The delayed evolution of high-mass white dwarfs: the Q branch and double-white-dwarf mergers. (arXiv:1905.12710v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Cheng_S/0/1/0/all/0/1">Sihao Cheng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cummings_J/0/1/0/all/0/1">Jeffrey Cummings</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Menard_B/0/1/0/all/0/1">Brice M&#xe9;nard</a>

Studying high-mass white dwarfs (WDs) can shed light on the progenitors of
Type Ia supernovae. Recently, the unprecedented power of Gaia Data Release 2
(DR2) has revealed an enhancement of high-mass WDs on the H-R diagram, called
the Q branch. This branch is located at the high-mass end of the
crystallisation branch identified by Tremblay et al. (2019). However,
investigating its properties, we find that the number density and the fraction
of fast-moving WDs on the Q branch cannot be explained by crystallisation
alone, suggesting the existence of an extra cooling delay. To explore the
properties of this delay, we compare two WD age indicators — the dynamical age
reflected by transverse velocity and the photometric age — for more than one
thousand high-mass WDs (1.08-1.23 $M_odot$). We show that, in addition to
crystallisation and merger delays, an 8-Gyr cooling delay is required on the Q
branch, which affects about $7%$ of high-mass WDs. $^{22}$Ne settling in some
WDs may account for this extra delay. We also show that $20pm6%$ of high-mass
WDs originate from double-WD mergers, corresponding to a merger rate of
$(2.1pm0.6)times10^{-14}M_odot^{-1}yr^{-1}$ in this mass range. This is a
direct observational constraint on the rate of double-WD mergers, which is a
promising channel of Type Ia supernova explosion.

Studying high-mass white dwarfs (WDs) can shed light on the progenitors of
Type Ia supernovae. Recently, the unprecedented power of Gaia Data Release 2
(DR2) has revealed an enhancement of high-mass WDs on the H-R diagram, called
the Q branch. This branch is located at the high-mass end of the
crystallisation branch identified by Tremblay et al. (2019). However,
investigating its properties, we find that the number density and the fraction
of fast-moving WDs on the Q branch cannot be explained by crystallisation
alone, suggesting the existence of an extra cooling delay. To explore the
properties of this delay, we compare two WD age indicators — the dynamical age
reflected by transverse velocity and the photometric age — for more than one
thousand high-mass WDs (1.08-1.23 $M_odot$). We show that, in addition to
crystallisation and merger delays, an 8-Gyr cooling delay is required on the Q
branch, which affects about $7%$ of high-mass WDs. $^{22}$Ne settling in some
WDs may account for this extra delay. We also show that $20pm6%$ of high-mass
WDs originate from double-WD mergers, corresponding to a merger rate of
$(2.1pm0.6)times10^{-14}M_odot^{-1}yr^{-1}$ in this mass range. This is a
direct observational constraint on the rate of double-WD mergers, which is a
promising channel of Type Ia supernova explosion.

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