D. Vanbeveren

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 Evolution of Very Massive Stars

Core collapse of dense massive star clusters is unavoidable, and this leads to the formation of massive objects, with masses of up to 1000 M☉ and even larger. When these objects become stars, stellar wind mass loss determines their evolution and final fate, and decides on whether they form black holes (with normal mass or with intermediate mass) or explode as a pair-instability supernova. In this paper we discuss the evolution of very massive stars and present a convenient evolution recipe that can be implemented in a gravitational N-body code to study the dynamics of dense massive clusters.

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 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.

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⊙.

A comparison between the orbital masses of early type binary components and masses predicted by stellar evolution

In this paper we show that for eight components of early type binary systems (including three supergiants) the orbital mass corresponds reasonably well with masses resulting from evolutionary computations.

Binary Populations and Stellar Dynamics in Young Clusters

We first summarize work that has been done on the effects of binaries on theoretical population synthesis of stars and stellar phenomena. Next, we highlight the influence of stellar dynamics in young clusters by discussing a few candidate UFOs (unconventionally formed objects) like intermediate mass black holes, η Car, ζ Pup, γ 2 Velorum and WR 140.