Hyper-runaway and hypervelocity white dwarf candidates in \textit{Gaia} Data Release 3: possible remnants from Ia/Iax supernova explosions or dynamical encounters

TL;DR

This study uses Gaia DR3 data to identify hypervelocity white dwarfs and sub-main-sequence stars, providing insights into supernova explosion mechanisms and the origins of hypervelocity stars.

Contribution

The paper reports the discovery of 46 new hypervelocity star candidates, including unbound white dwarfs and sub-main-sequence stars, and analyzes their implications for supernova models.

Findings

Extreme velocity ejections are rare among Type Ia supernovae.

The rate of hypervelocity star ejections aligns with the hybrid WD reverse-detonation model.

Lower-velocity hypervelocity WDs may originate from Type Iax supernovae or dynamical encounters.

Abstract

Type Ia and other peculiar supernovae (SNe) are thought to originate from the thermonuclear explosions of white dwarfs (WDs). Some of the proposed channels involve the ejection of a partly exploded WD (e.g. Iax SN remnant) or the companion of an exploding WD at extremely high velocities ($>400$ km\,s$^{-1}$). Characterisation of such hyper-runaway/hypervelocity (HVS) WDs might therefore shed light on the physics and origins of SNe. Here we analyse the \textit{Gaia} DR3 data to search for HVS WDs candidates, and peculiar sub-main-sequence (sub-MS) objects. We retrieve the previously identified HVSs, and find 46 new HVS candidates. Among these we identify two new unbound WDs and two new unbound sub-MS candidates. The remaining stars are hyper-runaway WDs and hyper-runaway sub-MS stars. The numbers and properties of the HVS WD and sub-MS candidates suggest that extreme velocity ejections ($>1000$ km\,s$^{-1}$) can accompany at most a small fraction of type Ia SNe, disfavouring a significant contribution of the D6-scenario to the origin of Ia SNe. The rate of HVS ejections following the hybrid WD reverse-detonation channel could be consistent with the identified HVSs. The numbers of lower-velocity HVS WDs could be consistent with type Iax SNe origin and/or contribution from dynamical encounters. We also searched for HVS WDs related to known SN remnants, but identified only one such candidate.