High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey^*,**

II. DB and DBA stars

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany e-mail: voss@ari.uni-heidelberg.de
² Institut für Theoretische Physik und Astrophysik, Universität Kiel, Leibnizstraße 15, 24098 Kiel, Germany
³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁴ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 13 February 2007
Accepted: 30 May 2007

Abstract

Context.We present a detailed spectroscopic analysis of the stars with helium-dominated spectra in the ESO Supernova Ia Progenitor Survey (SPY).

Aims.Atmospheric parameters, masses, and abundances of trace hydrogen are determined and discussed in the context of spectral evolution of white dwarfs.

Methods.The spectra are compared with theoretical model atmospheres using a  fitting technique, leading to determinations of effective temperature, surface gravity, and hydrogen abundance.

Results.Our final sample contains 71 objects, of which 6 are new detections and 14 are reclassified from DB to DBA because of the presence of H lines. One is a cool DO with weak He ii lines, 2 are composite DB+dM. 55% of the DB sample show hydrogen and are thus DBA, a significantly higher fraction than found before.

Conclusions.The large incidence of DBA, and the derived total hydrogen masses are compatible with the scenario that DBs “reappear” around 30 000 K from the DB gap by mixing and diluting a thin hydrogen layer of the order of 10^-15 . This hydrogen mass is then during the evolution continuously increased by interstellar accretion. There are indications that the accretion rate increases smoothly with age or decreasing temperature, a trend which continuous even below the current low temperature limit (Dufour 2006). A remaining mystery is the low accretion rate of H compared to that of Ca observed in the DBZA, but a stellar wind extending down to the lowest temperatures with decreasing strength might be part of the solution.