Cool White Dwarfs Revisited: New Spectroscopy and Photometry

In this paper we present new and improved data on 38 cool white dwarfs identified in 2001 by Oppenheimer and coworkers as candidate dark-halo objects. Using the high-resolution spectra obtained with LRIS on Keck I, we measure precise radial velocities for 13 white dwarfs that show an Hα absorption line. We show that accounting for radial velocities on average decreases the U-V plane velocities by only 6%. In two cases, accounting for the radial velocities put original halo candidates below the Oppenheimer and coworkers velocity cut. The radial velocity sample has a velocity dispersion in the direction perpendicular to the Galactic plane of σW = 59 km s-1, between the values typically associated with the thick-disk and stellar-halo populations. We also see indications of the presence of two populations by analyzing the velocities in the U-V plane. In addition, we present CCD photometry for half of the sample, and with it recalibrate the photographic photometry of the remaining white dwarfs. Using the new photometry in standard bands and applying the appropriate color-magnitude relations for hydrogen and helium atmospheres, we obtain new distance estimates. By recalibrating the distances of the white dwarfs that were not originally selected as halo candidates, we obtain 13 new candidates (and lose two original ones). On average, the new distances produce velocities in the U-V plane that are larger by 10%, with already fast objects gaining more. Using the new data while applying the same U-V plane velocity cut (94 km s-1) and methods of analysis as did Oppenheimer and coworkers, we find a density of cool white dwarfs of 1.7 × 10-4 pc-3, confirming their value. In addition, we derive the density as a function of the U-V plane velocity cutoff. The density (corrected for losses due to higher U-V plane velocity cuts) starts to flatten out at 150 km s-1 (0.4 × 10-4 pc-3) and is minimized (thus minimizing the possible nonhalo contamination) at 190 km s-1 (0.3 × 10-4 pc-3). These densities are in rough agreement with the estimates for the stellar-halo white dwarfs, corresponding to values a factor of 1.9 and 1.4 higher.

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