The non-explosive stellar merging origin of the ultra-massive carbon-rich white dwarfs

TL;DR

This study investigates the origin of ultra-massive, carbon-rich white dwarfs, proposing that they form through non-explosive white dwarf mergers, which has implications for understanding Type Ia supernovae.

Contribution

It provides observational and population synthesis evidence supporting white dwarf mergers as the origin of certain carbon-rich white dwarfs, linking their formation to supernova mechanisms.

Findings

Nearly half of the studied white dwarfs have kinematics consistent with old stellar populations.

Population synthesis supports the merger hypothesis for these white dwarfs.

Implications for Type Ia supernovae progenitor models.

Abstract

We have investigated the origin of a sub-class of carbon-polluted white dwarfs (DQ) originally identified as the ``hot DQ" white dwarfs. These objects are relatively hot (10 000 < T_eff < 25 000 K), have markedly higher carbon abundance (C-enriched), are more massive (M > 0.8 M_Sun) than ordinary DQs (M ~ 0.6 M_Sun), and display high space velocities. Hence, despite their young appearance their kinematic properties are those of an old white dwarf population. The way out of this dilemma is to assume that they formed via the merging of two white dwarfs. In this paper we examine the observed characteristics of this population of ``C-enriched" DQ white dwarfs and confirm that nearly half of the 63 known objects have kinematic properties consistent with those of the Galactic thick disc or halo. We have also conducted population synthesis studies and found that the merging hypothesis is indeed compatible with observations. Studies of this sub-class of white dwarfs have important implications for our understanding of Type Ia Supernovae (SNeIa), commonly used to determine the expansion history of the universe, since the same formation channel applies to both kinds of objects. Hence probing the properties of these white dwarfs that failed to explode may yield important constraints to the modelling of the mechanisms leading to a thermonuclear runaway.