With our galactic evolutionary code that contains a detailed intermediate-mass and massive binary population model, we study the temporal evolution of the galactic population of double neutron star binaries, mixed systems with a neutron star and black hole component, and double black hole binaries. We computed the merger rates of these relativistic binaries and translate them into LIGO II detection rates. We demonstrate that accounting for the uncertainties in the relation "initial mass-final mass" predicted by massive close binary evolution and due to the possible effect of large stellar wind mass loss during the luminous blue variable phase of a star with initial mass higher than 30−40 M and during the red supergiant phase of a star with initial mass lower than 30−40 M when such a star is a binary component, the double black hole merger rate may be very low, contrary to predictions made by other groups. Hydrodynamic computations of the r-process chemical yields ejected during the relativistic binary merger process have recently become available. With our galactic code that includes binaries, it is then straightforward to calculate the temporal galactic evolution of the r-process elements ejected by these mergers. We conclude that except for the earliest evolutionary phase of the Galaxy (∼the first 100 Myr), double compact star mergers may be the major production sites of r-process elements, and it is probable that the mixed systems dominate this production over double neutron star binary mergers.

Aims. We investigate the contribution of different formation scenarios for type Ia supernovae in elliptical galaxies. The singledegenerate scenario (a white dwarf accreting from a late main-sequence or red giant companion) is tested against the doubledegenerate scenario (the spiral-in and merging of two white dwarfs through the emission of gravitational wave radiation). Methods. We use a population-number synthesis code that incorporates the latest physical results in binary evolution and allows us to differentiate between certain physical scenarios (e.g. description of common-envelope evolution) and evolutionary parameters (e.g. mass-transfer efficiency during Roche-lobe overflow). The obtained theoretical distributions of the delay times of type Ia supernovae are compared, both in morphological shape and in absolute number of events, to those which are observed. The critical dependency of these distributions on certain parameters is used to constrain the values of the latter. Results. We find that the single-degenerate scenario alone cannot explain the morphological shape of the observational delay-time distribution, while the double-degenerate scenario (or a combination of both) can. Most of these double-degenerate type Ia supernovae are created through a normal quasi-conservative Roche-lobe overflow followed by a common-envelope phase, not through two successive common-envelope phases. This may cast doubt on the use in other studies of analytical formalisms to determine delay times. In terms of absolute number, theoretical supernova Ia rates in old elliptical galaxies lie a factor of at least three below the observed ones. We propose a solution involving the effect of rotation on the evolution of intermediate mass binaries.

Context. Binary population synthesis (BPS) modelling is a very effective tool to study the evolution and properties of various types of close binary systems. The uncertainty in the parameters of the model and their effect on a population can be tested in a statistical way, which then leads to a deeper understanding of the underlying (sometimes poorly understood) physical processes involved. Several BPS codes exist that have been developed with different philosophies and aims. Although BPS has been very successful for studies of many populations of binary stars, in the particular case of the study of the progenitors of supernovae Type Ia, the predicted rates and ZAMS progenitors vary substantially between different BPS codes. Aims. To understand the predictive power of BPS codes, we study the similarities and differences in the predictions of four different BPS codes for low-and intermediate-mass binaries. We investigate the differences in the characteristics of the predicted populations, and whether they are caused by different assumptions made in the BPS codes or by numerical effects, e.g. a lack of accuracy in BPS codes. Methods. We compare a large number of evolutionary sequences for binary stars, starting with the same initial conditions following the evolution until the first (and when applicable, the second) white dwarf (WD) is formed. To simplify the complex problem of comparing BPS codes that are based on many (often different) assumptions, we equalise the assumptions as much as possible to examine the inherent differences of the four BPS codes. Results. We find that the simulated populations are similar between the codes. Regarding the population of binaries with one WD, there is very good agreement between the physical characteristics, the evolutionary channels that lead to the birth of these systems, and their birthrates. Regarding the double WD population, there is a good agreement on which evolutionary channels exist to create double WDs and a rough agreement on the characteristics of the double WD population. Regarding which progenitor systems lead to a single and double WD system and which systems do not, the four codes agree well. Most importantly, we find that for these two populations, the differences in the predictions from the four codes are not due to numerical differences, but because of different inherent assumptions. We identify critical assumptions for BPS studies that need to be studied in more detail.Key words. binaries: close -stars: evolution -white dwarfs IntroductionBinary population synthesis (BPS) codes enable the rapid calculation of the evolution of a large number of binary stars over the course of the binary lifetime. With such models, we can study the diverse properties of binary populations, e.g. the chemical enrichment of a region, or the frequency of an astrophysical event (for a review, see Han et al. 2001). We can learn about and study the formation and evolution of stellar systems that are important for a wide range of astronomical topics: novae, X-ray binaries,...

