High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey. II. DB and DBA stars
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 χ2 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 M⊙. 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.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- August 2007
- DOI:
- 10.1051/0004-6361:20077285
- Bibcode:
- 2007A&A...470.1079V
- Keywords:
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- stars: white dwarfs;
- methods: observational;
- stars: fundamental parameters;
- stars: atmospheres