Ca hnk: Calcium-rich Transient SN 2016hnk from the Helium Shell Detonation of a Sub-Chandrasekhar White Dwarf

TL;DR

SN 2016hnk is a calcium-rich supernova resulting from a helium-shell double-detonation on a white dwarf, exhibiting unique spectral and luminosity features that distinguish it from typical thermonuclear supernovae.

Contribution

This paper provides the first detailed modeling of SN 2016hnk as a helium-shell double-detonation, expanding understanding of Ca-rich supernovae origins.

Findings

SN 2016hnk produced about 0.03 solar masses of nickel-56.

It had a low peak luminosity of -15.4 mag and a fast rise time of 15 days.

Spectra show high-velocity calcium absorption and line blanketing.

Abstract

We present observations and modeling of SN 2016hnk, a Ca-rich supernova (SN) that is consistent with being the result of a He-shell double-detonation explosion of a C/O white dwarf. We find that SN 2016hnk is intrinsically red relative to typical thermonuclear SNe and has a relatively low peak luminosity ($M_B = -15.4$ mag), setting it apart from low-luminosity Type Ia supernovae (SNe Ia). SN 2016hnk has a fast-rising light curve that is consistent with other Ca-rich transients ($t_r = 15$ d). We determine that SN 2016hnk produced $0.03 \pm 0.01 M_{\odot}$ of ${}^{56}\textrm{Ni}$ and $0.9 \pm 0.3 M_{\odot}$ of ejecta. The photospheric spectra show strong, high-velocity Ca II absorption and significant line blanketing at $\lambda < 5000$ Angstroms, making it distinct from typical (SN 2005E-like) Ca-rich SNe. SN 2016hnk is remarkably similar to SN 2018byg, which was modeled as a He-shell double-detonation explosion. We demonstrate that the spectra and light curves of SN 2016hnk are well modeled by the detonation of a $0.02 \ M_{\odot}$ helium shell on the surface of a $0.85 \ M_{\odot}$ C/O white dwarf. This analysis highlights the second observed case of a He-shell double-detonation and suggests a specific thermonuclear explosion that is physically distinct from SNe that are defined simply by their low luminosities and strong [Ca II] emission.