Bulk viscosity in neutron stars from hyperons

The contribution from hyperons to the bulk viscosity of neutron star matter is calculated. Compared to previous works we use for the weak interaction the one-pion exchange model rather than a current-current interaction, and include the neutral current nn-->-->nLambda process. Also the sensitivity to details of the equation of state is examined. Compared to previous works we find that the contribution from hyperons to the bulk viscosity is about two orders of magnitude smaller.