple encounters that can involve both single and binary stars.Keywords
All Related Versions
This publication has 0 references indexed in Scilit: