NOAO > Library > NOAO Preprint Series > Preprint 894
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NOAO > Library > NOAO Preprint Series > Preprint 894 |
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Author(s): Nelson, C.A., Cook, K.H., Axelrod, T.S., Mould, J.R., Alcock, C., Freeman, K.C.
Title: 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.
Date Received:
January 02, 2002
Publication Info:
ApJ 573:644-661
Telescopes:
HST, Hubble Space Telescope
Downloadable version (3629 Kb PDF)
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NOAO > Library > NOAO Preprint Series > Preprint 894 |
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