Probing the low-mass end of core-collapse supernovae

The fate of stars in the zero-age main-sequence (ZAMS) range 8-12 M_sun that undergo supernovae is not well understood. The number of detection is too sparse for any observational constraints. In this paper, we present a sample of nine Ca-rich type IIb Core-Collapse Supernovae (CCSNe) detected by the Zwicky Transient Facility with progenitor mass lying in this mass range. We perform a holistic analysis of the spectroscopic and photometric properties of the sample. Ca-rich implies oxygen-poor. Comparing the measured oxygen flux with existing models for CCSNe, we find that the progenitor mass for these explosions should be less than 12 M_sun. We find that the ejecta properties, including very low ejecta mass (< 1 M_sun) are consistent with that expected for He stars with initial mass < 3 M_sun. The mentioned properties, along with the presence of hydrogen in the photospheric phase, indicate that they are a distinct class of core-collapse supernova. The ejecta and spectral properties also suggest ultra-stripped supernovae origin that form neutron stars in compact binary systems. The observed properties will help constraint theoretical models in this low mass CCSNe regime which have been lacking observational constraints.