DDO Photometry of M71: Carbon and Nitrogen Patterns among Evolving Giants

We present V, B-V, and DDO C(41–42) and C(42–45) photometry for a sample of 75 red giants down to MV = +2 in the relatively metal-rich Galactic globular cluster M71. The C(41–42) colors reveal a bimodal distribution of CN band strengths generally anticorrelated with CH band strength as measured by the C(42–45) color. Both DDO colors agree well with those found in 47 Tucanae, a nearby globular cluster of similar metallicity, and suggest nearly identical C and N abundance patterns among the giants of both clusters. A comparison with synthetic DDO colors demonstrates that little change in surface C or N abundance is required to match the colors of the M71 giants over the entire luminosity range observed. Apparently like 47 Tuc (a cluster of much greater mass and central concentration), M71 exhibits an abundance pattern that cannot be solely the result of internal mixing.