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1–5 Jul 2019
Geological Museum
Europe/Copenhagen timezone

High-Mass White Dwarfs in Gaia DR2: the Q Branch and Double-WD Mergers

3 Jul 2019, 10:30
20m
Main Auditorium (Geological Museum)

Main Auditorium

Geological Museum

Øster Voldgade 5-7 DK-1350 Copenhagen

Speaker

Sihao Cheng (Johns Hopkins University)

Description

Recently, Gaia DR2 has revealed an enhancement of high-mass WDs on the H-R diagram, called the Q branch. The distribution of photometric ages and velocities of WDs around the branch suggest an extra cooling delay beyond current WD cooling models. To explore the properties of this delay, we statistically compare two age indicators -- the dynamical age reflected by transverse velocity and the photometric ages calculated from WD cooling models -- for more than one thousand high-mass WDs ($1.08-1.23\rm\,M_\odot$). We show that, in addition to crystallization and merger delays, an extra 8-Gyr cooling delay is required on the Q branch, which affects only about $7\,\%$ of high-mass WDs. $\rm^{22}Ne$ settling in previously-metal-rich double-WD merger products may account for this extra delay. Independent of the explanation for the Q branch, we also show that $20\pm6\,\%$ of high-mass WDs originate from double-WD mergers, corresponding to a merger rate of $(2.1\pm0.6)\times10^{-14}\rm\,M_\odot^{-1}\,yr^{-1}$ in our mass range. This is a direct observational constraint on the rate of double-WD mergers. In future, our method may be used to constrain the delay time distribution of double-WD mergers.

Primary authors

Sihao Cheng (Johns Hopkins University) Dr Jeffrey Cummings (Johns Hopkins University) Prof. Brice Ménard (Johns Hopkins University)

Presentation materials