Stars in the Hubble Ultra Deep Field

We identified 46 unresolved source candidates in the Hubble Ultra Deep Field,down to i775 = 29.5. Unresolved objects were identified using a parameter S,which measures the deviation from the curve-of-growth of a point source.Extensive testing of this parameter was carried out, including the effects ofdecreasing signal-to-noise and of the apparent motions of stars, whichdemonstrated that stars brighter than i775 = 27.0 could be robustly identified.Low resolution grism spectra of the 28 objects brighter than i775 = 27.0identify 18 M and later stellar type dwarfs, 2 candidate L-dwarfs, 2 QSOs, and4 white dwarfs. Using the observed population of dwarfs with spectral type M4or later, we derive a Galactic disk scale height of 400 \pm 100 pc for M and Lstars. The local white dwarf density is computed to be as high as (1.1 \pm 0.3)x10^(-2) stars/pc^3. Based on observations taken 73 days apart, we determinedthat no object in the field has a proper motion larger than 0.027"/year (3sigma detection limit). No high velocity white dwarfs were identified in theHUDF, and all four candidates appear more likely to be part of the Galacticthick disk. The lack of detected halo white dwarfs implies that, if the darkmatter halo is 12 Gyr old, white dwarfs account for less than 10% of the darkmatter halo mass.Comment: 35 pages, 11 figures, accepted by Ap