W. Van Rensbergen

The WR and O-type star population predicted by massive star evolutionary synthesis

Abstract Evolutionary calculations of massive single stars and of massive close binaries that we use in the population number synthesis (PNS) code are presented. Special attention is given to the assumptions/uncertainties influencing these stellar evolutionary computations (and thus the PNS results). A description is given of the PNS model together with the initial statistical distributions of stellar parameters needed to perform number synthesis. We focus on the population of O-type stars and WR stars in regions where star formation was continuous in time and in starburst regions. We discuss the observations that have to be explained by the model. These observations are then compared to the PNS predictions. We conclude that: 1. probably the majority of the massive stars are formed as binary components with orbital period between 1 day and 10 yr; most of them interact. 2. at most 8% of the O-type stars are runaways due to a previous supernova explosion in a binary; recent studies of pulsar space velocities and linking the latter to the effect of asymmetrical supernova explosions, reveal that only a small percentage of these runaways will have a neutron star companion. 3. with present day stellar evolutionary computations, it is difficult to explain the observed WR/O number ratio in the solar neighbourhood and in the inner Milky Way by assuming a constant star formation rate, with or without binaries. The observed ratio for the Magellanic Clouds is better reproduced. 4. the majority of the single WR stars may have had a binary past. 5. probably merely 2–3% (and certainly less than 8%) of all WR stars have a neutron star companion. 6. a comparison between theoretical prediction and observations of young starbursts is meaningful only if binaries and the effect of binary evolution are correctly included. The most stringent feature is the rejuvenation caused by mass transfer.

Spin up and hot spots can drive mass out of a binary

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. ×

Mass loss out of close binaries The formation of Algol-type systems, completed with case B RLOF

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.

The effect of close binary evolution on photoionization models for evolving starbursts

Abstract Using a population number synthesis code and detailed massive star evolutionary calculations, Van Bever & Vanbeveren (1998) investigated the effect of close binary evolution on the WR and O-type star population of starburst regions. It was concluded that the age determination based on O-type features of a starburst which is older than 4 million years could be erroneous if observations are interpreted from the point of view of single star evolution only. Most important is the formation of rejuvenated OB-type Blue Stragglers when significant matter lost by the binary primary during the Roche lobe overflow (RLOF), is accreted by the companion. The present paper illustrates the latter statement. We consider the most frequently used age determinators, i.e., the strength of the Hα and Hβ emission lines of starburst regions. We distinguish the effects of the OB-type Blue Stagglers, of WR single stars and of hydrogen deficient core helium burning binary components in the post-RLOF phase of evolution (including the WR binary components) and the effects of X-radiation produced in standard high mass X-ray binaries.

The effects of binaries on the evolution of UV spectral features in massive starbursts

In this paper we investigate the effects of binaries having an initial period between 1 day and 10 years on the theoretical simulation of the evolution of UV spectral features in massive starbursts. The binary evolutionary processes that dominate the evolution of the considered spectral features are the Roche lobe overflow in Case Br systems, the mass transfer rate and the merger rate. They cause UV spectral rejuvenation in starbursts that are older than 5 Myr.

Blue supergiant progenitor models of type II supernovae

We show that within all the uncertainties that govern the process of Roche-lobe overflow in case Br-type massive binaries, it cannot be excluded that a significant fraction of them merge and become single stars. We demonstrate that at least some of them will spend most of their core helium-burning phase as hydrogen-rich blue stars, populating the massive blue supergiant region and/or the massive Be-type star population. The evolutionary simulations lead us to suspect that these mergers will explode as luminous blue hydrogenrich stars, and it is tempting to link them to at least some superluminous supernovae.

Mass loss out of close binaries Case A Roche lobe overflow

Liberal evolution of interacting binaries has been proposed previously by several authors in order to meet various observed binary characteristics better than conservative evolution does. Since Algols are eclipsing binaries the distribution of their orbital periods is precisely known. 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. In a recent paper (Van Rensbergen et al. 2010, A&A) 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 hydrogen shell burning and it is our aim to reproduce the observed distributions of the system parameters of Algol-type semi-detached systems. Our calculations reveal the amount of time that an Algol binary lives with a well defined value of mass ratio and orbital period. We use these data to simulate the distribution of mass ratios and orbital periods of Algols. Binaries with a late B-type initial primary hardly lose any mass whereas those with an early B primary evolve in a non-conservative 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. Observed orbital periods of Algol binaries larger 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.

Mass and angular momentum loss during RLOF in Algols

We present a set of evolutionary computations for binaries with a B‐type primary at birth. Some liberal computations including loss of mass and angular momentum during binary evolution are added to an extensive grid of conservative calculations. Our computations are compared statistically to the observed distributions of orbital periods and mass ratios of Algols. Conservative Roche Lobe Over Flow (RLOF) reproduces the observed distribution of orbital periods decently but fails to explain the observed mass ratios in the range ∈ [0.4–1]. In order to obtain a better fit the binaries have to lose a significant amount of matter, without transferring too much angular momentum.

Accretion disks in Algols: Progenitors and evolution

There are only a few Algols with measured accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems, narrowing down the initial parameter space. We investigate the origin and evolution of 6 Algol systems with accretion disks to find the initial parameters and evolutionary constraints for them. With a modified binary evolution code, series of close binary evolution are calculated to obtain the best match for observed individual systems. Initial parameters for 6 Algol systems with accretion disks were determined matching both the present system parameters and the observed disk characteristics. When RLOF starts during core hydrogen burning of the donor, the disk lifetime was found to be short. The disk luminosity is comparable to the luminosity of the gainer during a large fraction of the disk lifetime.

On the origin and evolutionary state of RZ Cassiopeiae, KO Aquilae, and S Equulei (Research Note)

Aims. Determination of the present evolutionary state and the restrictions on the initial mass ratios of RZ Cas, KO Aql and S Equ. Methods. Comparison of mass gaining stars with evolutionary models of single stars with the same mass and subsequent comparison with accretion tracks from simultaneous conservative binary evolution. Results. The gainers are in an early main sequence stage (Xc > 0:5), with KO Aql being almost unevolved (assuming quasi-thermal equilibrium). The initial donor/gainer mass ratios Mdi=Mgi must be larger than three to obtain the present mass and luminosity of the gainers.Aims. We determine the present evolutionary state and the restrictions on the initial mass ratios of RZ Cas, KO Aql and S Equ. Methods. We compare mass-gaining stars with evolutionary models of single stars with the same mass and subsequently compare them with accretion tracks from simultaneous conservative binary evolution. Results. The gainers are in an early main sequence stage (Xc > 0.5), with KO Aql being almost unevolved (assuming quasi-thermal equilibrium). The initial donor/gainer mass ratios Mdi/Mgi must be larger than three to obtain the present mass and luminosity of the gainers.