A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DBA and DB Stars

TL;DR

This study measures the gravitational redshift of helium-atmosphere white dwarfs to estimate their mean masses, extending previous methods to WDs without visible hydrogen lines and comparing results with spectroscopic data.

Contribution

It introduces a gravitational redshift approach for helium-dominated white dwarfs and compares the derived mean masses with those from spectroscopic methods.

Findings

DBA white dwarfs have an average mass around 0.71 Msun.

DB white dwarfs have an average mass around 0.74 Msun.

The gravitational redshift method yields larger mean masses than spectroscopic estimates.

Abstract

We measure apparent velocities (v_app) of absorption lines for 36 white dwarfs (WDs) with helium-dominated atmospheres -- 16 DBAs and 20 DBs -- using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). We find a difference of 6.9+/-6.9 km/s in the average apparent velocity of the H-alpha lines versus that of the HeI 5876AA for our DBAs. This is a measure of the blueshift of this He line due to pressure effects. By using this as a correction, we extend the gravitational redshift method employed by Falcon et al. (2010) to use the apparent velocity of the HeI 5876AA line and conduct the first gravitational redshift investigation of a group of WDs without visible hydrogen lines. We use biweight estimators to find an average apparent velocity, <v_app>_BI, (and hence average gravitational redshift, <v_g>_BI) for our WDs; from that we derive an average mass, <M>_BI. For the DBAs, we find <v_app>_BI = 40.8+/-4.7 km/s and derive <M>_BI = 0.71 +0.04 -0.05 Msun. Though different from <v_app> of DAs (32.57 km/s) at the 91% confidence level and suggestive of a larger DBA mean mass than that for normal DAs derived using the same method (0.647 +0.013 -0.014 Msun; Falcon et al. 2010), we do not claim this as a stringent detection. Rather, we emphasize that the difference between <v_app>_BI of the DBAs and <v_app> of normal DAs is no larger than 9.2 km/s, at the 95% confidence level; this corresponds to roughly 0.10 Msun. For the DBs, we find <v_app>^He_BI = 42.9+/-8.49 km/s after applying the blueshift correction and determine <M>_BI = 0.74 +0.08 -0.09 Msun. The difference between <v_app>^He_BI of the DBs and <v_app> of DAs is less than or equal to 11.5 km/s (~0.12 Msun), at the 95% confidence level. The gravitational redshift method indicates much larger mean masses than the spectroscopic determinations of the same sample by Voss et al. (2007)...