# Browsing by Subject "cool"

Now showing 1 - 2 of 2

- Results Per Page
1 5 10 20 40 60 80 100

- Sort Options
Ascending Descending

Item A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs: DBA And DB Stars(2012-10) Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.; Williams, Kurtis A.; Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.Show more We measure apparent velocities (nu(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. We find a difference of 6.9 +/- 6.9 kms(-1) in the average apparent velocity of the H alpha lines versus that of the He I 5876 angstrom lines 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. to use the apparent velocity of the He I 5876 angstrom 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, (BI), (and hence average gravitational redshift, (BI)) for our WDs; from that we derive an average mass, < M >(BI). For the dbAs, we find (BI) = 40.8 +/- 4.7 kms(-1) and derive < M >(BI) = 0.71(-0.05)(+0.04) M-circle dot. Though different from 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(-0.014)(+0.013) M-circle dot; Falcon et al.), we do not claim this as a stringent detection. Rather, we emphasize that the difference between (BI) of the dbAs and of normal DAs is no larger than 9.2 kms(-1), at the 95% confidence level; this corresponds to roughly 0.10 M-circle dot. For the dbs, we find (BI) = 42.9 +/- 8.49 km s(-1) after applying the blueshift correction and determine < M >(BI) = 0.74(-0.09)(+0.08) M-circle dot. The difference between (BI) of the dbs and of DAs is <= 11.5 kms(-1) (similar to 0.12 M-circle dot), 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. 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.Show more Item The ZZ Ceti Red Edge(2002-06) Kanaan, A.; Kepler, S. O.; Winget, D. E.; Winget, D. E.Show more With a time-series CCD photometric survey, we have demonstrated clearly that the observed red edge for the ZZ Ceti stars instability strip at 11 000 K is real, with the pulsation amplitude decreasing at least by a factor of 50. Previous surveys for variability among hydrogen atmosphere white dwarfs around 11 000 K have been carried out using time-series photoelectric photometry, not differential photometry, insensitive for small amplitude periodicities of 15 min and longer. In our survey we constantly monitor the sky brightness as well as one or more comparison stars through the same color filter, reducing the adverse effects of differential extinction and sky fluctuations, obtaining true differential photometry.Show more