Blue Straggler Stars in the Inner Region of the Globular Cluster M 15

Abstract

M15 (NGC 7078) is the prototype of core collapse globular clusters (Djorgovski & King 1986) with a resolved core of 2.2″ = 0.13 pc radius (Lauer et al. 1991) The short relaxation time in the core implies that it is most likely in a post-collapse phase of its evolution (Peterson et al. 1989). In such a scenario the M 15 core and its vicinity would be expected to be an ideal location for binaries (formed by multiple star interactions, see Fusi Pecci et al. 1992) and their by-products the Blue Stragglers Stars (BSS) which, being more massive than the average member of the cluster (Nemec, 1989) would settle within a few core radii of the center.

BSS have not been found in the outer parts of Ml5 observed so far (Battistini et al. 1985) and the core is too dense and the cluster too distant (12.8 kpc) for it to be resolved from the ground. The Faint Object Camera (FOC) on the Hubble Space Telescope (HST), already very successful in locating BSS in the core of the globular clusters 47 Tuc (Paresce et al. 1991) and NGC 6397 (Burgarella et al. 1992) with its high angular resolution and UV sensitivity, was used to search for BSS in and around the core of M15.

Here we present an Ultraviolet Color Magnitude Diagram (UV-CMD) of ~ 300 stars in the inner region of M 15. A conspicuous group of blue, hot stars has been identified. These stars define a sequence almost parallel to the HB which is consistent with the simulated BSS sequence obtained using the extrapolation of the Main Sequence locus (Fahlman et al. 1985) as mapped onto our UV-CMD.

To be conservative, we consider only the nine brightest stars of this population as the most probable BSS candidates. But a large number of these stars are visible at fainter magnitudes; they could be low luminosity BSS lying just above the turn-off, and could explain the radial color difference detected in the inner regions of M 15 (Baylin et al. 1989; Cederbloom et al. 1992; Stetson 1991)

The complete results of this investigation is presented in (1993).