Compact Binary Populations

Ulrich Kolb is a Senior Lecturer in the Department of Physics and Astronomy at The Open University, in Milton Keynes, Buckinghamshire. His main research interests are in the area of compact binaries and this was the subject of his talk to us. He has written a book called Interacting Binary Stars. He is also now chairing the production of the 2nd level OU residential school course Observing the Universe, hosted by the Observatori Astroṇmic de Mallorca.

Compact binaries are two stars which orbit each other. One star is enormous and the other compact like a white dwarf, a neutron star or even a black hole. Their orbits are very small as they are close to each other and the larger star starts to donate matter to the smaller star. This creates an accretion disk, which is a disk of gas that forms around the smaller object as material spirals from the massive object.

Mass exchange in a binary system can occur by three modes:

1. Roche-lobe overflow of the primary star. For a low-mass X-ray binary (where a compact object is bound to a star whose mass is similar to or less than that of our Sun), the only way to achieve enough mass transfer to create large fluxes of X-rays is through Roche-lobe overflow. The Roche-lobe is the location between the two stars in a binary where the gravitational pull from one star is equal and opposite to that of the other star. If the binary system is "close", i.e. the orbital radius is small, this point can occur near the surface of the normal star. Thus a funnel is created for significant mass to flow out toward the compact star for accretion. In the case of Roche-lobe overflow, the angular momentum of the accreting material will tend to form a differentially rotating disk around the secondary. The material in this accretion disk is then slowly spiralled into the intense gravitational well of the compact object. It heats up to temperatures over 1,000,000 degrees and, therefore, shines brightly in the X-ray spectrum.

2. Enhanced stellar wind from the primary star. A number of X-ray binaries are known to consist of a massive primary emitting a stellar wind driven by the primary's radiation pressure, orbited by a neutron star or black hole. Such a system is called a HMXRB, or high-mass X-ray binary; the primary typically has a mass 10 times or more than that of our Sun. The compact object captures a fraction of the wind and converts the potential energy of the accreted plasma into X-rays. While qualitatively feasible, X-ray production by accretion from an undisturbed spherical wind can fall several orders of magnitude below the observed luminosity in the case of some binary systems. However, simple modifications to the basic theory brought the observations closer to prediction. Such modifications include the effect of the X-ray emission on the velocity of the incoming wind and an angular dependence of the primary's mass loss.

3. Capture of circumstellar material from a star. In a Be star/neutron star binary, the behaviour of the Be star controls the X-ray characteristics of the system. A Be star is a B star which rotates so rapidly that an instability results via which material streams out from the equatorial plane and an expanding atmosphere is formed. This introduces strong emission lines of hydrogen and neutral helium into the stellar spectrum. Furthermore, these stars are known to throw off large amounts of matter from their equatorial regions at apparently random intervals. The capture and accretion of this material by the secondary is known to be the source of many of the observed X-ray transients.

A compact binary cannot be resolved with an amateur optical telescope, as the stars are too close together. Professional astronomers measure the light difference, as one star passes in front of the other to deduce the difference. The accretion is the most efficient energy source in the universe. Accretion onto a neutron star has the efficiency of 10%, compared with a hydrogen bomb which has only the efficiency of 0.7%. If the transfer is more than the smaller star can sustain then jets can be seen as outflow from the centre of the accretion disk.