H. Belkus

The Wolf-Rayet Population Predicted by Massive Single Star and Massive Binary Evolution

We discuss the differences between massive single star and massive close binary population number synthesis predictions of WR stars. We show that the WC/WN number ratio as function of metallicity depends significantly on whether or not binaries are included. Furthermore, the observed WC(+OB)/WN(+OB) number ratio in the solar neighborhood seems to indicate that the WR mass-loss rates are lower by another factor 2 compared to recently proposed clumping-corrected formalisms. We then demonstrate that the observed lower luminosity distribution of single WN stars can be explained in a satisfactory way by massive single star evolutionary computations, in which the red supergiant phase is calculated using a stellar wind mass-loss rate formalism that is based on recent observations.

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 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.

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.

Synthetic UV Spectra of Starburst Regions with Massive Close Binaries

We present the evolution of synthetic ultraviolet spectra of starburst regions having a significant fraction of binaries. The population synthesis program uses an extended library of stellar evolutionary tracks of single stars and binaries computed with the most recent stellar wind mass loss rates during RSG, LBV and WR stages. In the case of binaries we account in detail for the effects of Roche lobe overflow, mass transfer and mass accretion, common envelope evolution, the spiral-in process, asymmetric kicks to neutron stars as a result of their supernova explosion, etc. The most important synthesized line profiles are those of Si IV λ1400 and C IV λ1550, two P Cygni lines prominent in O stars. The main effect of the close binaries is to extend the UV bright lifetime of the starburst due to the appearance of accretion stars. But more importantly, our computations show that a spectrum of a single star model of about 6 Myr old is effectively undistinguishable from that of a binary rich model of about 10 Myr old. This indicates a possible age degeneracy, reminiscent of the influence of binaries on the commonly used optical age indicator W(Hβ).

The influence of close binary evolution on the age determination of starburst regions

Using a population number synthesis code and detailed massive star evolutionary calculations, we investigate the effect of close binary evolution on H α and H β emission-line strength variation in starburst regions. The H β line strength especially, is considered as a good age indicator. We demonstrate that the formation of rejuvenated accretion stars (Blue Stragglers) through RLOF lengthens the life-time of the H β flux and makes the age determination of a starburst regions ambiguous. We separately calculate the effect of Wolf-Rayet stars and of X-ray binaries on H α and H β and conclude that both are of minor importance.