SN 2013ai: a link between hydrogen-rich and hydrogen-poor core-collapse supernovae

TL;DR

SN 2013ai exhibits characteristics bridging hydrogen-rich and hydrogen-poor core-collapse supernovae, with unique spectral velocities, light curve features, and progenitor properties, suggesting a continuum between different supernova types.

Contribution

This study provides detailed spectral and light curve analysis of SN 2013ai, proposing it as a link between SNe II and SNe IIb, with implications for supernova progenitor models.

Findings

SN 2013ai has a long rise time and high expansion velocities.

Spectral features resemble SN IIb but lack strong helium lines.

Progenitor estimated at ~17 solar masses with significant nickel synthesis.

Abstract

We present a study of optical and near-infrared (NIR) spectra along with the light curves of SN 2013ai. These data range from discovery until 380 days after explosion. SN 2013ai is a fast declining type II supernova (SN II) with an unusually long rise time; $18.9\pm2.7$d in $V$ band and a bright $V$ band peak absolute magnitude of $-18.7\pm0.06$ mag. The spectra are dominated by hydrogen features in the optical and NIR. The spectral features of SN 2013ai are unique in their expansion velocities, which when compared to large samples of SNe II are more than 1,000 kms faster at 50 days past explosion. In addition, the long rise time of the light curve more closely resembles SNe IIb rather than SNe II. If SN 2013ai is coeval with a nearby compact cluster we infer a progenitor ZAMS mass of $\sim$17 M$_\odot$. After performing light curve modeling we find that SN 2013ai could be the result of the explosion of a star with little hydrogen mass, a large amount of synthesized $^{56}$Ni, 0.3-0.4 M$_\odot$, and an explosion energy of $2.5-3.0\times10^{51}$ ergs. The density structure and expansion velocities of SN 2013ai are similar to that of the prototypical SN IIb, SN 1993J. However, SN 2013ai shows no strong helium features in the optical, likely due to the presence of a dense core that prevents the majority of $\gamma$-rays from escaping to excite helium. Our analysis suggests that SN 2013ai could be a link between SNe II and stripped envelope SNe.