Context. The observed distribution of orbital periods of Algols with a B-type primary at birth agrees fairly well with the prediction from conservative theory. Conservative evolution fails, however, to produce the rather large fraction of Algols observed with a high mass-ratio, especially: q ∈ [0.4-0.6]. Aims. In order to keep Algols for a longer time with a higher mass-ratio without disturbing the distribution of orbital periods too much, interacting binaries have to lose a significant fraction of their total mass without losing much angular momentum before or during Algolism. We propose a mechanism that meets both requirements. Methods. In the case of direct impact the gainer spins up: sometimes up to critical velocity. Equatorial material on the gainer is therefore less bound. A similar statement applies to material located at the edge of an accretion disc. The incoming material moreover creates a hot spot in the area of impact. The sum of the rotational and radiative energy of hot spot material depends on the masstransfer-rate. The sum of both energies overcomes the binding energy at a well defined critical value of the mass-transfer-rate. As long as the transfer-rate is smaller than this critical value RLOF happens conservatively. But as soon as the critical rate is exceeded the gainer will acquire no more than the critical value and RLOF runs into a liberal era. Results. Low-mass binaries never achieve mass-transfer-rates larger than the critical value. Intermediate-mass binaries evolve mainly conservatively but mass will be blown away from the system during the short era of rapid mass-transfer soon after the onset of RLOF. We have calculated the evolution of binaries with a 9 M primary and a 5.4 M companion over a range of initial orbital periods, covering case-A RLOF. Mass-loss from the system is achieved during direct impact only. Conclusions. We find systems that show Algolism for more than ten million years. RLOF occurs almost always conservatively. Only during some 20 000 years the gainer is not capable of grasping all the material that comes from the donor. The mass-ratio q ∈ [0.4-0.6] which was hardly populated by conservative evolution now contains Algols for a significant fraction of their existence.

Context. Several authors have previously introduced liberal evolution of interacting binaries, with the purpose of meeting various observed binary characteristics better than with conservative evolution. Since Algols are eclipsing binaries, the distribution of their orbital periods is known precisely. The distribution of their mass ratios contains, however, more uncertainties. We try to reproduce these two distributions theoretically using a liberal scenario in which the gainer star can lose mass into interstellar space as a consequence of its rapid rotation and the energy of a hot spot. Aims. In a recent paper we calculated the liberal evolution of binaries with a B-type primary at birth where mass transfer starts during core hydrogen burning of the donor. In this paper we include the cases where mass transfer starts during shell hydrogen burning, and it is our aim to reproduce the observed distributions of the system parameters of Algol-type semidetached systems. Methods. Our calculations reveal the amount of time that an Algol binary lives with a well-defined value of mass ratio and orbital period. We used these data to simulate the distribution of mass ratios and orbital periods of Algols. Results. Binaries with a late B-type initial primary hardly lose any mass, whereas those with an early B primary evolve in a nonconservative way. Conservative binary evolution predicts only ∼12% of Algols with a mass ratio q above 0.4. This value is raised up to ∼17% using our scenario of liberal evolution, which is still far below the ∼45% that is observed. Conclusions. Observed orbital periods of Algol binaries longer than one day are faithfully reproduced by our liberal scenario. Mass ratios are reproduced better than with conservative evolution, but the resemblance is still poor.