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 \pm 6.9 km s ^ { -1 } in the average apparent velocity of the H \alpha lines versus that of the He i 5876 Å 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 to use the apparent velocity of the He i 5876 Å 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 , \langle v _ { app } \rangle _ { BI } , ( and hence average gravitational redshift , \langle v _ { g } \rangle _ { BI } ) for our WDs ; from that we derive an average mass , \langle M \rangle _ { BI } .
For the DBAs , we find \langle v _ { app } \rangle _ { BI } = 40.8 \pm 4.7 km s ^ { -1 } and derive \langle M \rangle _ { BI } = 0.71 ^ { +0.04 } _ { -0.05 } M _ { \odot } .
Though different from \langle v _ { app } \rangle of DAs ( 32.57 km s ^ { -1 } ) 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 } M _ { \odot } ; ) , we do not claim this as a stringent detection .
Rather , we emphasize that the difference between \langle v _ { app } \rangle _ { BI } of the DBAs and \langle v _ { app } \rangle of normal DAs is no larger than 9.2 km s ^ { -1 } , at the 95 % confidence level ; this corresponds to roughly 0.10 M _ { \odot } .
For the DBs , we find \langle v _ { app } ^ { He } \rangle _ { BI } = 42.9 \pm 8.49 km s ^ { -1 } after applying the blueshift correction and determine \langle M \rangle _ { BI } = 0.74 ^ { +0.08 } _ { -0.09 } M _ { \odot } .
The difference between \langle v _ { app } ^ { He } \rangle _ { BI } of the DBs and \langle v _ { app } \rangle of DAs is \leq 11.5 km s ^ { -1 } ( \sim 0.12 M _ { \odot } ) , at the 95 % confidence level .
The gravitational redshift method indicates much larger mean masses than the spectroscopic determinations of the same sample by .
Given the small sample sizes , it is possible that systematic uncertainties are skewing our results due to the potential of kinematic substructures that may not average out .
We estimate this to be unlikely , but a larger sample size is necessary to rule out these systematics .