Radial Color Gradient and Main Sequence Mass Segregation in M30 (NGC 7099)

Abstract

It has long been known that the post-core-collapse globular cluster M30 has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giants does not fully account for this gradient. This study uses Hubble Space Telescope WFPC2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the non-cluster background contribution, and finds Δ(B-V) ~ +0.3 mag for the overall cluster starlight over the range r = 2" to >~ 1'. The slope of the color profile is: Δ(B-V)/Δ\log(r) = 0.20 +/- 0.07 mag dex^{-1}. 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 due entirely to 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 (1992) and from D'Antona et al. 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".