Cool White Dwarfs Revisited: New Spectroscopy and Photometry

In this paper we present new and improved data on 38 cool white dwarfsidentified by Oppenheimer et al. 2001 (OHDHS) as candidate dark halo objects.Using the high-res spectra obtained with LRIS, we measure radial velocities for13 WDs that show an H alpha line. We show that the knowledge of RVs decreasesthe UV-plane velocities by only 6%. The radial velocity sample has a W-velocitydispersion of sig_W = 59 km/s--in between the values associated with the thickdisk and the stellar halo. We also see indications for the presence of twopopulations by analyzing the velocities in the UV plane. In addition, wepresent CCD photometry for half of the sample, and with it recalibrate thephotographic photometry of the remaining WDs. Using the new photometry instandard bands, and by applying the appropriate color-magnitude relations for Hand He atmospheres, we obtain new distance estimates. New distances of the WDsthat were not originally selected as halo candidates yield 13 new candidates.On average, new distances produce velocities in the UV plane that are larger by10%, with already fast objects gaining more. Using the new data, while applyingthe same UV-velocity cut (94 km/s) as in OHDHS, we find a density of cool WDsof 1.7e-4 pc^-3, confirming the value of OHDHS. In addition, we derive thedensity as a function of the UV-velocity cutoff. The density (corrected forlosses due to higher UV cuts) starts to flatten out at 150 km/s (0.4e-4 pc^-3),and is minimized (thus minimizing a possible non-halo contamination) at 190km/s (0.3e-4 pc^-3). These densities are in a rough agreement with theestimates for the stellar halo WDs, corresponding to a factor of 1.9 and 1.4higher values.Comment: Accepted to ApJ. New version contains some additional data. Results unchange