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

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 ≈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 (0.23−0.78) × 10³⁸ erg s⁻¹ and [Ca II] luminosity of (3.4−9.1) × 10³⁸ 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 ≈0.005−0.05 M_⊙. 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_⊙, corresponding to He core masses of ≈1.8−2.0 M_⊙. 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 ≳0.02 M_⊙ of hydrogen in the progenitor at the time of explosion. The presence of H, together with the large [Ca II]/[O I] ratio (≈10−15) in the nebular phase is consistent with SN 2019ehk being a Type IIb core-collapse supernova from a stripped low mass (≈9−9.5 M_⊙) 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.