A Proper Motion Survey For White Dwarfs With WFPC2
Abstract
We have performed a search for halo white dwarfs as high proper motion objects in a second epoch WFPC2 image of the Groth-Westphal strip. The survey covers 74.8 square arcmin, and is complete to V~26.5. We identify 24 high proper motion objects with mu > 0.014''/y. Five of these high proper motion objects are identified as strong white dwarf candidates on the basis of their color, (V-I) < 1.4. We also identify two marginal candidates whose photometric errors are within ~1 sigma of our color cutoff. We create a model of the Milky Way thin disk, thick disk and stellar halo and find that this sample of white dwarfs is clearly an excess above the ~1 detections expected from these known stellar populations. The origin of the excess signal is less clear. Possibly, the excess cannot be explained without invoking a fourth galactic component: a white dwarf dark halo. Previous work of this nature has separated white dwarf samples into various galactic components based on kinematics; distances, and thus velocities, are unavailable for a sample this faint. Therefore, we present a statistical separation of our sample into the four components using only the directly observable variables, V, (V-I), mu. We perform a maximum likelihood analysis to find the most likely local white dwarf densities suggested by the observations. Our results imply a 7% white dwarf halo and six times the canonical value for the thin disk white dwarf density (at marginal statistical significance). These results depend sensitively on our assumptions about the age and initial mass function of the dark halo component.Keywords
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