Superradiance in rotating stars and pulsar-timing constraints on dark photons
Tien-Tien Yu (CERN & YITP Stony Brook)

Rotational superradiance was theoretically shown to occur in black hole spacetimes; in the presence of massive bosonic degrees of freedom, superradiance triggers an instability that leads to peculiar gravitational-wave signatures and black hole distribution in the spin-mass plane, which in turn can impose stringent constraints on ultralight fields. In this talk, I will demonstrate that a similar effect occurs with rotating conducting spheres, and I will discuss rotational superradiance effects around conducting stars. This has interesting consequences for dark photons, as massive dark photons would cause stars to spin down due to superradiant instabilities. The time scale of the spindown depends on the mass of the dark photon, and on the rotation rate, compactness, and conductivity of the star. Existing measurements of the spindown rate of pulsars place direct constraints on models of dark sectors.