  Using the new state-of-the-art core-collapse supernova (CCSN) code
F{\sc{ornax}}, we have simulated the three-dimensional dynamical evolution of
the cores of 9-, 10-, 11-, 12-, and 13-M$_{\odot}$ stars from the onset of
collapse. Stars from 8-M$_{\odot}$ to 13-M$_{\odot}$ constitute roughly 50% of
all massive stars, so the explosive potential for this mass range is important
to the overall theory of CCSNe. We find that the 9-, 10-, 11-, and
12-M$_{\odot}$ models explode in 3D easily, but that the 13-M$_{\odot}$ model
does not. From these findings, and the fact that slightly more massive
progenitors seem to explode \citep{vartanyan2019}, we suggest that there is a
gap in explodability near 12-M$_{\odot}$ to 14-M$_{\odot}$ for non-rotating
progenitor stars. Factors conducive to explosion are turbulence behind the
stalled shock, energy transfer due to neutrino-matter absorption and
neutrino-matter scattering, many-body corrections to the neutrino-nucleon
scattering rate, and the presence of a sharp silicon-oxygen interface in the
progenitor. Our 3D exploding models frequently have a dipolar structure, with
the two asymmetrical exploding lobes separated by a pinched waist where matter
temporarily continues to accrete. This process maintains the driving neutrino
luminosty, while partially shunting matter out of the way of the expanding
lobes, thereby modestly facilitating explosion. The morphology of all 3D
explosions is characterized by multiple bubble structures with a range of
low-order harmonic modes. Though much remains to be done in CCSN theory, these
and other results in the literature suggest that, at least for these lower-mass
progenitors, supernova theory is converging on a credible solution.
