E. De Donder

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.

The delay-time distribution of Type Ia supernovae: a comparison between theory and observation

Aims. We investigate the contribution of different formation scenarios for type Ia supernovae in elliptical galaxies. The single-degenerate scenario (a white dwarf accreting from a late main-sequence or red giant companion) is tested against the double-degenerate 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.

The galactic evolution of the supernova rates

Supernova rates (hypernova, type II, type Ib/c and type Ia) in a particular galaxy depend on the metallicity (i.e. on the galaxy age), on the physics of star formation and on the binary population. In order to study the time evolution of the galactic supernova rates, we use our chemical evolutionary model that accounts in detail for the evolution of single stars and binaries. In particular, supernovae of type Ia are considered to arise from exploding white dwarfs in interacting binaries and we adopt the two most plausible physical models: the single degenerate model and the double degenerate model. Comparison between theoretical prediction and observations of supernova rates in different types of galaxies allows to put constraints on the population of intermediate mass and massive close binaries. The temporal evolution of the absolute galactic rates of different types of SNe (including the SN Ia rate) is presented in such a way that the results can be directly implemented into a galactic chemical evolutionary model. Particularly for SNIa the inclusion of binary evolution leads to results considerably different from those in earlier population synthesis approaches, in which binary evolution was not included in detail.

The chemical evolution of the solar neighbourhood: the effect of binaries

In this paper we compute the time evolution of the elements (4He, 12C, 14N, 16O, 20Ne, 24Mg, 28Si, 32S, 40Ca and 56Fe) and of the supernova rates in the solar neighbourhood by means of a galactic chemical evolutionary code that includes in detail the evolution of both single and binary stars. Special attention is payed to the formation of black holes. Our main conclusions: in order to predict the galactic time evolution of the different types of supernovae, it is essential to compute in detail the evolution of the binary population, the observed time evolution of carbon is better reproduced by a galactic model where the effect is included of a significant fraction of intermediate mass binaries, massive binary mass exchange provides a possible solution for the production of primary nitrogen during the very early phases of galactic evolution, chemical evolutionary models with binaries or without binaries but with a detailed treatment of the SN Ia progenitors predict very similar age-metallicity relations and very similar G-dwarf distributions whereas the evolution of the yields as function of time of the elements 4He, 16O, 20Ne, 24Mg, 28Si, 32S and 40Ca differ by no more than a factor of two or three, the observed time evolution of oxygen is best reproduced when most of the oxygen produced during core helium burning in ALL massive stars serves to enrich the interstellar medium. This can be used as indirect evidence that (massive) black hole formation in single stars and binary components is always preceded by a supernova explosion.Abstract In this paper we compute the time evolution of the elements (4He, 12C, 14N, 16O, 20Ne, 24Mg, 28Si, 32S, 40Ca and 56Fe) and of the supernova rates in the solar neighbourhood by means of a galactic chemical evolutionary code that includes in detail the evolution of both single and binary stars. Special attention is payed to the formation of black holes. Our main conclusions: • in order to predict the galactic time evolution of the different types of supernovae, it is essential to compute in detail the evolution of the binary population, • the observed time evolution of carbon is better reproduced by a galactic model where the effect is included of a significant fraction of intermediate mass binaries, • massive binary mass exchange provides a possible solution for the production of primary nitrogen during the very early phases of galactic evolution, • chemical evolutionary models with binaries or without binaries but with a detailed treatment of the SN Ia progenitors predict very similar age–metallicity relations and very similar G-dwarf distributions whereas the evolution of the yields as function of time of the elements 4He, 16O, 20Ne, 24Mg, 28Si, 32S and 40Ca differ by no more than a factor of two or three, • the observed time evolution of oxygen is best reproduced when most of the oxygen produced during core helium burning in ALL massive stars serves to enrich the interstellar medium. This can be used as indirect evidence that (massive) black hole formation in single stars and binary components is always preceded by a supernova explosion.

The chemical evolution of the Galaxy: the importance of stars with an initial mass larger than 40 M⊙

Abstract In the present paper we investigate in how far stars with an initial mass larger than 40 M⊙ affect the chemical enrichment of the Galaxy. We illustrate the importance for chemical yields of a most up-to-date treatment of the various stellar wind mass loss episodes in stellar evolutionary codes and we discuss the effects of a possible supernova-like outburst prior to massive black hole formation.

The influence of neutron star mergers on the galactic chemical enrichment of r-process elements

Abstract A population number synthesis code follows in detail the evolution of a population of single stars and of close binaries. We use our code to simulate the population of neutron star–neutron star and black hole–neutron star binaries. We then combine our population number synthesis code with a galactic chemical evolutionary model in order to follow the time evolution of the formation and merger rate of these double compact star binaries and the resulting chemical enrichment of r-process elements, over the whole Galactic lifetime. It can be concluded that the neutron star/black hole merger process is able to reproduce the observed r-process enrichment of the Galaxy. However, we show that the latter conclusion depends critically on the physics of case BB Roche lobe overflow in binaries with a neutron star component and a hydrogen deficient core helium/helium shell burning star with a mass between 2.6M⊙ and 6M⊙.

The effect of binaries on the space velocity distribution of single pulsars

Abstract In this paper we determine the theoretically expected space velocity distribution of single pulsars resulting from real single stars and from disrupted binaries. Hereto we use a population number synthesis (PNS) code that contains a detailed model for single and binary evolution. The binary period evolution and the effect of an asymmetric supernova on the binary parameters are followed in detail. If the progenitor of the pulsar was in a binary, disrupted by an asymmetric supernova explosion, the influence of the orbital velocity of the progenitor and of the binary potential is incorporated. We conclude that: (i) the shape of the pulsar space velocity distribution is mainly determined by the input kick distribution; (ii) the importance of binary evolution depends on the relative fraction of small and large kick velocities.

Very massive close binaries and the puzzling temporal evolution of 14N in the Solar Neighbourhood

Abstract Low metallicity very massive stars with an initial mass between 140 M ⊙ and 260 M ⊙ can be subdivided into two groups: those between 140 M ⊙ and 200 M ⊙ which produce a relatively small amount of Fe, and those with a mass between 200 M ⊙ and 260 M ⊙ where the Fe-yield ejected during the supernova explosion is enormous. We first demonstrate that the inclusion of the second group into a chemical evolutionary model for the Solar Neighbourhood predicts an early temporal evolution of Fe, which is at variance with observations whereas it cannot be excluded that the first group could have been present. We then show that a low metallicity binary with very massive components (with a mass corresponding to the first group) can be an efficient site of primary 14 N production through the explosion of a binary component that has been polluted by the pair instability supernova ejecta of its companion. When we implement these massive binary 14 N yields in a chemical evolution model, we conclude that very massive close binaries may be important sites of 14 N enrichment during the early evolution of the Galaxy.