FURTHER EVIDENCE FOR A COLOR GRADIENT IN M15

Using data obtained at the Canada-France-Hawaii Telescope (CFHT) in subarcsecond seeing (0.5"-0.6" FWHM), we construct color maps in B - V and H-alpha - R of the central regions of the globular cluster M15. Comparison in B - V of the region within 6.6" (0.31 pc) of the cluster center with a coadded sample taken from the surrounding region demonstrates a color gradient, in the sense that the central region is bluer than the surrounding region at a high level of statistical significance. This is in agreement with the previous finding of a U - B color gradient in M15 by Bailyn et al. [AJ, 98, 882 (1989)] based on a color map with lower angular resolution. Analysis of the H-alpha - R color map indicates that the central region is "H-alpha-deficient," i.e., has a stronger H-alpha-absorption line on average than the coadded region. This result argues against the possibility that the excess blue light from the central region is due to cataclysmic variables. Instead, two possible causes of the color gradient are (1) it may be due to a centrally concentrated population of faint blue stars, possibly resulting from binary-single star interactions, and (2) it may result from a central deficit of low-mass main-sequence stars.