Type Ia supernovae from non-accreting progenitors
  Type Ia supernovae (SNe Ia) are manifestations of stars deficient of hydrogen
and helium disrupting in a thermonuclear runaway. While explosions of
carbon-oxygen white dwarfs are thought to account for the majority of events,
part of the observed diversity may be due to varied progenitor channels. We
demonstrate that helium stars with masses between $\sim$1.8 and 2.5 M$_{\odot}$
may evolve into highly degenerate, near-Chandrasekhar mass cores with
helium-free envelopes that subsequently ignite carbon and oxygen explosively at
densities $\sim(1.8-5.9)\times 10^{9}$g cm$^{-3}$. This happens either due to
core growth from shell burning (when the core has a hybrid CO/NeO composition),
or following ignition of residual carbon triggered by exothermic electron
captures on $^{24}$Mg (for a NeOMg-dominated composition). We argue that the
resulting thermonuclear runaways is likely to prevent core collapse, leading to
the complete disruption of the star. The available nuclear energy at the onset
of explosive oxygen burning suffices to create ejecta with a kinetic energy of
$\sim$10$^{51}$ erg, as in typical SNe Ia. Conversely, if these runaways result
in partial disruptions, the corresponding transients would resemble SN Iax
events similar to SN 2002cx. If helium stars in this mass range indeed explode
as SNe Ia, then the frequency of events would be comparable to the observed SN
Ib/c rates, thereby sufficing to account for the majority of SNe Ia in
star-forming galaxies.
