The Cooling of Massive White Dwarfs from Gaia EDR3

Abstract

We determine the distribution of cooling ages of massive Gaia EDR3 white dwarfs identified with over 90% probability within 200 pc and with mass in the range 0.95-1.25 $M_\odot$. Using three sets of publicly available models, we consider sub-samples of these white dwarfs sorted into three equally spaced mass bins. Under the assumption of a constant white dwarf formation rate, we find an excess of white dwarfs both along the Q branch and below it, corresponding respectively to stars that are in the process of freezing and those that are completely frozen. We compare the cooling age distributions for each of these bins to the recently determined time-varying star formation rate of Gaia DR2 main sequence stars. For white dwarfs in the two lightest mass bins, spanning the mass range 0.95-1.15 $M_\odot$, we find that the cumulative cooling age distribution is statistically consistent with the expectation from the star formation rate. For white dwarfs in the heaviest mass bin, 1.15-1.25 $M_\odot$, we find that their cumulative distribution is inconsistent with the star formation rate for all of the models considered; instead, we find that their cooling age distribution is well fitted by a linear combination of the distribution expected for single stellar evolution products and the distribution expected for double white dwarf merger products when approximately 40-50% of the 1.15-1.25 $M_\odot$ white dwarfs that formed over the past 4 Gyr are produced through double white dwarf mergers.