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

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 χ 2 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.

DB white dwarfs in the Sloan Digital Sky Survey data release 10 and 12

“…For the averaging, we thus use the intervals between 16 000 and 22 000 K, with many objects and a constant mass throughout, which results in a lower mean mass of 0.606 ± 0.004 M compared to 0.671 M in BW11. The mean masses of Voss et al (2007) agree approximately with our current result, but since different mixing-length parameters and different intervals for the averaging were used, the comparison is not very meaningful. We emphasize again that, as long as the reason for the log g increase at the low T eff end is not understood, it is very important to only compare mean masses for well-defined T eff intervals.…”

“…The percentage of DBAs increases very significantly with increasing S/N, from 37% for the subsample with z ≤ 250 pc to 75% at the highest S/N. This is even higher than the values of 44% found by BW11 and 55% by Voss et al (2007), regarded as being lower limits in those papers. Our findings suggest that practically all DBs show some trace of hydrogen if the resolution and S/N are high enough.…”

Evolutionary and pulsational properties of white dwarf stars

“…It is expected that a modest accretion rate of about 10 −19 − 10 −21 M ⊙ /yr could be enough to explain the presence of H in the DBs. The effective temperatures of DB stars derived from model atmospheres that contain H are considerably lower than those obtained in the case in which H is neglected Castanheira et al 2006;Voss et al 2007). Thus, the presence of (unknown) quantities of H in the He-rich atmospheres of DB stars has strong impact on the determination of the effective temperature and gravity.…”

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey

“…Koester et al (2001) derived temperatures and gravities from a preliminary sample of about 200 objects; 71 helium-rich stars of spectral type DB and DBA were studied in Voss et al (2007), and approximately 60 objects with detected Ca ii resonance lines were discussed in Koester et al (2005). Pauli et al (2003Pauli et al ( , 2006 studied the 3D kinematics using the SPY data, new DO and PG1159 stars have been identified by Werner et al (2004).…”