The HST large programme on NGC 6752 -- IV. The White Dwarf Sequence

TL;DR

This study uses Hubble data to analyze the white dwarf cooling sequence in NGC 6752, confirming models and estimating the cluster's age around 13 Gyr, while exploring the effects of multiple populations and model uncertainties.

Contribution

First comprehensive analysis of the white dwarf luminosity function in NGC 6752 using HST data, validating theoretical models and refining age estimates.

Findings

White dwarf luminosity function peaks at m_F606W=29.3

Cluster age estimated between 12.7 and 13.5 Gyr

Hydrogen envelope models with new opacities yield younger WD ages

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 m_F606W = 29.3+/-0.1, consistent within uncertainties with what has been previously reported, and is now complete down to m_F606W~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 m_F606W 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 timescales of models in the magnitude range 28.1 < m_F606W < 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.