Abstract

Carbon in hot DB white dwarfs

Detlev Koester (University of Kiel, Germany)

J. Provencal, University of Delaware, USA B.T. Gaensicke, University of Warwick, UK

In this paper we report the discovery of two DBs at 25000K with photospheric carbon and no other heavy elements present. The lower limits on the C/Si ratio are 600 and 2500. We argue that the only plausible explanation is dredge-up of carbon, and derive a He layer mass fraction of approximately 2e-9. Upon further cooling such a thin layer would be transformed into a carbon-rich atmosphere by the deepening convection zone. We discuss this scenario in relation to other possible explanations for the carbon detections.

Mode of presentation: oral