HST star counts at high galactic latitudes

We use star counts from 13 deep HST fields imaged with the {\it Wide FieldCamera - 2} in order to constrain the amount of dark matter in the Galaxy thatcan be in the form of low-mass main sequence stars or white-dwarfs. Based onthe number of red stars found in our fields, we exclude the possibility thatmore than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs;fairly massive ($M \sim 0.2 M_\odot$) and yet extremely faint ($M_I$ \gtsima$14.5$) stellar candidates would have to be invoked in order to make theobserved number of stars compatible with that predicted by a stellar dark halo.White dwarfs must also be intrinsically very faint ($M_I$ \gtsima $14$) inorder to be consistent with the observed number of faint stars in the HSTfields. We also rule out an increasing or flat stellar luminosity functionbeyond $M_V \sim 13$. The inferred slope of the disk luminosity function isintermediary between local, volume-limited surveys and ground-based photometricones. Finally, the magnitude counts are well fitted by existing models for thestructure of the Galaxy, with only small changes in the fiducial values of themodel parameters. The colour distribution, however, is not well reproduced bythe models. It is unclear at present if this reflects inadequacies of theavailable models or uncertainties in the colour-magnitude diagrams for lowmetallicity stars and in the photometric calibration.Comment: 18 pages plus 6 figure