If, throughout a GMC, the ambipolar diffusion time
tAD were to instantaneously
drop to a value less than the local free-fall time, the bomb would
certainly ignite.
Since tAD depends directly
on the strength of the magnetic field, one might imagine a
scenario by which the B-field inside the GMC decreases during
a galaxy-galaxy interaction ... but you would be hard-pressed to
find anyone who would buy such a scenario!
But it recently has dawned on us that,
via the ionization rate zCR,
the ambipolar diffusion time also depends on the flux of cosmic
rays in the vicinity of the GMC, and