EDP Sciences
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Volume 371, Number 3, June I 2001
Page(s) 921 - 931
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20010388

A&A 371, 921-931 (2001)
DOI: 10.1051/0004-6361:20010388

On the white dwarf distances to galactic globular clusters

M. Salaris1, 2, S. Cassisi3, 2, E. GarcíaBerro4, 5, J. Isern6, 5 and S. Torres4

1  Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
2  Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany
3  Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
4  Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Jordi Girona Salgado s/n, Módul B-4, Campus Nord, 08034 Barcelona, Spain
5  Institut d'Estudis Espacials de Catalunya
6  Institut de Ciències de l'Espai (CSIC), Edifici Nexus, Gran Capitá 2-4, 08034 Barcelona, Spain

(Received 11 January 2001 / Accepted 9 March 2001)

We analyze in detail various possible sources of systematic errors on the distances of globular clusters derived by fitting a local template DA white dwarf sequence to the cluster counterpart (the so-called WD-fitting technique). We find that the unknown thickness of the hydrogen layer of white dwarfs in clusters plays a non negligible role. For reasonable assumptions -supported by the few sparse available observational constraints -about the unknown mass and thickness of the hydrogen layer for the cluster white dwarfs, a realistic estimate of the systematic error on the distance is within $\pm$0.10 mag. However, particular combinations of white dwarf masses and envelope thicknesses -which at present cannot be excluded a priori -could produce larger errors. Contamination of the cluster DA sequence by non-DA white dwarfs introduces a very small systematic error of about -0.03 mag in the MV/(V-I) plane, but in the MV/(B-V) plane the systematic error amounts to ~+0.20 mag. Contamination by white dwarfs with helium cores should not influence appreciably the WD-fitting distances. Finally, we obtain a derivative $\Delta
(m-M)_{V}/\Delta E(B-V)\sim -5.5$ for the WD-fitting distances, which is very similar to the dependence found when using the Main Sequence fitting technique.

Key words: distance scale -- globular clusters: general -- stars: white dwarfs

Offprint request: M. Salaris, ms@astro.livjm.ac.uk

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