Group of Evolution in Binary Systems

We present a numerical code intended for calculating stellar evolution in close binary systems. In doing so, we consider that mass transfer episodes occur when the stellar size overflows the corresponding Roche lobe. In such a situation we equate the radius of the star to the equivalent radius of the Roche lobe. This equation is handled implicitly together with those corresponding to the whole structure of the star. We describe in detail the necessary modifications to the standard Henyey technique for treating the mass-loss rate implicitly together with thin outer-layer integrations. We have applied this code to the calculation of the formation of low-mass, helium white dwarfs in low-mass close binary systems. We find that the global numerical convergence properties are fairly good. In particular, the onset and end of mass transfer episodes are computed automatically.

We calculate the evolution of close binary systems (CBSs) formed by a neutron star (behaving as a radio pulsar) and a normal donor star, which evolve either to a helium white dwarf (HeWD) or to ultra-short orbital period systems. We consider X-ray irradiation feedback and evaporation due to radio pulsar irradiation. We show that irradiation feedback leads to cyclic mass transfer episodes, allowing CBSs to be observed in between episodes as binary radio pulsars under conditions in which standard, non-irradiated models predict the occurrence of a low-mass X-ray binary. This behavior accounts for the existence of a family of eclipsing binary systems known as redbacks. We predict that redback companions should almost fill their Roche lobe, as observed in PSR J1723-2837. This state is also possible for systems evolving with larger orbital periods. Therefore, binary radio pulsars with companion star masses usually interpreted as larger than expected to produce HeWDs may also result in such quasi-Roche lobe overflow states, rather than hosting a carbon–oxygen WD. We found that CBSs with initial orbital periods of Pi

In close binary systems composed of a normal donor star and an accreting neutron star, the amount of material received by the accreting component is, so far, a real intrigue. In the literature, there are available models that link the accretion disc surrounding the neutron star with the amount of material it receives, but there is no model linking the amount of matter lost by the donor star to that falling on to the neutron star. In this paper, we explore the evolutionary response of these close binary systems when we vary the amount of material accreted by the neutron star. We consider a parameter β which represents the fraction of material lost by the normal star that can be accreted by the neutron star. β is considered as constant throughout the evolution. We have computed the evolution of a set of models considering initial donor star masses Mi/M⊙ between 0.5 and 3.50, initial orbital periods Pi/d between 0.175 and 12, initial masses of neutron stars (MNS)i/M⊙ of 0.80, 1.00, 1.20 and 1.40 and several values of β. We assumed solar abundances. These systems evolve to ultracompact or to open binary systems, many of which form low-mass helium white dwarfs. We present a grid of calculations and analyse how these results are affected upon changes in the value of β. We find a weak dependence of the final donor star mass on β. In most cases, this is also true for the final orbital period. The most sensitive quantity is the final mass of the accreting neutron star. As we do not know the initial mass and rotation rate of the neutron star of any system, we find that performing evolutionary studies is not helpful for determining β.