Radial color gradient and main-sequence mass segregation in M30 (NGC 7099)

It has long been known that the post-core-collapse globular cluster M30 (NG C 7099) has a bluer-inward color gradient, and recent work suggests that th e central deficiency of bright red giant stars does not fully account for t his gradient. This study uses Hubble Space Telescope Wide Field Planetary C amera 2 images in the F439W and F555W bands, along with ground-based CCD im ages with a wider field of view for normalization of the noncluster backgro und contribution, and finds Delta(B - V) similar to 0.3 mag for the overall cluster starlight over the range 2 " to greater than or similar to 1' in r adius. The slope of the color profile in this radial range is Delta(B - V)/ Delta log r = +0.20 +/- 0.07 mag dex(-1), where the quoted uncertainty acco unts for Poisson fluctuations in the small number of bright evolved stars t hat dominate the cluster light. We explore various algorithms for artificia lly redistributing the light of bright red giants and horizontal-branch sta rs uniformly across the cluster. The traditional method of redistribution i n proportion to the cluster brightness profile is shown to be inaccurate. T here is no significant residual color gradient in M30 after proper uniform redistribution of all bright evolved stars; thus, the color gradient in M30 's central region appears to be caused entirely by post-main-sequence stars . Two classes of plausible dynamical models, Fokker-Planck and multimass Ki ng models, are combined with theoretical stellar isochrones from Bergbusch & VandenBerg and from D'Antona and collaborators to quantify the effect of mass segregation of main-sequence stars. In all cases, mass segregation of main-sequence stars results in Delta(B - V) similar to -0.06 to +0.02 mag o ver the range r = 20 "-80 "; this is consistent with M30's residual color g radient within measurement error. The observed fraction of evolved-star lig ht in the B and V bands agrees with the corresponding model predictions at small radii but drops below it for r greater than or similar to 20 ".