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Astron. Astrophys. 363, 1040-1050 (2000)


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Cool helium-rich white dwarfs from the Hamburg/ESO survey * **

S. Friedrich 1,2, D. Koester 1, N. Christlieb 3, D. Reimers 3 and L. Wisotzki 3

1 Universität Kiel, Institut für Theoretische Physik und Astrophysik, 24098 Kiel, Germany
2 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
3 Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Received 23 May 2000 / Accepted 20 September 2000

Abstract

We present an analysis of 40 cool helium-rich white dwarfs found in the Hamburg/ESO survey. They were selected for follow-up spectroscopy because of their [FORMULA] colour below -0.18, the absence of strong absorption lines, and a continuum shape similar to that of a quasar. Effective temperatures for individual stars were determined by fitting model atmospheres of nearly pure helium with a small admixture of hydrogen. As a consequence of the selection criteria all but one sample stars have [FORMULA] below 20000 K. Four stars clearly show helium and hydrogen lines in their spectra. In the spectra of another three, helium, hydrogen, and metal lines can be detected. For these stars hydrogen and metal abundances were also determined by fitting appropriate model atmospheres containing these elements. Seven sample stars most likely have helium-rich atmospheres but do not show any helium lines. They either have featureless spectra or show calcium lines.

Key words: surveys – stars: fundamental parameters – stars: white dwarfs

* Based on observations collected at the European Southern Observatory, La Silla, Chile. This research has made use of the Simbad database operated at CDS, Strasbourg, France, and of the Digitized Sky Survey, produced at the Space Telescope Science Institute under US Government grant NAG W-2166.
** Tables 2 and 3, and Figs. 1-6 are also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)

Send offprint requests to: S. Friedrich

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000

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