BINARY PULSAR PSR J1012- EVOLUTION, AGES, AND MASSES OF COMPANIONS(53)

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 scenar io, we explore the most plausible range of masses each component would have when the orbital parameters of the system achieve the observed v alues. We demonstrate that a neutron star of initial mass 1.4 M. can a ccrete, at most, 0.75 M. during the whole evolution of this binary sys tem and may have increased its gravitational mass to 2.0 M.. Also, our evolutionary calculations reveal that, for all relevant scenarios, th e lowest mass the helium white dwarf in this system can reach at the l atest stages of binary evolution is 0.15 M.. We present estimates of t he age of the neutron star in this system that consistently account fo r all phases of detached and semidetached binary evolution, including the rather prolonged phase of a white dwarf cooling. For the system PS R 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 o f very low mass helium white dwarfs, companions of millisecond pulsars .