It's my understanding the general mechanism of core collapse involves the adiabatic constant of the material, gamma. This is the exponent in the relation P V^(gamma) = constant.
For a normal, non-relativistic gas in which the particles have no internal degrees of freedom, gamma is 5/3. As a gas becomes more relativistic, and as photon pressure becomes more important, gamma declines toward 4/3.
For gamma = 4/3, a self-gravitating gas will be marginally stable: the energy needed to compress a sphere of the gas will be equal to the gravitational potential energy liberated by the compression. So, any effect that pushes gamma below 4/3 makes it unstable against collapse.
In a conventional core collapse SN this is photodissociation of nuclei, where energy gets soaked up in breaking apart nuclei into alpha particles and then free nucleons. In a pair-instability SN, this is increasing conversion of photons to electron-positron pairs.
The temperature involved for this is well above that needed for production of electron-positron pairs, so one may ask why that doesn't happen. I think it's because the cores of such stars are dense enough the electrons are degenerate, and there simply isn't "room" for new electrons to be added at an energy that would make pair production possible.
BTW, I don't think largeness is needed for photodisintegration to occur; this should happen even in garden variety type-II SN.
For a normal, non-relativistic gas in which the particles have no internal degrees of freedom, gamma is 5/3. As a gas becomes more relativistic, and as photon pressure becomes more important, gamma declines toward 4/3.
For gamma = 4/3, a self-gravitating gas will be marginally stable: the energy needed to compress a sphere of the gas will be equal to the gravitational potential energy liberated by the compression. So, any effect that pushes gamma below 4/3 makes it unstable against collapse.
In a conventional core collapse SN this is photodissociation of nuclei, where energy gets soaked up in breaking apart nuclei into alpha particles and then free nucleons. In a pair-instability SN, this is increasing conversion of photons to electron-positron pairs.