Globular Cluster Photometry with the Hubble Space Telescope. VII. Color Gradients and Blue Stragglers in the Central Region of M30 from Wide Field Planetary Camera 2 Observations* **

摘要

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 Hubble Space Telescope Wide Field Planetary Camera 2 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; they are found to be strongly concentrated toward the cluster center. The specific frequency of blue stragglers, FBSS ≡ N(BSS)/N(V < VHB + 2), is 0.25 ± 0.05 in the inner region of M30 (r < 20''), significantly higher than the frequency found in other clusters: FBSS = 0.05–0.15. The shape of M30's blue straggler luminosity function resembles the prediction of the collisional formation model, and is inconsistent with the binary merger model of Bailyn & Pinsonneault. An unusually blue star (B = 18.6, B-V = -0.97), possibly a cataclysmic variable based on its color, is found about 12 from the crowded cluster center; the photometric uncertainty for this star is large, however, because of the presence of a very close neighbor. Bright red giant stars (B < 16.6) appear to be depleted by a factor of 2–3 in the inner r < 10'' relative to fainter giants, subgiants, and main-sequence turnoff stars (95% significance). We confirm that there is a radial gradient in the color of the overall 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 red giants is responsible for about half of the observed color gradient; the rest of the gradient is caused by the relative underabundance of faint red main-sequence stars near the cluster 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 (3.8 σ effect), roughly independent of red giant brightness over the range MV = -2 to +2.