An analysis of the blue straggler population in the Sgr dSph globular cluster Arp 2★

Abstract

We present and discuss new BVI CCD photometry in the field of the globular cluster Arp 2, which is considered a member of the Sagittarius dwarf spheroidal galaxy. The main goal of this investigation is to study the statistics and spatial distribution of blue straggler stars in the cluster. Blue stragglers are stars observed to be hotter and bluer than other stars with the same luminosity in their environments. As such, they appear to be much younger than the rest of the stellar population. Two main channels have been suggested to produce such stars: (1) collisions between stars in clusters; or (2) mass transfer between, or merger of, the components of primordial short-period binaries. The spatial distribution of these stars inside a star cluster, compared with the distribution of stars in different evolutionary stages, can cast light on the most efficient production mechanism at work. In the case of Arp 2, we found that blue straggler stars are significantly more concentrated than main-sequence stars, while they show the same degree of concentration as evolved stars (either red giants or horizontal branch stars). Since Arp 2 is not a very concentrated cluster, we suggest that this high central concentration is an indication that blue stragglers are mostly primordial binary stars.