The first direct measurement of the mass of a blue straggler in the core of a globular cluster: BSS 19 in 47 Tucanae

Hot, young blue straggler stars are observed by the Hubble Space Telescope to exist in the cores of very old globular clusters. Theory maintains that these stars must be more massive than hydrogen-burning cluster members that are as old as the clusters themselves. We have, for the first time, verified this theoretical claim by directly measuring the mass of the blue straggler BSS 19 in the core of 47 Tucanae. The derived mass, M = 1.7 +/- 0.4 M., is nearly twice that of the oldest primordial hydrogen-burning cluster star. BSS 19 is found to be rotating rapidly, with a derived projected rotational velocity of v sin i = 155 +/- 55 km s(-1). This large value for the rotational velocity tends to support a model for the formation of BSS 19 in which the blue straggler was formed from a binary system via a noncollisional mechanism (e.g., the coalescence of a contact binary).