The peculiar Ca-rich SN 2019ehk: Evidence for a Type IIb core-collapse supernova from a low mass stripped progenitor

Abstract

The nature of the peculiar `Ca-rich' SN 2019ehk in the nearby galaxy M100 remains unclear. Its origin has been debated as either a stripped core-collapse supernova or a thermonuclear helium detonation event. Here, we present very late-time photometry of the transient obtained with the Keck I telescope at $\approx 280$ days from peak light. Using the photometry to perform accurate flux calibration of a contemporaneous nebular phase spectrum, we measure an [O I] luminosity of $(0.19-1.08)\times 10^{38}$ erg s${}^{-1}$ and [Ca II] luminosity of $(2.7-15.6)\times 10^{38}$ erg s${}^{-1}$ over the range of the uncertain extinction along the line of sight. We use these measurements to derive lower limits on the synthesized oxygen mass of $\approx 0.004-0.069$ M${}_{\odot }$. The oxygen mass is a sensitive tracer of the progenitor mass for core-collapse supernovae, and our estimate is consistent with explosions of very low mass CO cores of $1.45-1.5$ M${}_{\odot }$, corresponding to He core masses of $\approx 1.8-2.0$ M${}_{\odot }$. We present high quality peak light optical spectra of the transient and highlight features of hydrogen in both the early (`flash') and photospheric phase spectra, that suggest the presence of $\gtrsim 0.02$ M${}_{\odot }$ of hydrogen in the progenitor at the time of explosion. The presence of H, together with the large [Ca II]/[O I] ratio ($\approx 10-15$) in the nebular phase is consistent with SN 2019ehk being a Type IIb core-collapse supernova from a stripped low mass ($\approx 9-9.5$ M${}_{\odot }$) progenitor, similar to the Ca-rich SN IIb iPTF 15eqv. These results provide evidence for a likely class of `Ca-rich' core-collapse supernovae from stripped low mass progenitors in star forming environments, distinct from the thermonuclear Ca-rich gap transients in old environments.

12 pages, 5 figures, Accepted for publication in ApJL