## Abstract

A grid of synthetic horizontal-branch (SHB) models based on horizontal-branch (HB) evolutionary tracks with improved physics has been constructed to reconsider the theoretical calibration of the dependence of M_{v}(RR) on metallicity in globular clusters and the slope of the mean 〈M_{v}(RR)〉-[Fe/H] relation. The SHB models confirm Lee's earlier finding that the slope of the 〈M_{v}(RR)>-[Fe/H] relation is itself a function of the metallicity range considered and that, in addition, for a given [Fe/H] RR Lyrae luminosities depend on HB morphology. This is due to the fact that HB stars pass through the RR Lyrae instability strip at different evolutionary stages, depending on their original position on the HB. At [Fe/H] = -1.9, and for HB type 0, the models yield M_{v}(RR) = 0.47 ± 0.10. The mean slope for the zero-age HB models is 0.204. Since there is no simple universal relation between M_{v}(RR) and metallicity that is applicable to all globular clusters, the HB morphology of each individual cluster must be taken into account, in addition to [Fe/H], in deriving the appropriate M_{v}(RR). Taking HB morphology into account, we find that the slope of the mean 〈M_{v}(R.R)>-[Fe/H] relation varies between 0.36 for the clusters with Galactocentric distances R_{gc}. less than 6 kpc and 0.22 for clusters with 6 kpc < R_{gc} ≤ 20 kpc. Implications for interpreting observations of field RR Lyrae variables and for absolute globular cluster ages and Galactic chronology are briefly discussed.

Original language | English |
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Pages (from-to) | 1398-1404 |

Number of pages | 7 |

Journal | Astronomical Journal |

Volume | 119 |

Issue number | 3 |

DOIs | |

Publication status | Published - 2000 Mar |

## All Science Journal Classification (ASJC) codes

- Astronomy and Astrophysics
- Space and Planetary Science