Final answer:
The increase in rotational kinetic energy comes from the conservation of energy principle.
Step-by-step explanation:
The increase in rotational kinetic energy comes from the conservation of energy principle, which states that energy cannot be created or destroyed, but can only change form. In this case, the increase in rotational kinetic energy is due to a decrease in gravitational potential energy as the core collapses. As the core's mass is 1.3 times that of our Sun, it has a greater gravitational potential energy. As the core collapses, this potential energy is converted into rotational kinetic energy.