Observational constraints on the progenitor metallicities of core-collapse supernovae

TL;DR

This study directly measures the metallicities of supernova progenitors using local spectra, revealing only a slight difference between types Ibc and II, suggesting metallicity is not the main factor in supernova classification.

Contribution

It provides the first direct spectroscopic measurements of progenitor metallicities at supernova sites, challenging previous indirect estimates and their implications.

Findings

Mean metallicity difference between Ibc and II is only 0.06 dex.

Metallicity distributions are marginally consistent with being from the same population.

Progenitor metallicity is likely not the primary factor in supernova type determination.

Abstract

We present constraints on the progenitor metallicities of core-collapse supernovae. To date, nearly all metallicity constraints have been inferred from indirect methods such as metallicity gradients in host galaxies, luminosities of host galaxies, or derived global galaxy metallicities. Here, progenitor metallicities are derived from optical spectra taken at the sites of nearby supernovae, from the ratio of strong emission lines found in their host HII regions.We present results from the spectra of 74 host HII regions and discuss the implications that these have on the nature of core-collapse supernova progenitors. Overall, while we find that the mean metallicity of type Ibc environments is higher than that of type II events, this difference is smaller than observed in previous studies. There is only a 0.06 dex difference in the mean metallicity values, at a statistical significance of ~1.5 sigma, while using a KS-test we find that the two metallicity distributions are marginally consistent with being drawn from the same parent population (probability >10%). This argues that progenitor metallicity is not a dominant parameter in deciding supernovae type, with progenitor mass and/or binarity playing a much more significant role.