Raquel Salmeron’s childhood interest in space blossomed into a study of how things spin out among the stars, as Tim Wetherell explains.

It’s remarkable how the smallest of things can spark a lifetime’s passion. Dr Raquel Salmeron from the Research School of Astronomy and Astrophysics recalls that her interest in astronomy began at age six, when her mother read her a story that mentioned the planet Earth. When asked what a planet was, her mother explained that the place we lived was in orbit around the sun, as were several other planets. Raquel remembers laying awake that night thinking about how awesome this concept was, and from that point has had an interest in astronomy and flight. As is so often the case, her path to becoming a research scientist in astronomy was somewhat indirect.
Pure astronomy wasn’t a study option for Salmeron in her native Venezuela, so instead she studied aeronautical engineering, during which she developed an interest in mathematical modelling of fluid flows. When it came time to undertake her PhD at the University of Sydney, she recognised that the mathematics describing air flow around aircraft winds could be easily adapted to model plasma flows in astronomy. Having completed her PhD, Salmeron worked at the University of Chicago for two years before taking up a postdoctoral fellowship at the Research School of Astronomy and Astrophysics at ANU.

Salmeron’s research focuses on the process of star formation within the vast clouds of dust and gas astronomers called nebulae. Within such nebulae, the interplay between turbulent flow, magnetic fields and shock waves from nearby supernova explosions sometimes results in the formation of a region with slightly higher density than the surrounding nebula. The gravitational attraction of the extra mass in this new dense region then begins to draw in still more material from the cloud in a process known as accretion. Eventually enough mass becomes concentrated in one place to create a star.

But the process is far more complex than it might appear at first sight. The gas in nebulae is in motion such that the system typically has 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. The various processes involved in doing this are a hot topic in modern astronomy. As with so much ground breaking work in astronomy, observations made with telescopes are only half the story. The observational data need to be related to an accurate theoretical model, which is where mathematical modellers such as Salmeron come in.

The researcher’s current project is the development of a novel model of accretion that incorporates a more comprehensive range of processes than has previously been used. She explains: “Angular momentum lies at the core of disc 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 disc surrounding a protostar are ionised by interstellar cosmic rays, or radiation from the central object or a nearby star. The motion of these charged particles – ions, electrons, charged dust grains – alters the geometry and strength of magnetic fields that, 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 the charged particles and neutral atoms also cause indirect linkage between neutral atoms and the magnetic field. 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.

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, whereas two herds of elephants simply can’t cross paths without mayhem resulting. According to Salmeron, all three diffusion processes are often at work in different regions within a stellar accretion disk, and 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 disc 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 disc wind. One interesting feature of disc 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 discs and jets in some nearby forming stars but with current technology telescopes, resolving the details of the process is tantalizingly out of reach. 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.

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ANU Reporter 
Spring 2007