On the white dwarf distances to galactic globular clusters

We analyze in detail various possible sources of systematic errors on the d istances of globular clusters derived by fitting a local template DA white dwarf sequence to the cluster counterpart (the so-called WD-fitting techniq ue). We find that the unknown thickness of the hydrogen layer of white dwar fs in clusters plays a non negligible role. For reasonable assumptions - su pported by the few sparse available observational constraints - about the u nknown mass and thickness of the hydrogen layer for the cluster white dwarf s, a realistic estimate of the systematic error on the distance is within /-0.10 mag. However, particular combinations of white dwarf masses and enve lope thicknesses - which at present cannot be excluded a priori - could pro duce larger errors. Contamination of the cluster DA sequence by non-DA whit e dwarfs introduces a very small systematic error of about -0.03 mag in the M-v/(V-I) plane, but in the M-v/(B-V) plane the systematic error amounts t o similar to +0.20 mag. Contamination by white dwarfs with helium cores sho uld not influence appreciably the WD-fitting distances. Finally, we obtain a derivative Delta (m-M)(v)/DeltaE(B-V) similar to -5.5 for the WD-fitting distances, which is very similar to the dependence found when using the Mai n Sequence fitting technique.