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

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.

💡 Research Summary

The paper presents a comprehensive observational study of SN 2005cz, a helium‑rich (Type Ib) supernova that shares several unusual characteristics with the previously reported Ca‑rich, He‑rich event SN 2005E. Photometric monitoring shows that SN 2005cz reached a modest peak absolute magnitude (M_V ≈ –15 mag) and faded extremely rapidly, with a half‑life of roughly ten days. Such a faint, fast‑declining light curve implies a very small amount of radioactive ^56Ni (≤ 0.01 M_⊙) was synthesized in the explosion.

Spectroscopic analysis reveals strong He I λ5876 and λ6678 lines at early times, confirming the helium‑rich nature of the ejecta. In the nebular phase (≥ +50 days) the spectrum is dominated by Ca II near‑infrared triplet and the forbidden