HST star counts at high galactic latitudes

We use star counts from 17 deep fields imaged with the Wide Field Camera 2 (WFC-2) as part of the HST Medium Deep Survey key project in order to constrain the amount of dark matter in the Galaxy that can be in the form of low-mass main-sequence stars or white dwarfs. Based on the number of red stars found in our fields, we exclude the possibility that more than 15 per cent of the massive dark halo in the Galaxy is made up of M dwarfs or subdwarfs; fairly massive (M similar to 0.2 M.) and yet extremely faint (M(I) greater than or similar to 14.5) stellar candidates would have to be invoked in order to make the observed number of stars compatible with that predicted by a stellar dark halo. White dwarfs must also be intrinsically very faint (M(I) greater than or similar to 14) in order to be consistent with the observed number of faint stars in the HST fields. We rule out an increasing or flat stellar luminosity function beyond M(V) similar to 13. The inferred disc luminosity function is intermediate between that derived with complete parallax samples of nearby stars and the results of photometric studies of more distant ones. Finally, the magnitude counts are well fitted by existing models for the structure of the Galaxy, except for an excess of stars with 23 < I < 23.5. The colour distribution, however, is not as well reproduced by the models, especially if the thick disc component is neglected. However, the overall agreement between models and the data precludes the need to add any significant new stellar population to the ones assumed to exist.