Binary Pulsar PSR J1012+53: Evolution, Ages, and Masses of Companions

We investigate the evolution of the close binary system consisting of the millisecond radio pulsar PSR J1012+53 and its low-mass companion, presumably a helium white dwarf. Within a standard evolutionary scenario, we explore the most plausible range of masses each component would have when the orbital parameters of the system achieve the observed values. We demonstrate that a neutron star of initial mass 1.4 M☉ can accrete, at most, 0.75 M☉ during the whole evolution of this binary system and may have increased its gravitational mass to 2.0 M☉. Also, our evolutionary calculations reveal that, for all relevant scenarios, the lowest mass the helium white dwarf in this system can reach at the latest stages of binary evolution is 0.15 M☉. We present estimates of the age of the neutron star in this system that consistently account for all phases of detached and semidetached binary evolution, including the rather prolonged phase of a white dwarf cooling. For the system PSR J1012+53, we demonstrate the occurrence of thermal hydrogen flashes on a low-mass helium white dwarf at the latest stages of its evolution. We discuss the impact of this system on the issue of the mass limit for a neutron star and on the problem of the structure and evolution of very low mass helium white dwarfs, companions of millisecond pulsars.