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Astron. Astrophys. 320, 823-830 (1997)

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The distance scale to globular clusters through new horizontal branch models

V. Caloi 1, F. D'Antona 1, 2 and I. Mazzitelli 1

1 Istituto di Astrofisica Spaziale CNR, C.P. 67, I-00044 Frascati, Italy
(caloi@victoria.ias.fra.cnr.it; aton@hyperion.ias.fra.cnr.it)
2 Osservatorio Astronomico di Roma, via dell' Osservatorio 2, I-00040 Monteporzio Catone (RM), Italy

Received 25 June 1996 / Accepted 7 October 1996


As a completion of preceding work (Mazzitelli et al. 1995), we present new horizontal branch models with mass fractions of heavy element content Z = [FORMULA] and [FORMULA] ; we discuss the complete set of models with Z from [FORMULA] to [FORMULA], and their implications for the ages of galactic globular clusters.

Models at low Z are [FORMULA] mag more luminous than equivalent models of the previous generation, implying a reduction of globular cluster ages of [FORMULA] Gyr. Half of this effect depends on the core masses at the helium flash, which are larger by 0.007 [FORMULA] at Z = [FORMULA] ; the remaining half depends on the new microphysical inputs (equation of state and opacities) in present models.

The distance scale resulting from these new models for [FORMULA] [FORMULA] is consistent with Sandage (1993) and Walker (1992) scales, but not with Carney et al. (1992) scale. The relation [FORMULA] (ZAHB) vs. [FORMULA] is not linear, so that the notion of slope turns out to be an elusive one, and in any case depending on the range of [FORMULA] considered.

New computations of the core mass at the helium flash are presented and discussed. We finally show that the luminosity at the tip of the giant branch obtained in our computations is compatible with the maximum luminosity of giants observed in globular clusters by Frogel et al. (1983).

Key words: stars: evolution – stars: horizontal branch models – globular clusters – distance scale

Send offprint requests to: F. D'Antona

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998