  The final explosive fate of massive stars, and the nature of the compact
remnants they leave behind (black holes and neutron stars), are major open
questions in astrophysics. Many massive stars are stripped of their outer
hydrogen envelopes as they evolve. Such Wolf-Rayet (W-R) stars emit strong and
rapidly expanding (v_wind>1000 km/s) winds indicating a high escape velocity
from the stellar surface. A fraction of this population is also helium
depleted, with spectra dominated by highly-ionized emission lines of carbon and
oxygen (Types WC/WO). Evidence indicates that the most commonly-observed
supernova (SN) explosions that lack hydrogen and helium (Types Ib/Ic) cannot
result from massive WC/WO stars, leading some to suggest that most such stars
collapse directly into black holes without a visible supernova explosions.
Here, we present observations of supernova SN 2019hgp, discovered about a day
after explosion. The short rise time and rapid decline place it among an
emerging population of rapidly-evolving transients (RETs). Spectroscopy reveals
a rich set of emission lines indicating that the explosion occurred within a
nebula composed of carbon, oxygen, and neon. Narrow absorption features show
that this material is expanding at relatively high velocities (>1500 km/s)
requiring a compact progenitor. Our observations are consistent with an
explosion of a massive WC/WO star, and suggest that massive W-R stars may be
the progenitors of some rapidly evolving transients.
