iPTF 16hgs: A double-peaked Ca-rich gap transient in a metal poor, star forming dwarf galaxy

Abstract: Calcium rich gap transients represent an intriguing new class of faint and fast evolving supernovae that exhibit strong [Ca II] emission in their nebular phase spectra. In this paper, we present the discovery and follow-up observations of iPTF 16hgs -- an intermediate luminosity and fast evolving transient that exhibited a double peaked light curve. Exhibiting a typical Type Ib spectrum in the photospheric phase and an early transition to a [Ca II] dominated nebular phase, we show that iPTF 16hgs shows properties consistent with the class of Ca-rich gap transients, with two interesting exceptions. First, while the second peak of the light curve is similar to other Ca-rich gap transients (suggesting $M_{ej}$ of 0.4 M$<em>\odot$ and peak luminosity of $3 \times 10<sup>{41}$ ergs s$<sup>{-1}$), we show that the first blue and fast declining (over $2$ days) peak is unique to this source. Second, with Integral Field Unit observations of the host galaxy, we find that iPTF 16hgs occurred in the outskirts (projected offset of $6$ kpc $= 1.9 R</em>{eff}$) of a low metallicity (0.4 Z$<em>\odot$), star forming, dwarf spiral galaxy. Using deep late-time VLA and uGMRT observations, we place stringent limits on the local environment of the source, ruling out a large parameter space of circumstellar densities and mass loss environments of the progenitor. If iPTF 16hgs shares explosion physics with the class of Ca-rich gap transients, the presence of the first peak can be explained by enhanced mixing of 0.01 M$</em>\odot$ of $<sup>{56}$Ni into the outer layers the ejecta, reminiscent of some models of He-shell detonations on WDs. On the other hand, if iPTF 16hgs is physically unrelated to the class, the first peak is consistent with shock cooling emission (of an envelope with a mass of 0.08 M$<em>\odot$ and radius of 13 R$</em>\odot$) associated with a core-collapse explosion of a highly stripped massive star in a close binary system.