Slowly cooling white dwarfs in M13 from stable hydrogen burning

Abstract

White dwarfs (WDs) are the final evolutionary product of the vast majority of stars in the Universe. They are electron-degenerate structures characterized by no stable thermonuclear activity, and their evolution is generally described as a pure cooling process. Their cooling rate is adopted as cosmic chronometer to constrain the age of several Galactic populations, including the disk, globular and open clusters. By analysing high-resolution photometric data of two very similar Galactic globular clusters (M3 and M13), we find a clear-cut and unexpected overabundance of bright WDs in M13. Theoretical models suggest that, consistent with the horizontal branch morphology, this overabundance is due to a slowing down of the cooling process in ~70% of the WDs in M13, caused by stable thermonuclear burning in their residual hydrogen-rich envelope. The presented observational evidence of quiescent thermonuclear activity occurring in cooling WDs brings new attention on the use of the WD cooling rate as cosmic chronometer for low-metallicity environments.