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Description
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 SN2016hnk is intrinsically red relative to typical thermonuclear SNe and has a relatively low peak luminosity (M_B_=-15.4mag), setting it apart from low-luminosity SNe Ia. SN 2016hnk has a fast-rising light curve that is consistent with other Ca-rich transients (t_r_=15days). We determine that SN 2016hnk produced 0.03+/-0.01M_{sun}_ of ^56^Ni and 0.9+/-0.3M_{sun}_ of ejecta. The photospheric spectra show strong, high-velocity CaII absorption and significant line blanketing at {lambda}<5000{AA}, 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_{sun}_ helium shell on the surface of a 0.85M_{sun}_ 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 [CaII] emission.
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