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

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 $0.03\pm 0.01M_{\odot }$ of ${}^{56}\text{Ni}$ and $0.9\pm 0.3M_{\odot }$ 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 $0.02M_{\odot }$ helium shell on the surface of a $0.85M_{\odot }$ 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.

arXiv comment33 pages, 18 Figures. Accepted for publication in ApJ

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