Luminous Long-Period Variables in Globular Clusters and the Galactic Bulge: Their Dependence on Metallicity

We derive the frequency of occurrence of luminous long-period variables (LLPVs) in globular clusters and in the Baade's window field of the Galactic bulge. LLPVs occur only in clusters with [Fe/H] ≳ -1.0. In these clusters their frequency of occurrence relative to the number of giant stars appears to be independent of metallicity. Integrated over all metallicities, Baade's window appears to be deficient in LLPVs. We estimate [Fe/H] values for Baade's window LLPVs from their period and a log P versus [Fe/H] relation, derived from the cluster variables, and find that this deficiency is due primarily to the absence of LLPVs with [Fe/H] ≳ 0.0. We propose that this is due to enhanced mass-loss rates in these LLPVs with a consequently abbreviated lifetime compared with lower metallicity LLPVs. A typical lifetime for cluster LLPVs is about 3 × 105 yr. Finally, we emphasize the need for a much more complete survey for LLPVs in globular clusters.

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