G. J. Savonije

Evolutionary status of bright, low-mass x-ray sources/sup 1/

A model of bright, low-mass X-ray binaries is proposed which features a lower giant-branch star losing mass on a nuclear time scale to an accreting compact companion. Simple numerical models show that mass transfer rates > or =10/sup -9/ M/sub sun/ yr/sup -1/ are sustained at very nearly a constant rate until the envelope of the donor star is exhausted. The model predicts orbital periods in the range 1/sup d/-200/sup d/ and X-ray to optical luminosity ratios L/sub x//L/sub opt/roughly-equal200-1000 for these sources. It accounts in a natural way for the large fraction of the total galactic bulge luminosity emitted by a few bright (> or =10/sup 37/ ergs s/sup -1/) sources. It also accords very well with the observed X-ray and optical properties of the halo source Cyg X-2 and also with those of 2S 0921-63, provided this latter system contains a massive accreting white dwarf rather than a neutron star. Problems of the prior evolution of low-mass X-ray sources are also briefly delineated.

On the evolutionary status of bright, low-mass X-ray sources

Congregated near the nuclear bulge of our own Galaxy, and also that of M31, are a handful of bright (≳1037 ergs s-1) persistent X-ray sources. These sources account for most of the total X-ray luminosity of each of these bulges, and their space distribution conforms closely to that of an old disk population (Markers et al. 1977; Van Speybroeck et al. 1979). They have (at least in our own Galaxy) very similar X-ray flux distributions (Jones 1977), and lack the X-ray pulsations frequently found in massive X-ray binaries. Presumably, they are old, low-mass binaries in which the magnetic fields of the neutron stars have decayed.

The evolution of naked helium stars with a neutron-star companion in close binary systems

The evolution of helium stars with masses of 1.5 – 6.7 M⊙ in binary systems with a 1.4 M⊙ neutron-star companion is presented. Such systems are assumed to be the remnants of Be/X-ray binaries with B-star masses in the range of 8 – 20 M⊙ which underwent a case B or case C mass transfer and survived the common-envelope and spiral-in process. The orbital period is chosen such that the helium star fills its Roche lobe before the ignition of carbon in the centre. We distinguish case BA (in which mass transfer is initiated during helium core burning) from case BB (onset of Rochelobe overflow occurs after helium core burning is terminated, but before the ignition of carbon). We found that the remnants of case BA mass transfer from 1.5 – 2.9 M⊙ helium stars are heavy CO white dwarfs. This implies that a star initially as massive as 12 M⊙ is able to become a white dwarf. CO white dwarfs are also produced from case BB mass transfer from 1.5 – 1.8 M⊙ helium stars, while ONe white dwarfs are formed from 2.1 – 2.5 M⊙helium stars. Case BB mass transfer from more-massive helium stars with a neutron-star companion will produce a double neutron-star binary. We are able to distinguish the progenitors of type Ib supernovae (as the high-mass helium stars or systems in wide orbits) from those of type Ic supernovae (as the lower-mass helium stars or systems in close orbits). Finally, we derive a ”zone of avoidance” in the helium star mass vs. initial orbital period diagram for producing neutron stars from helium stars.

Orbital evolution by dynamical tides in solar type stars. Application to binary stars and planetary orbits

We study the tidal evolution of eccentric binary systems consisting of a solar type main sequence star accompanied by either another solar type star, or by a planet with a mass similar to Jupiters mass. The tidal dissipation which takes place in the solar type star(s) is calculated in the framework of dynamical tides, and resonant interaction with the g-mode and quasi-toroidal oscillation eigenmodes of the stellar component(s) is included in the orbital calculations. It appears that in a system of two solar type stars intervals during which harmonic components of the perturbing tidal potential become locked onto resonances with stellar oscillation modes appear ubiquitously, signicantly enhancing the eciency of the tidal coupling. In our calculations stellar binaries become circularized during the main sequence lifetime for orbital periods up to about 10 days, or 16 days in the case of very slow stellar rotation. Ecient resonance locking causes signicant tidal decay in weakly eccentric planetary binaries with orbital periods up to approximately 5 days in case the solar type star is a slow rotator, and very large increase of the orbital period and eccentricity in case the star is rotating rapidly. The dynamical tide with inclusion of the eects of close resonances with the stellar oscillation modes provides considerably more ecient tidal coupling than the equilibrium tide with viscous damping of turbulent eddies.

Tidal evolution of eccentric orbits in massive binary systems - II. Coupled resonance locking for two rotating main sequence stars

We extend our study of the tidal evolution of elliptic binary systems to the case of a system consisting of two

The origin of the runaway high-mass X-ray binary HD 153919/4U1700-37

10 M_\odot

Tidal interaction of a rotating 1 M star with a binary companion

uniformly rotating main sequence stars. Previous work showed that in a system consisting of a

One-armed oscillations in Be star discs

1.4 M_\odot

Visibility of unstable oscillation modes in a rapidly rotating B star

compact object in orbit about a

Non-radial oscillations of the rapidly rotating Be star HD 163868

10 M_\odot