Does the mixing length parameter depend on metallicity?
Further tests of evolutionary sequences using homogeneous databases
Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: palmieri,piotto; firstname.lastname@example.org
2 European Southern Observatory, 3107 Alonso de Cordova, Casilla 19001, Santiago 19, Chile e-mail: email@example.com
3 Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34131 Trieste, Italy e-mail: firstname.lastname@example.org
4 Dipartimento di Fisica, Università di Pisa, Piazza Torricelli 2, 56100 Pisa, Italy e-mail: email@example.com
Corresponding author: G. Piotto, firstname.lastname@example.org
Accepted: 3 May 2002
This paper is a further step in the investigation of the morphology of the color-magnitude diagram of Galactic globular clusters, and the fine-tuning of theoretical models, made possible by the recent observational efforts to build homogeneous photometric databases. In particular, we examine here the calibration of the morphological parameter WHB vs. metallicity, originally proposed by Brocato et al. ([CITE]; B98), which essentially measures the color position of the red-giant branch. We show that the parameter can be used to have a first-order estimate of the cluster metallicity, since the dispersion around the mean trend with [Fe/H] is compatible with the measurement errors. The tight WHB-[Fe/H] relation is then used to show that variations in helium content or age do not affect the parameter, whereas it is strongly influenced by the mixing-length parameter α (as expected). This fact allows us, for the first time, to state that there is no trend of α with the metal content of a cluster. A thorough examination of the interrelated questions of the α-elements enhancement and the color-Teff transformations, highlights that there is an urgent need for an independent assessment of which of the two presently accepted metallicity scales is the true indicator of a cluster's iron content. Whatever scenario is adopted, it also appears that a deep revision of the -temperature relations is needed.
Key words: stars: evolution / stars: Hertzsprung-Russell (HR) and C-M diagrams / stars: horizontal-branch / stars: Population II / Galaxy: globular clusters: general / Galaxy: halo
© ESO, 2002