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Issue A&A
Volume 400, Number 1, March II 2003
Page(s) L13 - L16
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20030132



A&A 400, L13-L16 (2003)
DOI: 10.1051/0004-6361:20030132

Letter

Model atmosphere analysis of the extreme DQ white dwarf GSC2U J131147.2+292348

D. Carollo1, D. Koester2, A. Spagna1, M. G. Lattanzi1 and S. T. Hodgkin3

1  INAF, Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
2  Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
3  Cambridge Astronomical Survey Unit, Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK

(Received 12 January 2003 / Accepted 28 January 2003 )

Abstract
A new model atmosphere analysis for the peculiar DQ white dwarf discovered by Carollo et al. (2002) is presented. The effective temperature and carbon abundance have been estimated by fitting both the photometric data ( $\it UB_{J}VR_{F}I_{N}JHK$) and a low resolution spectrum ( $3500<\lambda<7500$ Å) with a new model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We estimate $T_{\rm eff}\simeq 5120\pm 200$ K and $\rm log[C/He]\simeq -5.8 \pm 0.5$, which make GSC2U J131147.2+292348 the coolest DQ star ever observed. This result indicates that the hypothetical transition from C 2 to C 2H molecules around $T_{\rm eff} $ = 6000 K, which was inferred to explain the absence of DQ stars at lower temperatures, needs to be reconsidered.


Key words: white dwarfs -- techniques: spectroscopic -- stars: kinematics -- stars: individual: GSC2U J131147.2+292348

Offprint request: D. Carollo, carollo@to.astro.it

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