The final fates of massive stars shape the chemical enrichment of galaxies, the formation of compact remnants, and the birth environments of new generations of stars. These fates depend sensitively on initial mass, mass loss, binary interaction, and late-stage stellar structure. At the low-mass boundary of core collapse, stars with initial masses of ≈8-12M☉ occupy a critical transition region: below this range, stars are expected to end as white dwarfs, while above it, they undergo core collapse to form neutron stars or black holes. Yet the location and outcome of this boundary remain uncertain. For a typical initial mass function, around 50% of potential core-collapse supernova (CCSN) progenitors lie in this mass range, but their explosions are rarely identified. This deficit likely reflects the connection between these low-mass progenitors and low-luminosity SNe that are difficult to detect at larger distances. In this thesis, I address these observational challenges by leveraging the depth and cadence of the Zwicky Transient Facility (ZTF). I led the largest volume-limited supernova surveys to date: the ZTF Census of the Local Universe (CLU) survey and the Complete Astronomical Transient Survey within 150 Mpc (CATS150). I also contributed to the commissioning and science validation of the Next Generation Palomar Spectrograph, enabling efficient and systematic spectroscopic studies of faint and nearby transients. Together, these efforts provide a robust observational foundation for probing the low-mass end of core collapse. This thesis systematically searches for low-mass CCSNe across the full sequence of envelope stripping, beginning with hydrogen-rich Type II explosions and extending to partially and fully stripped systems. Using the CLU survey, I construct the largest systematic sample of Type IIP SNe to date and show that low-luminosity Type IIP events account for only a small fraction, 8+1-2%, of the core-collapse population. Lightcurve modeling places these events at the low-56Ni, low-energy, low-ejecta-mass end of the hydrogen-rich population. Nebular spectroscopy identifies at most two plausible electron-capture SN candidates, indicating that this channel occupies a very narrow super-asymptotic giant branch (sAGB) progenitor mass window, ΔM ≲ 0.02-0.06M☉. I then show that binary-stripped explosions also populate the low-mass regime, arising from low-mass helium stars with inflated radii during the final stages of stellar evolution. This population includes Ca-rich Type IIb and double-peaked Type Ibc SNe, and extends to ultra-stripped and interaction-powered extremes such as SNe 2023zaw and 2018erx. These events demonstrate that stripped-envelope explosions can reach extremely low radioactive yields and may be underrepresented in current samples unless their emission is boosted by shock cooling or circumstellar interaction. Taken together, these results show that neither hydrogen-rich nor stripped low-mass channels alone can account for all explosions expected from 8-12M☉ progenitors. This discrepancy suggests that the effective lower-mass boundary for successful core collapse may be higher than commonly assumed, that some events remain undetected due to dust obscuration, or that a fraction of progenitors collapse directly to form black holes.
Kaustav K. Das (Wed,) studied this question.