Neutron stars with submillisecond periods: A population of high-mass objects?

Rapidly spinning neutron stars, recycled in low-mass binaries, may have acc reted a substantial amount of mass. The available relativistic measurements of neutron star masses, all clustering around 1.4 M., however, refer mostl y to slowly rotating neutron stars that accreted a tiny amount of mass duri ng evolution in a massive binary system. We develop a semianalytical model for studying the evolution of the spin pe riod P of a magnetic neutron star as a function of the baryonic mass load M -ac; evolution is followed down to submillisecond periods, and the magnetic field is allowed to decay significantly before the end of recycling. We us e different equations of state and include rotational deformation effects a nd the presence of a strong gravitational field and of a magnetosphere. For the nonmagnetic case, comparison with numerical relativistic codes shows t he accuracy of our description. The minimum accreted mass requested to spin up a magnetized 1.35 M. neutron star at a few milliseconds is similar to 0.05 M., while this value doubles for an unmagnetized neutron star. Below 1 ms, the request is for at least similar to 0.25 M.. Only highly nonconservative scenarios for the binary ev olution could prevent the transfer of such a mass to the compact object. Un less a physical mechanism limits the rotational period, there may exist a y et undetected population of massive submillisecond neutron stars. The disco very of a submillisecond neutron star would imply a lower limit for its mas s of about 1.7 M..