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

¹ Yunnan Observatories, Chinese Academy of Sciences, 650216 Kunming, PR China
e-mail: xiangcunmeng@ynao.ac.cn; zhanwenhan@ynao.ac.cn
² Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650216 Kunming, PR China

Received: 9 July 2014
Accepted: 28 October 2014

Abstract

Context. 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. A pair of low-mass CO + He white dwarfs (WD) has been suggested to be the progenitor of CRTs. A helium shell is accumulated onto the CO WD by accretion, and then a helium-shell detonation is ignited when the helium shell reaches a critical mass, which could lead to the second detonation in the center of the CO WD.

Aims. Taking the birth rate of CRTs into consideration, we examine whether and if yes, which type of low-mass CO + He WD pairs fulfill the constraints of being of an old population and of the birth rate derived from observations.

Methods. We carried out a series of binary population syntheses and present four different channels in which CO + He WD pairs can be formed. We selected the systems that fulfill the constraints of being of an old population and of the birth rate from all the CO + He WD pairs by constraining the component mass of the WD pairs.

Results. For the four channels, the stable Roche lobe overflow (RLOF) could significantly influence the formation of the WD pairs. Based on their position on the M_CO-M_He plane, the mass-transfer between the components for most of the CO + 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_⊙ and the He WDs are less massive than 0.25 M_⊙ if CO + He WD pairs are to fulfill the constraints of being of an old population and of the birth rate of CRTs. However, the He WD mass is lower than the ejecta mass of the CRTs derived from observations, while the total mass of the low-mass WD pairs is higher than this.

Conclusions. 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, it needs to be examined 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 bound remnant is left after the thermonuclear explosion.