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

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)...