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Register a new userClose binary evolution I. The tidally induced shear mixing in rotating binaries
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We study how tides in a binary system induce some specific internal shear mixing, able to substantially modify the evolution of close binaries prior to mass transfer. We construct numerical models accounting for tidal interactions, meridional circulation, transport of angular momentum, shears and horizontal turbulence and consider a variety of orbital periods and initial rotation velocities. Depending on orbital periods and rotation velocities, tidal effects may spin down (spin down Case) or spin up (spin up Case) the axial rotation. In both cases, tides may induce a large internal differential rotation. The resulting tidally induced shear mixing (TISM) is so efficient that the internal distributions of angular velocity and chemical elements are greatly influenced. The evolutionary tracks are modified, and in both cases of spin down and spin up, large amounts of nitrogen can be transported to the stellar surfaces before any binary mass transfer. Meridional circulation, when properly treated as an advection, always tends to counteract the tidal interaction, tending to spin up the surface when it is braked down and vice versa. As a consequence, the times needed for the axial angular velocity to become equal to the orbital angular velocity may be larger than given by typical synchronization timescales. Also, due to meridional circulation some differential rotation remains in tidally locked binary systems.
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It has long been suspected that tidal forces in close binary stars could modify the orientation of the pulsation axis of the constituent stars. Such stars have been searched for, but until now never detected. Here we report the discovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 in TESS space photometry data. The system contains a Delta Scuti pulsator in a 1.6-d orbit, whose pulsation mode amplitude is strongly modulated at the orbital frequency, which can be explained if the pulsations have a much larger amplitude in one hemisphere of the star. We interpret this as an obliquely pulsating distorted dipole oscillation with a pulsation axis aligned with the tidal axis. This is the first time that oblique pulsation along a tidal axis has been recognized. It is unclear whether the pulsations are trapped in the hemisphere directed towards the companion or in the side facing away from it, but future spectral measurements can provide the solution. In the meantime, the single-sided pulsator HD 74423 stands out as the prototype of a new class of obliquely pulsating stars in which the interactions of stellar pulsations and tidal distortion can be studied.
A set of 27 evolutionary models of cool close binaries was computed under the assumption that their evolution is influenced by the magnetized winds. Initial periods of 1.5, 2.0 and 2.5 d were considered. For each period three values of 1.3, 1.1 and 0.9 solar mass were taken as the initial masses of the more massive components. Here the results of the computations of the first evolutionary phase are presented, which starts from the initial conditions and ends when the more massive component reaches its critical Roche lobe. In all considered cases this phase lasts for several Gyr. For binaries with the higher total mass and/or longer initial periods this time is equal to, or longer than the main sequence life time of the more massive component. For the remaining binaries it amounts to a substantial fraction of this life time. From the statistical analysis of models, the predicted period distribution of detached binaries with periods shorter than 2 d was obtained and compared to the observed distribution from the ASAS data. An excellent agreement was obtained under the assumption that the period distribution in this range is determined solely by the mass and angular momentum loss due to the magnetized winds. This result indicates, in particular, that virtually all cool detached binaries with periods of a few tenths of a day, believed to be the immediate progenitors of W UMa-type stars, were formed from detached systems with periods around 2-3 d and that magnetic braking is the dominant formation mechanism of cool contact binaries. It operates on the time scale of several Gyr rendering them rather old, with age of 6-10 Gyr. The results of the present analysis will be used as input data to investigate the subsequent evolution of the binaries, through the mass exchange phase and contact or semi-detached configuration till the ultimate merging of the components.
[Abridged] We test the evolutionary model of cool close binaries on the observed properties of near contact binaries (NCBs). Those with a more massive component filling the Roche lobe are SD1 binaries whereas in SD2 binaries the Roche lobe filling component is less massive. Our evolutionary model assumes that, following the Roche lobe overflow by the more massive component (donor), mass transfer occurs until mass ratio reversal. A binary in an initial phase of mass transfer, before mass equalization, is identified with SD1 binary. We show that the transferred mass forms an equatorial bulge around the less massive component (accretor). Its presence slows down the mass transfer rate to the value determined by the thermal time scale of the accretor, once the bulge sticks out above the Roche lobe. It means, that in a binary with a (typical) mass ratio of 0.5 the SD1 phase lasts at least 10 times longer than resulting from the standard evolutionary computations neglecting this effect. This is why we observe so many SD1 binaries. Our explanation is in contradiction to predictions identifying the SD1 phase with a broken contact phase of the Thermal Relaxation Oscillations model. The continued mass transfer, past mass equalization, results in mass ratio reversed. SD2 binaries are identified with this phase. Our model predicts that the time scales of SD1 and SD2 phases are comparable to one another. Analysis of the observations of 22 SD1 binaries, 27 SD2 binaries and 110 contact binaries (CBs) shows that relative number of both types of NCBs favors similar time scales of both phases of mass transfer. Total masses, orbital angular momenta and orbital periods of SD1 and SD2 binaries are indistinguishable from each other whereas they differ substantially from the corresponding parameters of CBs. We conclude that the results of the analysis fully support the model presented in this paper.
One of the main uncertainties in evolutionary calculations of massive stars is the efficiency of internal mixing. It changes the chemical profile inside the star and can therefore affect the structure and further evolution. We demonstrate that eclipsing binaries, in which the tides synchronize the rotation period of the stars and the orbital period, constitute a potentially strong test for the efficiency of rotational mixing. We present detailed stellar evolutionary models of massive binaries assuming the composition of the Small Magellanic Cloud. In these models we find enhancements in the surface nitrogen abundance of up to 0.6 dex.
We calculate the evolution of close binary systems (CBSs) formed by a neutron star (behaving as a radio pulsar) and a normal donor star, evolving either to helium white dwarf (HeWD) or 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 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 {it quasi - Roche Lobe Overflow} states, rather than hosting a carbon-oxygen WD. We found that CBSs with initial orbital periods $mathrm{P_{i}<1}$ day evolve into redbacks. Some of them produce low mass HeWDs, and a subgroup with shorter $mathrm{P_{i}}$ become black widows (BWs). Thus, BWs descent from redbacks, although not all redbacks evolve into BWs. There is mounting observational evidence favoring that BW pulsars are very massive ($mathrm{gtrsim 2; M_{odot}}$). As they should be redback descendants, redback pulsars should also be very massive, since most of the mass is transferred before this stage.
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