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 (NGC 7099) has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giant stars does not fully account for this gradient. This study uses Hubble Space Telescope Wide Field Planetary Camera 2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the noncluster background contribution, and finds Δ(B-V) ∼ 0.3 mag for the overall cluster starlight over the range 2'' to ≳1' in radius. The slope of the color profile in this radial range is Δ(B-V)/Δ log r = +0.20 ± 0.07 mag dex^-1, where the quoted uncertainty accounts for Poisson fluctuations in the small number of bright evolved stars that dominate the cluster light. We explore various algorithms for artificially redistributing the light of bright red giants and horizontal-branch stars uniformly across the cluster. The traditional method of redistribution in proportion to the cluster brightness profile is shown to be inaccurate. There 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 King 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 Δ(B-V) ∼ -0.06 to +0.02 mag over the range r = 20''–80''; this is consistent with M30's residual color gradient within measurement error. The observed fraction of evolved-star light in the B and V bands agrees with the corresponding model predictions at small radii but drops below it for r ≳ 20''.