Metallicity Differences in Type Ia Supernova Progenitors Inferred from Ultraviolet Spectra

TL;DR

This study provides the first robust evidence that ultraviolet spectra of twin Type Ia supernovae reveal differences in progenitor metallicity, which influence their luminosity and nickel yields, advancing understanding of supernova progenitor properties.

Contribution

It demonstrates that UV spectral analysis can distinguish progenitor metallicities in Type Ia supernovae, linking metallicity to luminosity differences and nickel production.

Findings

UV spectra reveal metallicity differences in twin SNe Ia.

Higher metallicity progenitors produce lower luminosities.

Progenitor metallicities inferred to be above and below solar levels.

Abstract

Two "twin" Type Ia supernovae (SNe Ia), SNe 2011by and 2011fe, have extremely similar optical light-curve shapes, colors, and spectra, yet have different ultraviolet (UV) continua as measured in Hubble Space Telescope spectra and measurably different peak luminosities. We attribute the difference in the UV continua to significantly different progenitor metallicities. This is the first robust detection of different metallicities for SN Ia progenitors. Theoretical reasoning suggests that differences in metallicity also lead to differences in luminosity. SNe Ia with higher progenitor metallicities have lower 56Ni yields, and lower luminosities, for the same light-curve shape. SNe 2011by and 2011fe have different peak luminosities (Delta M_V = 0.6 mag), which correspond to different 56Ni yields: M_11fe(56Ni)/M_11by(56Ni) = 1.7^+0.7_-0.5. From theoretical models that account for different neutron to proton ratios in progenitors, the differences in 56Ni yields for SNe 2011by and 2011fe imply that their progenitor stars were above and below solar metallicity, respectively. Although we can distinguish progenitor metallicities in a qualitative way from UV data, the quantitative interpretation in terms of abundances is limited by the present state of theoretical models.