Does the mixing length parameter depend on metallicity?

Free access article

Issue		A&A Volume 392, Number 1, September II 2002


Page(s)		115 - 129
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20020840

A&A 392, 115-129 (2002)
DOI: 10.1051/0004-6361:20020840

Further tests of evolutionary sequences using homogeneous databases

R. Palmieri¹, G. Piotto¹, I. Saviane², L. Girardi^{3, 1} and V. Castellani⁴

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
    e-mail: palmieri,piotto; lgirardi@pd.astro.it
² European Southern Observatory, 3107 Alonso de Cordova, Casilla 19001, Santiago 19, Chile
    e-mail: isaviane@eso.org
³ Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34131 Trieste, Italy
    e-mail: lgirardi@ts.astro.it
⁴ Dipartimento di Fisica, Università di Pisa, Piazza Torricelli 2, 56100 Pisa, Italy
    e-mail: vittorio@astr18pi.difi.unipi.it

(Received 5 April (2002) / Accepted 3 May (2002))

Abstract
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 $W_{\rm HB}$ vs. metallicity, originally proposed by Brocato et al. (1998; 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 $W_{\rm HB}$ -[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 $\alpha$ (as expected). This fact allows us, for the first time, to state that there is no trend of $\alpha$ with the metal content of a cluster. A thorough examination of the interrelated questions of the $\alpha$ -elements enhancement and the color- $T_{\rm eff}$ 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 V-I-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

Offprint request: G. Piotto, piotto@pd.astro.it

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.