Evolution of low mass close binary systems with a neutron star: its dependence with the initial neutron star mass

We construct a set of binary evolutionary sequences for systems composed by a normal, solar composition, donor star together with a neutron star. We consider a variety of masses for each star as well as for the initial orbital period corresponding to systems that evolve to ultra-compact or millisecond pulsar-helium white dwarf pairs. Specifically, we select a set of donor star masses of 0.50, 0.65, 0.80, 1.00, 1.25, 1.50, 1.75, 2.00, 2.25, 2.50, 3.00 and 3.50 M⊙, whereas for the accreting neutron star we consider initial mass values of 0.8, 1.0, 1.2 and 1.4 M⊙. Because the minimum mass for a proto-neutron star is approximately 0.9 M⊙, the value of 0.8 M⊙ was selected in order to cover the whole range of possible initial neutron star masses. The considered initial orbital period interval ranges from 0.5 to 12 d. It is found that the evolution of systems, with fixed initial values for the orbital period and the mass of the normal donor star, heavily depends upon the mass of the neutron star. In some cases, varying the initial value of the neutron star mass, we obtain evolved configurations ranging from ultra-compact to widely separated objects. We also analyse the dependence of the final orbital period with the mass of the white dwarf. In agreement with previous expectations, our calculations show that the final orbital period–white dwarf mass relation is fairly insensitive to the initial neutron star mass value. A new period–mass relation based on our own calculations is proposed, which is in good agreement with period–mass relations available in the literature. As a consequence of considering a set of values for the initial neutron star mass, these models allow finding different plausible initial configurations (donor and neutron star masses and orbital period interval) for some of the best observed binary systems of the kind we are interested in here. We apply our calculations to analyse the case of PSR J0437−4715, showing that there is more than one possible set of initial parameters (masses, period and the fraction β of matter accreted by the neutron star) for this particular system.Fil: de Vito, María Alejandra. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; ArgentinaFil: Benvenuto, Omar Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentin

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 M i/M ⊙ between 0.5 and 3.50, initial orbital periods P i/d between 0.175 and 12, initial masses of neutron stars (M NS) 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.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Evolution of low-mass close binary systems with a neutron star: Its dependence with the initial neutron star mass

We construct a set of binary evolutionary sequences for systems composed by a normal, solar composition, donor star together with a neutron star. We consider a variety of masses for each star as well as for the initial orbital period corresponding to systems that evolve to ultra-compact or millisecond pulsar-helium white dwarf pairs. Specifically, we select a set of donor star masses of 0.50, 0.65, 0.80, 1.00, 1.25, 1.50, 1.75, 2.00, 2.25, 2.50, 3.00 and 3.50 M⊙, whereas for the accreting neutron star we consider initial mass values of 0.8, 1.0, 1.2 and 1.4 M⊙. Because the minimum mass for a proto-neutron star is approximately 0.9 M⊙, the value of 0.8 M⊙ was selected in order to cover the whole range of possible initial neutron star masses. The considered initial orbital period interval ranges from 0.5 to 12 d. It is found that the evolution of systems, with fixed initial values for the orbital period and the mass of the normal donor star, heavily depends upon the mass of the neutron star. In some cases, varying the initial value of the neutron star mass, we obtain evolved configurations ranging from ultra-compact to widely separated objects. We also analyse the dependence of the final orbital period with the mass of the white dwarf. In agreement with previous expectations, our calculations show that the final orbital period-white dwarf mass relation is fairly insensitive to the initial neutron star mass value. A new period-mass relation based on our own calculations is proposed, which is in good agreement with period-mass relations available in the literature. As a consequence of considering a set of values for the initial neutron star mass, these models allow finding different plausible initial configurations (donor and neutron star masses and orbital period interval) for some of the best observed binary systems of the kind we are interested in here. We apply our calculations to analyse the case of PSR J0437-4715, showing that there is more than one possible set of initial parameters (masses, period and the fraction β of matter accreted by the neutron star) for this particular system.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

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.Facultad de Ciencias Astronómicas y Geofísica

The evolutionary status of the white dwarf companion of the binary pulsar PSR J1713+0747

