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 non-cluster background contribution , and finds \Delta ( B - V ) \sim + 0.3 mag for the overall cluster starlight over the range 2 ^ { \prime \prime } to \gtrsim 1 ^ { \prime } in radius .
The slope of the color profile in this radial range is : \Delta ( B - V ) / \Delta \log ( r ) = 0.20 \pm 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 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 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 ) \sim - 0.06 to +0.02 mag over the range r = 20 ^ { \prime \prime } \ > – \ > 80 ^ { \prime \prime } ; 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 \gtrsim 20 ^ { \prime \prime } .