Globular cluster photometry with the Hubble Space Telescope. VI. WF/PC-I observations of the stellar populations in the core of M13 (NGC 6205)

We study the dense core of the globular cluster Messier 13 (NGC 6205) using pre-refurbishment Planetary Camera-I images obtained with the Hubble Space Telescope. Short exposures (60 s) through the F555W and F785LP filters (similar to Johnson V and I, respectively) have been used to obtain V and I photometry of 2877 stars brighter than V∼20 in a 1.25 arcmin² region of the cluster including its core and extending out to r∼66″ (2.3 pc) from its center. The sample is complete to V≃18.3 (the main sequence turnoff) and the 1 σ photometric error is about 0.1 mag. We find 15 blue straggler star candidates and 10 other possible blue stragglers in this region of M13. Their specific frequency is in the range F_BSS=0.04-0.07, comparable to what is observed near the centers of other dense clusters. A comparison between M13's observed V band stellar luminosity function and a theoretical model (Bergbusch & Vandenberg, 1992, ApJS, 81, 163) for the luminosity function of an old, metal-poor cluster shows that the model predicts too few of the brightest red giants (V∼12.5-15) by a factor of two relative to subgiants/turnoff stars (>6σ effect). The radial distributions of red giants, blue stragglers, and subgiants are consistent with one another, and are well fit by a King profile of core radius r_core=38″±6″ (90% confidence limits) or 1.3 pc. Stars in the blue horizontal branch of M13, however, appear to be centrally depleted relative to other stellar types. We combine data from three dense "King model clusters," M13, M3, and 47 Tuc, and two post core collapse clusters, M30 and M15, and compare the distributions of various stellar types as a function of (r/r_{half light}) and (r/r_core). The horizontal branch stars in the combined sample appear to be centrally depleted relative to the giants (97% significance) - this depletion is only a 1σ-2σ effect in each of the clusters taken individually. The blue stragglers in the combined sample are centrally concentrated relative to the giants.