Hyperon softening of the EOS of dense matter and the spin evolution of isolated neutron stars

The effect of the hyperon softening of the equation of state (EOS) of dense matter on the spin evolution of isolated neutron stars is studied for a broad set of hyperonic EOSs. We use a multidomain 2-D code based on a spectral method, and show how important the precision of solving the equations of stationary motion is for the stability analysis. For some EOSs, the hyperon softening leads to spin-up by the angular momentum loss, in the form of the back bending phenomenon, for a rather broad range of stellar baryon mass. We show that large segments of the evolutionary tracks exhibiting the back bending behaviour in the moment-of-inertia - rotation-frequency plane, are actually unstable and therefore not astrophysically relevant. We show also that during the spin-up - angular-momentum-loss epoch, an isolated neutron star (e.g., a radio pulsar) can lose a sizable part of its initial angular momentum without significantly changing its rotation period. We propose also simple arguments and criteria allowing to connect the presence of a back bending epoch with the mass-radius relations and the stiffness and/or softness of the nucleon and hyperon EOSs of the neutron star core.