Spectral types and masses of white dwarfs in globular clusters ^*,**,***

¹ Institut für Theoretische Physik und Astrophysik der Universität Kiel, 24098 Kiel, Germany e-mail: koester@astrophysik.uni-kiel.de
² Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany e-mail: heber@sternwarte.uni-erlangen.de;rn38@astro.le.ac.uk
³ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile e-mail: mzoccali@astro.puc.cl
⁴ European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei München, Germany e-mail: arenzini@eso.org
⁵ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: ferraro@bo.astro.it

Corresponding author: S. Moehler, moehler@astrophysik.uni-kiel.de

Received: 8 December 2003
Accepted: 2 March 2004

Abstract

White dwarfs in globular clusters offer additional possibilities to determine distances and ages of globular clusters, provided their spectral types and masses are known. We therefore started a project to obtain spectra of white dwarfs in the globular clusters NGC 6397 and NGC 6752. All observed white dwarfs show hydrogen-rich spectra and are therefore classified as DA. Analysing the multi-colour photometry of the white dwarfs in NGC 6752 yields an average gravity of  = 7.84 and 0.53  as the most probable average mass for globular cluster white dwarfs. Using this average gravity we try to determine independent temperatures by fitting the white dwarf spectra. While the stellar parameters determined from spectroscopy and photometry usually agree within the mutual error bars, the low resolution and S/N of the spectra prevent us from setting constraints stronger than those derived from the photometry alone. For the same reasons the white dwarf spectra obtained for NGC 6397 unfortunately do not provide an independent distance estimate of sufficient accuracy to distinguish between the long and short distance scale for globular clusters.