Interacting Binaries

Whilst single star evolution is itself a rich and varied field of study, there exist systems containing a pair of stars which are close enough for matter to flow from one star to the other. We term these systems "interacting binaries" and they are even more diverse and interesting than single stars, with many complex physical and conceptual problems still to be solved.

Compact Binaries

In a subclass of interacting binaries matter flows onto a compact object (a neutron star, black hole or white dwarf). Energy is released in the accretion flow, and often in nuclear reactions within the accreted matter.

Jan-Uwe Ness has produced a webpage showing and explaining his research into classical novae, which can occur in systems where the accreting object is a white dwarf.

Here we particularly concentrate on understanding the evolution of X-ray binaries (where the compact object is a neutron star or black hole, and much energy is released in X-rays). Though X-ray binaries are very rare objects due to the fine-tuning required in their evolutionary paths, their X-ray emission means that they are comparatively easy to observe, and the fact that they each contain such an extreme object as a neutron star or a black hole means that they have the potential to test physics that we cannot reproduce in a laboratory.

Low-mass X-ray binaries (LMXBs) are particularly good systems to study the evolution of as we believe that millisecond pulsars (MSPs) are produced by being spun-up in LMXBs, so we can try to match the population statistics of LMXBs and their evolution to the population of MSPs.

Artist's impression of a microquasar

This drawing of an interacting compact binary shows the donor star losing matter to an accretion disc around a compact object. In this object, the accretion-powered source is launching a jet, a process which is poorly understood but is seen in a wide variety of astrophysical objects.

During the LMXB phase, it is generally believed that the neutron star is spun up by accretion of matter, leaving a millisecond pulsar once the X-ray phase has ended. However, statistical comparisons between millisecond pulsars and LMXBs suggest that there are either too many millisecond pulsars relative to the number of LMXBs or that the duration of the LMXB phase has been overestimated by a large factor. This latter, more likely, possibility may be understood by irradiation effects which can dramatically change the structure of the irradiated normal star. In particular, the secondary may become inflated which leads to accelerated evolution and a shorter duration of the LMXB phase. The details of this process depend, however, on the circulation inside the secondary caused by the one-sidedness of the irradiation in a binary. This is an important problem which we are actively studying at the moment, developing both the theoretical framework and the numerical tools to tackle this problem.

One of the most important recent discoveries in this field is the realization that many stars in low-mass X-ray binaries originate from much more massive progenitors (e.g., Cyg X-2, Cyg X-3). Based on our recent calculations, it now seems that a large fraction, if not the majority, of low-mass X-ray binaries may actually belong to a much more massive, previously largely ignored class of intermediate-mass X-ray binaries (IMXBs), and that standard textbooks on the subject need to be rewritten.

Apart from modelling individual systems, we use binary populations synthesis techniques to model the population of X-ray binaries (with US collaborators) and keep active collaborations with observational groups to test our predictions and improve our modelling efforts. With them, we have concluded that if the IMXB systems are taken into account along with a simple approximation to irradiation effects then our current problems with LMXB evolution should be resolved (see Pfahl, Rappaport and Podsiadlowski, 2003). Our work on understanding irradiation effects properly will hopefully confirm this new picture.

Emphasising the extraordinary symmetry of the jet ejection mechanism in SS433, this filtered VLA radio image has been marked on both sides with coloured dots representing blobs of mass emitted at a given time.

SS433

SS433 was the first object identified in our galaxy as producing relativistic jets - it emits collimated beams of matter at a quarter of the speed of light. Other microquasars are now known, but they remain unexplained.

The corkscrew precession and symmetry of the jets in the microquasar SS433 is striking, even more so when it is realised that the symmetry of the jets extends to equal and opposite variations in jet speed (between 0.24c and 0.28c). This analysis that demonstrates this and the image to the left can be found in Blundell and Bowler (2004). We still have no definitive mechanism to explain such behaviour.

This system has been extensively studied, but the masses of its components are still unknown. The analysis above has given an independent distance measure to the system, and previous work here has shown that in addition to the spectacular and energetic polar jets, SS433 is ejecting matter in its equatorial plane at a rate of around 10^-4 solar masses per year. This high rate of mass loss implies that SS433 is in a very short-lived state.

Modelling Binary Evolution and Population Synthesis Methods

Population arguments are a powerful and commonly used technique in trying to understand the evolution of astrophysical objects. Population synthesis calculations in which the evolution tens of thousands of binary systems must be rapidly computed have tended to require greatly simplified physics to achieve the necessary calculation speed. We wish to be able to use the binary population sysnthesis technique but are also interested in improving the physics used in the binary models.

Hence we are taking both approaches. We are working with collaborators on improving binary population sysnthesis (we have been involved in the first population study in which each binary was directly calculated using a stellar evolution code rather than by using some analytic approximation or interpolation), but we are also determined to model accurately as well as quickly, for example by including the irradiation effects mentioned above.

We are also involved in extending population synthesis techniques to dense stellar systems (such as globular clusters) where gravitational interactions between different binary systems become important.