  We discuss the effect of nonlinear bulk viscosity and the associated
reheating on the evolution of newly born, rapidly rotating neutron stars with
r-modes destabilized through the Chandrasekhar-Friedman-Schutz (CFS) mechanism.
Bulk viscosity in these stars is due to the adjustment of the relative
abundances of different particle species as the density of a fluid element is
perturbed. It becomes nonlinear when the chemical potential difference
$\delta\mu$, measuring the chemical imbalance in the fluid element, becomes
larger than the temperature $T$, which is generally much smaller than the Fermi
energy. From this scale on, the bulk viscosity increases much faster with
$\delta\mu$ than predicted by the usual, linear approximation. This provides a
potential saturation mechanism for stellar oscillation modes at a small to
moderate amplitude. In addition, bulk viscosity dissipates energy, which can
lead to neutrino emission, reheating of the star, or both. This is the first
study to explicitly consider these effects in the evolution of the r-mode
instability. For stars with little or no hyperon bulk viscosity, these effects
are not strong enough to prevent the r-modes from growing to amplitudes
$\alpha\sim 1$ or higher, so other saturation mechanisms will probably set in
earlier. The reheating effect makes spin-down occur at a higher temperature
than would otherwise be the case, in this way possibly avoiding complications
associated with a solid crust or a core superfluid. On the other hand, stars
with a substantial hyperon bulk viscosity and a moderate magnetic field
saturate their mode amplitude at a low value, which makes them gravitational
radiators for hundreds of years, while they lose angular momentum through
gravitational waves and magnetic braking.