Recently Splaver et al. have measured the masses of the white dwarf and the neutron star (NS) components of the PSR J1713+0747 binary system pair by means of the general relativistic effect known as Shapiro delay with very high accuracy. Employing these data we attempt to find the original configuration that evolved to the observed system. For this purpose we perform a set of binary evolution calculations trying to simultaneously account for the masses of both stars and the orbital period. In doing so, we considered normal (donor) stars with an initial mass of 1.5M ⊙, while for the neutron star companion we assumed a mass of 1.4M ⊙. We assumed two metallicity values for the donor star (Z = 0.010 and 0.020) and that the initial orbital period was nearly 3 d. In order to get a good agreement between the masses of the models and observations we had to assume that the NS is only able to retain ≲0.10 of the matter transferred by the donor star. Calculations were performed employing the binary hydro code developed by Benvenuto & De Vito, that handles the mass transfer rate in a fully implicit way together with state-of-the-art physical ingredients and diffusion processes. Now our code also includes a detailed non-grey treatment for the atmospheres of white dwarfs (WDs). We compare the structure of the resulting WDs with the characteristic age of PSR J1713+0747 finding a nice agreement with observations by Lundgren et al. especially for the case of a donor star with Z = 0.010. This result indicates that, at least for the purposes of this paper, the evolution of this kind of binary system is fairly well understood. The models predict that, due to diffusion, the atmosphere of the white dwarf is an almost hydrogen-pure one. We find that such structures are unable to account for the colours measured by Lundgren et al. within their error bars. Thus, in spite of the very good agreement of the model with the main characteristics of the system, we find that some discrepancies in the WD emergent radiation remain to be explained.Facultad de Ciencias Astronómicas y Geofísica

The quasi-Roche lobe overflow state in the evolution of close binary systems containing a radio pulsar

We study the evolution of close binary systems formed by a normal (solar composition), intermediate-mass-donor star together with a neutron star. We consider models including irradiation feedback and evaporation. These nonstandard ingredients deeply modify the mass-transfer stages of these binaries. While models that neglect irradiation feedback undergo continuous, long-standing mass-transfer episodes, models including these effects suffer a number of cycles of mass transfer and detachment. During mass transfer, the systems should reveal themselves as low-mass X-ray binaries (LMXBs), whereas when they are detached they behave as binary radio pulsars. We show that at these stages irradiated models are in a Roche lobe overflow (RLOF) state or in a quasi-RLOF state. Quasi-RLOF stars have radii slightly smaller than their Roche lobes. Remarkably, these conditions are attained for an orbital period as well as donor mass values in the range corresponding to a family of binary radio pulsars known as "redbacks." Thus, redback companions should be quasi-RLOF stars. We show that the characteristics of the redback system PSR J1723-2837 are accounted for by these models. In each mass-transfer cycle these systems should switch from LMXB to binary radio pulsar states with a timescale of approximately one million years. However, there is recent and fast growing evidence of systems switching on far shorter, human timescales. This should be related to instabilities in the accretion disk surrounding the neutron star and/or radio ejection, still to be included in the model having the quasi-RLOF state as a general condition.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

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.Facultad de Ciencias Astronómicas y Geofísica

Identifying the formation mechanism of redback pulsars

We analyse the evolution of close binary systems containing a neutron star that lead to the formation of redback pulsars. Recently, there has been some debate on the origin of such systems and the formation mechanism of redbacks may still be considered as an open problem. We show that the operation of a strong evaporation mechanism, starting from the moment when the donor star becomes fully convective (or alternatively since the formation of the neutron star by accretion-induced collapse), produces systems with donor masses and orbital periods in the range corresponding to redbacks with donors appreciably smaller than their Roche lobes, i.e. they have low filling factors (lower than 0.75). Models of redback pulsars can be constructed assuming the occurrence of irradiation feedback. They have been shown to undergo cyclic mass transfer during the epoch at which they attain donor masses and orbital periods corresponding to redbacks, and stay in quasi-Roche lobe overflow conditions with high filling factors. We show that, if irradiation feedback occurs and radio ejection inhibits further accretion on to the neutron star after the first mass transfer cycle, the redback systems feature high filling factors. We suggest that the filling factor should be considered as a useful tool for discriminating among those redback formation mechanisms. We compare theoretical results with available observations and conclude that observations tend to favour models with high filling factors.Fil: de Vito, María Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Benvenuto, Omar Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Horvath, J. E.. Universidade de Sao Paulo; Brasi

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 M i/M ⊙ between 0.5 and 3.50, initial orbital periods P i/d between 0.175 and 12, initial masses of neutron stars (M NS) 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.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

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.Facultad de Ciencias Astronómicas y Geofísica