ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf

The detonation of a helium shell on a white dwarf (WD) has been proposed as a possible explosion triggering mechanism for SNe Ia. Here, we report ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent with being a helium-shell double-detonation. With a rise time of approximate to 18 days from explosion, the transient reached a peak absolute magnitude of M-R approximate to -18.2 mag, exhibiting a light curve akin to sub-luminous SN 1991bg-like SNe Ia, albeit with an unusually steep increase in brightness within a week from explosion. Spectra taken near peak light exhibit prominent Si absorption features together with an unusually red color (g - r approximate to 2 mag) arising from nearly complete line blanketing of flux blueward of 5000 angstrom. This behavior is unlike any previously observed thermonuclear transient. Nebular phase spectra taken at and after approximate to 30 days from peak light reveal evidence of a thermonuclear detonation event dominated by Fe-group nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are consistent with the detonation of a massive (approximate to 0.15 M.) helium shell on a sub-Chandrasekhar mass (approximate to 0.75 M-circle dot) WD after including mixing of approximate to 0.2 M-circle dot of material in the outer ejecta. These observations provide evidence of a likely rare class of thermonuclear supernovae arising from detonations of massive helium shells.