A pair of CO + He white dwarfs as the progenitor of 2005E-like supernovae?

TL;DR

This study explores the formation of CO + He white dwarf pairs through binary evolution channels and assesses their potential as progenitors of Ca-rich transients, highlighting the importance of mass constraints and explosion mechanisms.

Contribution

It introduces four binary formation channels for CO + He WD pairs and evaluates their compatibility with observed properties of Ca-rich transients, emphasizing the role of mass transfer stability.

Findings

CO WDs less than 0.6 M_sun and He WDs less than 0.25 M_sun are necessary for CRT progenitors.

Most WD pairs experience mixed stable and unstable mass transfer phases.

Low-mass WD pairs can produce ejecta masses consistent with observations of CRTs.

Abstract

Ca-rich transients (CRTs, 2005E-like supernovae) exhibit unusually strong Ca features during their nebular phase, and their distribution in their host galaxies indicates that they belong to a metal-poor old population. We carried out a series of binary population synthesis and presented four different channels of forming CO WD + He WD pairs. We selected the systems fulfilling the constraints of the old population and the birth rate from all the CO WD + He WD pairs by constraining the component mass of the WD pairs. For the four channels, the stable Roche lobe overflow (RLOF) could have a significant influence on the formation of the WD + WD pairs. Based on their position on the $M_{\rm CO}$-$M_{\rm He}$ plane, the mass-transfer between the components for most of the CO WD + He WD pairs is neither always unstable nor always stable. We found that it is necessary that the CO WDs are less massive than 0.6 $M_{\odot}$ and the He WDs are less massive than 0.25 $M_{\odot}$ if CO WD + He WD pairs fulfill the constraints from both the old population and the birth rate of CRTs. However, the He WD mass is lower than, while the total mass of the low-mass WD pairs is larger than{\bf ,} the ejecta mass of the CRTs derived from observations. Our results imply that the CO WDs participate in CRT explosions and at the same time, a bound remnant could be left after the CRT explosion if the low-mass WD pairs are the progenitors of CRTs. Therefore, whether or not the helium detonation on a low-mass CO WD may lead to the second detonation in the center of the CO WDs, and whether or not the binding remnant is left after the thermonuclear explosion should be examined carefully in the future.