A massive star origin for an unusual helium-rich supernova in an elliptical galaxy

TL;DR

This paper suggests that the unusual helium-rich supernovae SN 2005E and SN 2005cz originate from low-mass core-collapse supernovae of stars in elliptical galaxies, challenging previous white dwarf detonation models.

Contribution

It provides evidence that helium-rich, faint supernovae in elliptical galaxies can result from low-mass core-collapse events, expanding understanding of supernova progenitors.

Findings

SN 2005cz shares properties with SN 2005E, including helium richness and rapid fading.

The properties are best explained by a low-mass ($8-12 M_{\odot}$) core-collapse supernova.

Host galaxy NGC 4589 shows evidence of recent star formation, supporting the core-collapse origin.

Abstract

The unusual helium-rich (type Ib) supernova SN 2005E is distinguished from any supernova hitherto observed by its faint and rapidly fading light curve, prominent calcium lines in late-phase spectra and lack of any mark of recent star formation near the supernova location. These properties are claimed to be explained by a helium detonation in a thin surface layer of an accreting white dwarf (Perets et al. 2010). Here we report on observations of SN 2005cz appeared in an elliptical galaxy, whose observed properties resemble those of SN 2005E in that it is helium-rich and unusually faint, fades rapidly, shows much weaker oxygen emission lines than those of calcium in the well-evolved spectrum. We argue that these properties are best explained by a core-collapse supernova at the low-mass end ($8-12 M_{\odot}$) of the range of massive stars that explode (Smartt 2009). Such a low mass progenitor had lost its hydrogen-rich envelope through binary interaction, having very thin oxygen-rich and silicon-rich layers above the collapsing core, thus ejecting a very small amount of radioactive $^{56}$Ni and oxygen. Although the host galaxy NGC 4589 is an elliptical, some studies have revealed evidence of recent star-formation activity (Zhang et al. 2008), consistent with the core-collapse scenario.