P. Hellings

Phenomenological study of massive accretion stars

The effects of accretion during core-hydrogen burning and during shell hydrogen burning at constant rates of 3e−3M0yr−1 and 3e−4M0yr−1 on twelve massive stars of 10M0 and 30M0, with and without stellar wind mass loss are examined. For each case relaxation series are computed after 14M0 and after 29M0 have been accreted. The accretion series with the slower rate are well able to adjust their structure almost instantaneously to the increasing mass. The series with the rapid accretion rate show severe overluminosity, having envelopes strongly deviating from thermal equilibrium. After relaxation they return to the normal positions of their corresponding slow accretion models. The process of semi-convection enables the Main-Sequence models to increase their convective core and their core-hydrogen content. The hydrogen shell-burning models increase their intermediate fully convective zone both in size and in hydrogen content.

The post-RLOF structure of the secondary components in close binary systems, with an application to masses of Wolf-Rayet stars

The post-RLOF structure of the secondary after relaxation towards thermal equilibrium is calculated for a large grid of massive close binaries evolving through an early caseB of mass transfer. The initial primary masses range between 15 and 30Mo, the initial mass ratio between 0.3 and 0.9. The possibility that matter leaves the system during RLOF is included using an additional free parameter β. The calculations are based on the accretion and relaxation properties of massive accretion stars. Conclusions on the post-RLOF secondaries are presented in function of β, M1i, andqi, in the form of tables and figures on the post-RLOF positions in the HR diagram, the final masses, mass ratios, chemical profiles and the remaining core-hydrogen burning lifetime. It is found that all systems starting from initial conditions in the grid specified above evolve sequentially, i.e. the primary evolves into a supernova before the end of core H burning of the secondary. No WR+WR systems are encountered. The results are used to determine the masses of ten double lined spectroscopic WR+OB binaries. Most of the WR masses are in the range 8–14Mo, although the sample is subject to some important selection effects. One WR+OB binary has a WR mass between 4 and 5Mo. It is argued that mass determinations based only on the spectral type of the secondary yield WR masses that are too high up to a factor two.

Corrections for Hydrostatic Atmospheric Models: Radii and Effective Temperatures of Wolf Rayet Stars

With the assumption of planparallel hydrostatic atmospheres, used generally for the computation of evolutionary models, the radii of WR stars are seriously underestimated. The true atmospheres may be very extended, due to the effect of the stellar wind. Instead of these hydrostatic atmospheres we consider dynamical atmospheres adopting a velocity law. The equation of the optical depth is integrated outwards using the equation of continuity.

Wind Fed X-Ray Binaries

The effect of a supernova explosion on actually observed WR + 0 binaries is studied, and the transition of the resulting systemsinto X-ray sources is followed. In most cases the system remains bound and becomes a compact + 0 binary with a runaway velocity of the order of 10 km s-1, later evolving into a massive X-ray binary. The X-rays are at the beginning powered by accretion of matter lost by the OB component and later by Roche lobe overflow. When the Roche lobe overflow starts during the core hydrogen burning stage of the 0-type companion, the X-ray phase may last several 100 000 years; but during the hydrogen shell burning stage the X-ray phase takes only 5000 to 8000 years. The probability for case A is about 0.1, hence the duration of the X-ray lifetime for all massive X-ray binaries is about 105 years. This is in agreement with the observations which lead to a lifetime between 70 000 and 200 000 years.

Comments on V505 mon: One of the most massive binaries

From a discussion of the results or V505 Mon, we conclude that the spectral types contradict other parameters in the HR diagram.

On the Structure and Evolution of the OB-Companions in Wolf-Rayet Binaries

The treatment of close binary evolution changed from conservative (total mass and angular momentum constant) to non-conservative (a certain fraction of the matter expelled from the mass losing star, the primary, leaves the system, carrying away a fraction of the total angular momentum). Only the evolution of the mass losing star was computed in detail; the evolution of the accreting star was estimated in a very simple way by computing the evolution in the normal way just like for single stars, but taking into account the mass increase.