Letter to the Editor

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

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

Corresponding author: D. Carollo, carollo@to.astro.it

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. ([CITE]) is presented. The effective temperature and carbon abundance have been estimated by fitting both the photometric data () and a low resolution spectrum ( Å) with a new model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We estimate  K and , which make GSC2U J131147.2+292348 the coolest DQ star ever observed. This result indicates that the hypothetical transition from C₂ to C₂H molecules around  = 6000 K, which was inferred to explain the absence of DQ stars at lower temperatures, needs to be reconsidered.