On high proper motion white dwarfs from photographic surveys

The interpretation of high proper motion white dwarfs detected by Oppenheim er et al. (2001) was the start of a lively controversy. While the discovere rs identify a large fraction of their findings as dark halo members, others interpret the same sample as essentially made of disc and/or thick disc st ars. We use the comprehensive description of Galactic stellar populations p rovided by the "Besancon" model to produce a realistic simulation of Oppenh eimer et al. data, including all observational selections and calibration b iases. The conclusion is unambiguous: Thick disc white dwarfs resulting fro m ordinary hypotheses on the local density and kinematics are sufficient to explain the observed objects, there is no need for halo white dwarfs. This conclusion is robust to reasonable changes in model ingredients. The main cause of the misinterpretation seems to be that the velocity distribution o f a proper motion selected star sample is severely biased in favour of high velocities. This has been neglected in previous analyses. Obviously this d oes not prove that no such objects like halo white dwarfs can exist, but Op penheimer et al. observations drive their possible contribution in the dark matter halo down to an extremely low fraction.