The mass function of hydrogen-rich white dwarfs: robust observational evidence for a distinctive high-mass excess near 1 M⊙

The mass function of hydrogen-rich atmosphere white dwarfs has been frequently found to reveal a distinctive high-mass excess near 1 M-circle dot. However, a significant excess of massive white dwarfs has not been detected in the mass function of the largest white dwarf catalogue to date from the Sloan Digital Sky Survey (SDSS). Hence, whether a high-mass excess exists or not has remained an open question. In this work, we build the mass function of the latest catalogue of data release ten SDSS hydrogen-rich white dwarfs, including the cool and faint population (i.e. effective temperatures 6000 less than or similar to T-eff less than or similar to 12 000 K, equivalent to 12 mag less than or similar to M-bol less than or similar to 13 mag). We show that the high-mass excess is clearly present in our mass function, and that it disappears only if the hottest (brightest) white dwarfs (those with T-eff greater than or similar to 12 000 K, M-bol less than or similar to 12 mag) are considered. This naturally explains why previous SDSS mass functions failed at detecting a significant excess of high-mass white dwarfs. Thus, our results provide additional and robust observational evidence for the existence of a distinctive high-mass excess near 1 M-circle dot. We investigate possible origins of this feature and argue that the most plausible scenario that may lead to an observed excess of massive white dwarfs is the merger of the degenerate core of a giant star with a main-sequence or a white dwarf companion during or shortly after a common envelope event.