# Merger Products among White Dwarfs

Two white dwarfs (WDs) may merge and produce a new white dwarf with a higher mass. Compared to other high-mass white dwarfs, these merger products have higher velocity dispersion because they are older (older stars have higher velocity dispersion in the Milky-Way disc). We make use of this kinematic signature of double-WD merger products to estimate the number of them. This number can then be translate into the merger rate of WD binaries, which has implication for the progenitor problem of type-Ia supernovae. We show that this observed merger rate agrees well with that from binary simulations.

Our paper has been published on ApJ.

## High-mass white dwarfs

From Gaia Data Release 2, we select a 250 pc sample of high-mass, hot white dwarfs, avoiding those on the ‘Q branch’ (see here). In total we have got 1395 objects for our kinematic analysis.

## Velocity distribution

The distribution of transverse velocity distribution from Gaia data. If there is no double-WD merger products (f_m = 0), then the distribution should follow the black, dashed curves. The observed distribution (data points with error bars) shows clear excess and can be used to estimate f_m.

## Fraction of double-WD merger products (f_m)

Using the velocity distributions in five mass bins, we estimate the corresponding fractions of double-WD merger products (f_m).

## Merger rates

Then, we calculate the double-WD merger rate as a function of the product mass. Our estimates add significant precision and mass resolution to our knowledge of the merger rate, and are close to the binary population synthesis predictions. This supports the idea (in terms of event rate) that double-WD merger may contribute a significant fraction of type-Ia supernova.

s.cheng@jhu.edu

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