The HST large programme on NGC 6752 – IV. The White Dwarf sequence

Abstract

ABSTRACT We present our final study of the white dwarf cooling sequence (WD CS) in the globular cluster NGC 6752. The investigation is the main goal of a dedicated Hubble Space Telescope large program, for which all the observations are now collected. The WD CS luminosity function (LF) is confirmed to peak at mF606W ≃ 29.3 ± 0.1, consistent within uncertainties with what has been previously reported, and is now complete down to mF606W ≃ 29.7. We have performed robust and conclusive comparisons with model predictions that show how the theoretical LF for hydrogen envelope WD models closely follow the shape of the empirical LF. The magnitude of the peak of the observed LF is matched with ages between 12.7 and 13.5 Gyr, consistent with the cluster age derived from the main sequence turn off and subgiant branch. We also find that the impact of multiple populations within the cluster on the WD LF for mF606W below 27.3 is negligible, and that the presence of a small fraction of helium envelope objects is consistent with the data. Our analysis reveals a possible hint of an underestimate of the cooling time-scales of models in the magnitude range 28.1 < mF606W < 28.9. Finally, we find that hydrogen envelope models calculated with a new tabulation of electron conduction opacities in the transition between moderate and strong degeneracy provide WD ages that are too small in comparison to the main sequence turnoff age.