Populating the Galaxy with low-mass X-ray binaries

We perform binary population-synthesis calculations to investigate the incidence of low-mass X-ray binaries (LMXBs) and their birth rate in the Galaxy. We use a binary-evolution algorithm that models all the relevant processes including tidal circularization and synchronization. Parameters in the evolution algorithm that are uncertain and may affect X-ray binary formation are allowed to vary during the investigation. We agree with previous studies that under standard assumptions of binary evolution the formation rate and number of black hole (BH) LMXBs predicted by the model are more than an order of magnitude less than what is indicated by observations. We find that the common-envelope process cannot be manipulated to produce significant numbers of BH LMXBs. However, by simply reducing the mass-loss rate from helium stars adopted in the standard model, to a rate that agrees with the latest data, we produce a good match to the observations. Including LMXBs that evolve from intermediate-mass systems also leads to favourable results. We stress that constraints on the X-ray binary population provided by observations are used here merely as a guide as surveys suffer from incompleteness and much uncertainty is involved in the interpretation of results.     

The strong magnetic fields of the X-ray pulsars Hercules X-1 and 4U 1626–67 and their evolutionary scenarios

We consider the magnetic and spin evolution of the X-ray binary pulsars Her X-1 and 4U 1626–67, assuming that their magnetic fields are of crustal origin. We adopt the standard evolutionary model which implies that the neutron star passes through several phases in a binary system ('isolated pulsar' – propeller – wind accretion – Roche lobe overflow). In the framework of the model under consideration, the strong magnetic fields of relatively old pulsars like Her X-1 and 4U 1626–67 can naturally be understood if, at their birth, they had a sufficiently strong magnetic field, ∼3 × 10¹³ G, comparable to the maximal field observed in radio pulsars.     

Analytical solution for the evolution of a binary with stable mass transfer from a giant

We derive a simple analytical solution for the evolution of a close binary with nuclear time-scale driven mass transfer from a giant. This solution is based on the well-known fact that the luminosity and the radius of a giant scale to a good approximation as simple power laws of the mass M _c of the degenerate helium core. Comparison with results of numerical calculations by Webbink, Rappaport & Savonije show the analytical solution and the power-law approximation to be quite accurate. The analytical solution presented does also allow (in parametrized form) for non-conservative mass transfer. Furthermore, it is shown that the near constancy of the mass-transfer rate over most of the mass-transfer phase seen in the results by Webbink, Rappaport & Savonije is not a generic feature of this type of evolution but rather a consequence of a particular choice of parameters. The analytical solution also demonstrates that the level of mass transfer is largely set by the core mass of the giant at the onset of mass transfer. Finally, we show that the model is self-consistent and discuss its applicability to low-mass X-ray binaries.     

How abundant is the population of binary radio pulsars with black holes?

Using a 'scenario machine' we have carried out a population synthesis of radio pulsars with black hole binaries (BH+Psr) in the context of the most widespread assumptions concerning star mass loss during evolution, the mass ratio distribution of binary stars, the kick velocity and the envelope mass loss during collapse. Our purpose is to show that under any plausible parameters for the evolution scenario, the BH+Psr population should be abundant in the Galaxy. It is shown that in all models (including those evolved by Heger et al. and Woosley, Heger & Weaver), the expected number of black holes paired with radio pulsars is sufficient for such systems to be discovered within the next few years.     

Magnetic field evolution of accreting neutron stars – III

The evolutionary scenario of a neutron star magnetic field is examined assuming a spin-down induced expulsion of magnetic flux originally confined to the core, in a case in which the expelled flux undergoes ohmic decay. The nature of field evolution, for accreting neutron stars, is investigated incorporating the crustal microphysics and material movement resulting from accretion. This scenario may explain the observed field strengths of neutron stars but only if the crustal lattice contains a large amount of impurity, which is in direct contrast to the models that assume an original crustal field.     

The population of pulsars with interpulses and the implications for beam evolution

The observed fraction of pulsars with interpulses, their period distribution and the observed pulse width versus pulse period correlation are shown to be inconsistent with a model in which the angle α between the magnetic axis and the rotation axis is random. This conclusion appears to be unavoidable, even when non-circular beams are considered. Allowing the magnetic axis to align from a random distribution at birth with a time-scale of  ∼7 × 10⁷ yr  can, however, explain those observations well. The time-scale derived is consistent with that obtained via independent methods. The probability that a pulsar beam intersects the line of sight is a function of the angle α and therefore beam evolution has important consequences for evolutionary models and for estimations of the total number of neutron stars. The validity of the standard formula for the spin-down rate, which is independent of α, appears to be questionable.     

