HST WFPC2 Study of the Density Profile and Stellar Populations in M15

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We describe images of the center of the dense globular cluster M15 (NGC 7078) obtained with HST's WFPC2 instrument. Data taken in the F336W, F439W, and F555W filters (UBV) yield a color--magnitude diagram of ~ 3*E(4) stars in a 5 arcmin(2) region within r<2' (6.7pc) of the cluster center. Realistic simulated images have been used to estimate photometric error and incompleteness in the star counts. The measurement error in color is 1sigma la0 .05 mag for stars with V<18, increasing to 1sigma ~ 0.2 mag at V=20.5, which is 1.5 mag fainter than the main sequence turnoff. The larger number of stars detected with WFPC2 (compared to WF/PC-1) result in smaller Poisson errors in the determination of the star count profile. The projected radial distribution of stars (after correction for incompleteness and photometric bias/error) is well represented by a power law: N(r) ~ {r}(alpha ) , with slope alpha =-0.7+/-0.15, into radii less than 0\farcs3 (0.015pc). Any flat core of radius larger than 1'' (0.05pc) in the stellar distribution is ruled out at the 99% level. Only 30 blue straggler candidates are identified from the UBV data within rla30 '', and we use the simulations to correct for contamination by subgiant and turnoff stars. The specific frequency of blue straggler stars (relative to red giant branch [RGB] and subgiant stars) in the central region of M15 is f_BSS=0.05+/-0.02 which is significantly lower than in other dense clusters. The incompleteness-corrected radial distribution of 19

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