Globular Cluster Photometry with the Hubble Space Telescope VII. Color Gradients and Blue Stragglers in the Central Region of M30 (NGC 7099) from WFPC2 Observations

Abstract

We present F555W (V), F439W (B), and F336W (U) photometry of 9507 stars in the central 2' of the dense, post core collapse cluster M30 (NGC 7099) derived from HST WFPC2 images. These data are used to study the mix of stellar populations in the central region of the cluster. Forty eight blue straggler stars are identified and are found to be strongly concentrated towards the center. The specific frequency of blue stragglers, F_BSS\equiv N(BSS)/N(V<V_HB+2), is 0.25\pm 0.05 in the inner region of M30 (r<20''), significantly higher than the frequency found in other clusters. The shape of M30's BSS luminosity function resembles the prediction of the collisional formation model and is inconsistent with the binary merger model, of Bailyn and Pinsonneault. An unusually blue star (B=18.6,B-V=-0.97), possibly a cataclysmic variable based on its color, is found about 1.2'' from the center. Bright RG stars appear to be depleted by a factor of 2--3 in the inner r<10'' relative to fainter giants, subgiants, and MSTO stars (95% significance). We confirm that there is a radial gradient in the color of the cluster light, going from B-V=0.82 at r=1' to B-V=0.45 in the central 10''. The central depletion of the bright RG is responsible for about half of the observed color gradient; the rest of the gradient is caused by the relative underabundance of faint red MS stars near the center (presumably a result of mass segregation). The luminosity function of M30's evolved stars does not match the luminosity function shape derived from standard stellar evolutionary models: the ratio of the number of bright giants to the number of turnoff stars in the cluster is 30% higher than predicted by the model (3.8σeffect), roughly independent of RG brightness over the range M_V=-2 to +2. (abridged)