The evolution of the z distribution of normal neutron stars in the Galaxy

Under the two initial 1‐D one parameter velocity distribution forms (one is normal, the other is exponential), the z direction scale height evolution of normal neutron stars in the Galaxy is studied by numerical simulation. We do statistics for the cases at different time segments, also do statistics for the cumulative cases made of each time segment. The results show in the cumulative cases the evolution curves of the scale heights are smoother than in the each time segment, i.e., the cumulation improve the signal‐to‐noise ratio. Certainly the evolution cases are different at different Galactic disk locations, which also have very large difference from the average cases in the whole disk. In the initial stages of z evolution of normal neutron stars, after the beginning transient states, the cumulative scale heights increase linearly with time, and the cumulative scale height increasing rates have linear relationship with the initial velocity distribution parameters, which have larger fluctuation in the vicinity of the Sun than in the whole disk. We utilize the linear relationship of the cumulative scale height increasing rates vs. the initial velocity distribution parameters in the vicinity of the Sun to make comparison with the observation near the Sun. The results show if there is no magnetic decay, then the deserved initial velocity parameters are obvious lower than the present well known results from some authors; whereas if introducing magnetic decay, for the 1‐D normal case we can make consistence among concerning results using magnetic decay time values which are supported by some authors, while for the 1‐D exponential case the results show the lackness of young pulsar samples in the larger z in the vicinity of the Sun (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Absolute parameters of young stars – I. U Ophiuci

We have carried out an investigation of the early-type multiple star U Oph. We have used new high-resolution spectroscopy with the High Efficiency and Resolution Canterbury University Large Echelle Spectrograph (HERCULES) and 1-m McLellan Telescope of the University of Canterbury at Mt John University Observatory and literature-sourced optical and ultraviolet photometry. We applied the local reduction package [HERCULES Reduction Software Package (HRSP)] and other software to the spectroscopic data to find radial velocities. Information limit optimization techniques (ILOT) utilizing physically realistic fitting functions were applied to these data to yield new sets of absolute parameters:   M ₁= 5.13, M ₂= 4.56 (±2 per cent); R ₁= 3.41, R ₂= 3.08 (±1 per cent)  ; for the early-type eclipsing binary that dominates the system. We have combined times-of-minima photometry with other data for the triple system that makes up ADS 10428A, utilizing the wide orbit of Wolf et al. as well as HIPPARCOS astrometry of U Oph. ILOT techniques applied to the astrometric orbit yield a mass of the third star as  0.83 M_⊙  . We estimate an age of the system of around 30–40 Myr, from the isochrones of Bertelli, results given by Vaz, Andersen & Claret, as well as our own tests with an updated version of Paczyński's stellar modelling code. This age and other details are consistent with a possible origin in Gould's Belt. Such information for this, and comparable young multiple star systems, may help to clarify general properties of star formation and the subtle interactions of stars and their environment.     

Low- and intermediate-mass close binary evolution and the initial–final mass relation – III. Conservative case with convective overshooting and non-conservative case without overshooting

We have completed our series of calculation of case B evolution of Population I close binaries by including a conservative case with convective overshooting and a non-conservative case without convective overshooting in this paper. To apply the results easily to binary population synthesis, we fit two formulae for the remnant mass as a function of the initial mass of the primary, the radius of the primary at the onset of Roche lobe overflow (RLOF) and the initial mass ratio, with errors less than 1.9 per cent and 3.6 per cent for the conservative and non-conservative cases, respectively. We have also made comparisons between the results in this paper and those of previous studies in this series to examine the effects of our assumptions.
We find that the remnant mass depends on when RLOF begins in the Hertzsprung gap and the dependence increases with the mass of the primary. Helium–carbon–oxygen white dwarfs are formed in this series of calculation. Both the non-conservative assumption and convective overshooting make RLOF more stable: some binaries with initial mass ratios   q _i= 4.0  are dynamically stable during RLOF in all cases except for the conservative case without overshooting. From these models, we have discussed potential electron-degenerate oxygen–neon cores. Finally, we show the conditions under which one evolution scenario may be more likely than the others according to the results of previous authors.     

Pulsar spin–velocity alignment from single and binary neutron star progenitors

The role of binary progenitors of neutron stars (NSs) in the apparent distribution of space velocities and spin–velocity alignment observed in young pulsars is studied. We performed a Monte Carlo synthesis of pulsar populations originated from single and binary stars with different assumptions about the NS natal kick (kick–spin alignment, kick amplitude and kick reduction in electron-capture supernovae in binary progenitors with initial main-sequence masses from the range  8–11 M_⊙  which experienced mass exchange due to Roche lobe overflow). The calculated spin–velocity alignment in pulsars is compared with data inferred from radio polarization measurements. The observed space velocity of pulsars is found to be mostly affected by the natal kick velocity form and its amplitude; the fraction of binaries is not important here for reasonably large kicks. The natal kick–spin alignment is found to strongly affect the spin–velocity correlation of pulsars. Comparison with the observed pulsar spin–velocity angles favours a sizeable fraction of binary progenitors and kick–spin angles  ∼5°–20°  .