The complex processes creating accretion discs
Modern astronomers believe that stars are born in the dense cores of molecular clouds, essentially regions of space with relatively high density gas and dust, such as those in the famous Orion nebula. Within such nebulae, the interplay between turbulent flow, magnetic fields and shock waves from nearby supernovae explosions may result in the formation of a region with slightly higher density than the surrounding medium. This dense region then begins to gravitationally draw material from the cloud in a process astronomers call accretion.
However, the process is far more complex than it might at first sight appear. The individual particles in the cloud are all moving and the cloud generally has a net angular momentum. The laws of physics dictate that this angular momentum must be conserved which means that as material is drawn towards the forming protostar, it spins faster and faster like water flowing down a plug hole. It also means that the inflow occurs preferentially in the direction perpendicular to the plane of rotation, so the inwardly spiralling material forms a flattened disk with the protostar at the centre. However, there comes a point where the speed of rotation within the disk is so fast that centrifugal force prevents any further inward motion and the disc becomes rotationally supported. This is exactly the situation with all bodies in stable orbits including our own planet. The Earth is unable to move closer to the sun without shedding some of its angular momentum and fortunately for us, it has no way to do this. But it is exactly this kind of orbital stability that nature has to overcome if stars such as the sun are to form in the first place. How this is possible within physics of accretion disks is a question of special interest to Dr Raquel Salmeron of the ANU Research School of Astronomy and Astrophysics and Research School of Earth Sciences.
Dr Salmeron is developing a novel theoretical model of accretion that incorporates a more comprehensive range of processes than has previously been used. Dr Salmeron explains that angular momentum lies at the core of disk dynamics and in order to understand angular momentum transport, it is essential to look closely at the microphysics, in other words, at the detailed dynamical processes in the gas and the interaction of the gas with the magnetic field.
A very small number of the atoms in the accretion disk surrounding a protostar are ionised by interstellar cosmic rays or radiation from the central object and/or a nearby star. The motion of these charged particles (ions and electrons) leads to the generation of magnetic fields which in turn influence the paths of the charged particles themselves. The process is immensely complex and far from well understood, but astronomers know the disc to be weakly magnetised. Furthermore, collisions between ions and neutral atoms also cause indirect linkage between neutral atoms and the magnetic field. Dr Salmeron believes that understanding and accurately modelling these interactions is the key to answering fundamental questions about the physics of accretion.
Depending on the density of the gas and the number of charged particles within it, there are different kinds of diffusion processes (essentially the `slippage’ between the neutral gas and the magnetic field) that can occur. Two of them, in particular, have formed the basis for existing theoretical models. In very low density regions the charged ions and electrons can move with the magnetic field lines without much interaction with the surrounding neutral atoms because they hardly ever run into them - the so called ambipolar diffusion process. On the contrary, when the gas density is very high, they collide with neutrals so frequently that this process dominates their behaviour - the Ohmic diffusion limit.
Dr Salmeron’s own research focuses on incorporating a third and largely neglected diffusion process, Hall diffusion. This occurs at intermediate densities where the small, fast electrons are able to follow field lines relatively freely whilst the much larger ions experience multiple collisions with neutrals. It’s rather like the way an army of ants can move through a heard of elephants without bumping into too many of them, where as two herds of elephants simply can’t cross paths without mayhem resulting. According to Dr Salmeron all three diffusion processes are often at work in different regions within a stellar accretion disk, and that it is the interplay of these processes, driven by the magnetic field, that dictates the overall behaviour of the system.
The complex picture that emerges is of a swirling disk of matter surrounding a protostar, gradually offloading a large proportion of its angular momentum through complex ion/magnetic field interactions and collisions with neutral atoms. This leads to a small amount of disk matter moving outwards and carrying away the excess angular momentum, so that most of the mass can slow down and spiral inwards towards the forming star. Depending on the magnetic field strength, the matter can move radially out, like water spun out of washing, or can be ejected vertically in what is known as disk wind. One interesting feature of disk wind is that the ejected material often forms what are known as jets - intense energetic flows of matter at right angles to the system.
Astronomers can observe such disks and jets in some nearby forming stars but with current technology telescopes, resolving the details of the process is tantalisingly out of reach. Dr Salmeron hopes that completion of new generation instruments such as the Atacama Large Millimetre Array under construction in Chile may provide the observational data required to test and refine current accretion theories.
The accretion process underlies all star and planet formation in the universe and determines how matter enters black holes such as those believed to lie at the centre of many galaxies. Consequently, understanding accretion is one of the fundamental topics in astronomy today.