Where Are All the Fallback Disks? Constraints on Propeller Systems

Fallback disks are expected to form around new-born neutron stars following asupernova explosion. In almost all cases, the disk will pass through apropeller stage. If the neutron star is spinning rapidly (initial period $\sim10$ ms) and has an ordinary magnetic moment ($\sim 10^{30}$ G cm$^3$), therotational power transferred to the disk by the magnetic field of the neutronstar will exceed the Eddington limit by many orders of magnitude, and the diskwill be rapidly disrupted. Fallback disks can thus survive only aroundslow-born neutron stars and around black holes, assuming the latter do nottorque their surrounding disks as strongly as do neutron stars. This mightexplain the apparent rarity of fallback disks around young compact objects.Comment: Submitted to Astrophysical Journal Letter