M. A. De Vito

A code for stellar binary evolution and its application to the formation of helium white dwarfs

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.

EVOLUTIONARY TRAJECTORIES OF ULTRACOMPACT “BLACK WIDOW” PULSARS WITH VERY LOW MASS COMPANIONS

The existence of millisecond pulsars with planet-mass companions in close orbits is challenging from the stellar evolution point of view. We calculate in detail the evolution of binary systems self-consistently, including mass transfer, evaporation, and irradiation of the donor by X-ray feedback, demonstrating the existence of a new evolutionary path leading to short periods and compact donors as required by the observations of PSR J1719-1438. We also point out the alternative of an exotic nature of the companion planet-mass star.

The evolution of low-mass, close binary systems with a neutron star component: a detailed grid

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.

On the occurrence and detectability of Bose-Einstein condensation in helium white dwarfs

It has been recently proposed that helium white dwarfs may provide promising conditions for the occurrence of the Bose-Einstein condensation. The argument supporting this expectation is that in some conditions attained in the core of these objects, the typical De Broglie wavelength associated with helium nuclei is of the order of the mean distance between neighboring nuclei. In these conditions the system should depart from classical behavior showing quantum effects. As helium nuclei are bosons, they are expected to condense. In order to explore the possibility of detecting the Bose-Einstein condensation in the evolution of helium white dwarfs we have computed a set of models for a variety of stellar masses and values of the condensation temperature. We do not perform a detailed treatment of the condensation process but mimic it by suppressing the nuclei contribution to the equation of state by applying an adequate function. As the cooling of white dwarfs depends on average properties of the whole stellar interior, this procedure should be suitable for exploring the departure of the cooling process from that predicted by the standard treatment. We find that the Bose-Einstein condensation has noticeable, but not dramatic effects on the cooling process only for the most massive white dwarfs compatible with a helium dominated interior ( ≈ 0.50M⊙) and very low luminosities (say, Log(L/L⊙) < −4.0). These facts lead us to conclude that it seems extremely difficult to find observable signals of the Bose-Einstein condensation. Recently, it has been suggested that the population of helium white dwarfs detected in the globular cluster NGC 6397 is a good candidate for detecting signals of the Bose-Einstein condensation. We find that these stars have masses too low and are too bright to have an already condensed interior.

The formation of a helium white dwarf in a close binary system with diffusion

We study the evolution of a system composed of a 1.4-M⊙ neutron star and a normal, solar composition star of 2 M⊙ in orbit with a period of 1 d. Calculations were performed employing the binary hydro code presented by Benvenuto & De Vito that handle the mass transfer rate in a fully implicit way. We then included the main standard physical ingredients together with the diffusion processes and a proper outer boundary condition. We have assumed fully non-conservative mass transfer episodes. In order to study the interplay of mass loss episodes and diffusion we considered evolutionary sequences with and without diffusion in which all Roche lobe overflows (RLOFs) produce mass transfer. Another two sequences in which thermonuclearly driven RLOFs were not allowed to drive mass transfer have been computed with and without diffusion. As far as we are aware, this study represents the first binary evolution calculations in which diffusion is considered. The system produces a helium white dwarf of ∼0.21 M⊙ in an orbit with a period of ∼4.3 d for the four cases. We find that mass transfer episodes induced by hydrogen thermonuclear flashes drive a tiny amount of mass transfer. As diffusion produces stronger flashes, the amount of hydrogen-rich matter transferred is slightly higher than in the models without diffusion. We find that diffusion is the main agent in determining the evolutionary time-scale of low-mass white dwarfs even in the presence of mass transfer episodes.

An evolutionary model for the gamma-ray system PSR J1311−3430 and its companion

The most recent member of the millisecond pulsar with very low-mass companions and short orbital periods class, PSR J1311-3430 (Pletsch et al. 2012) is a remarkable object in various senses. Besides being the first discovered in gamma-rays, its measured features include the very low or absent hydrogen content. We show in this Letter that this important piece of information leads to a very restricted range of initial periods for a given donor mass. For that purpose, we calculate in detail the evolution of the binary system self-consistently, including mass transfer and evaporation, finding the features of the new evolutionary path leading to the observed configuration. It is also important to remark that the detailed evolutionary history of the system naturally leads to a high final pulsar mass, as it seems to be demanded by observations.

Evolution of redback radio pulsars in globular clusters

We study the evolution of close binary systems composed of a normal, intermediate mass star and a neutron star considering a chemical composition typical of that present in globular clusters (Z = 0.001). We look for similarities and differences with respect to solar composition donor stars, which we have extensively studied in the past. As a definite example, we perform an application on one of the redbacks located in a globular cluster. We performed a detailed grid of models in order to find systems that represent the so-called redback binary radio pulsar systems with donor star masses between 0.6 and 2.0 solar masses and orbital periods in the range 0.2 - 0.9 days. We find that the evolution of these binary systems is rather similar to those corresponding to solar composition objects, allowing us to account for the occurrence of redbacks in globular clusters, as the main physical ingredient is the irradiation feedback. Redback systems are in the quasi-RLOF state, that is, almost filling their corresponding Roche lobe. During the irradiation cycle the system alternates between semi-detached and detached states. While detached the system appears as a binary millisecond pulsar, called a redback. Circumstellar material, as seen in redbacks, is left behind after the previous semi-detached phase. The evolution of binary radio pulsar systems considering irradiation successfully accounts for, and provides a way for, the occurrence of redback pulsars in low-metallicity environments such as globular clusters. This is the case despite possible effects of the low metal content of the donor star that could drive systems away from redback configuration.