Planetary Nebulae in the Globular Cluster PAL 6 and NGC 6441

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Abstract

Jacoby & Fullton (1998, in preparation) recently surveyed the Galactic globular cluster system for planetary nebulae and identified excellent candidates in the disk clusters Pal 6 and NGC 6441. Velocities, abundances, and central star properties for these two nebulae are derived. There is a 0.5% probability that the nebula in Pal 6 is a chance superposition of a Galactic bulge nebula. Membership for the nebula in NGC 6441, though, is extremely likely based on common velocity, extinction, and proximity to the cluster center. The number of nebulae now known in globular clusters (4) represents a 3.1σ departure from the number expected (16) based on the theoretical stellar death rate (Renzini & Buzzoni 1986) and the depth of the survey. Possible rationales for this apparent discrepancy are discussed. In particular, the small number of observed PN in clusters can be explained by imposing the requirement that cluster PN form from binary progenitors. Statistically, a correlation exists between the presence of PN and the presence of x-ray sources in clusters, suggesting that PN are more likely to be found when the frequency of interacting binaries in a cluster is high. Oxygen abundances for these two PN, plus Ps 1 in M 15, suggest that the [0/Fe] ratios in cluster stars are not uniform from cluster to cluster. Values range from -0.7 (NGC 6441) to +0.8 (M15), as compared to +0.5 typically measured in low metallicity field stars.

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