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Дата изменения: Tue Dec 14 13:44:25 1999
Дата индексирования: Tue Oct 2 04:02:15 2012
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M.D. Smith, Research Astronomer

Dr Michael Smith has focused mainly on the earliest stages of stellar evolution: the processes leading to star formation from an initially unstable, collapsing cloud of gas, and the energetic outflows -- jets and winds -- associated with the formation and early evolution of protostars and pre-main-sequence stellar objects.

The observed events associated with star formation are related to diverse physical and dynamical mechanisms. Research has begun in Armagh to determine the relationships between these mechanisms and hence to uncover how low-mass stars like our Sun, as well as more massive stars, even star clusters and stellar systems, form.

The basic building sites for stars are gigantic clouds of cool molecules. The clouds are in limbo: their tendency to gravitational collapse is held up by strong magnetic fields and turbulent motions. The turbulence, revealed by spectroscopic observations, takes the form of chaotic motions at speeds exceeding the speed of sound. The balance is crucial to the appearance of our Universe; without it, the rate at which stars form would be (or, more correctly, would have been) about 30 times faster.

We have ideas about what may be the source of the turbulence (internal jet streams, external shock waves, stellar winds or the Galactic shear) but progress has been hampered by our ignorance of this brand of turbulence: supersonic molecular magnetohydrodynamics. Together with colleagues Mac Low (New York) and Burkert (Heidelberg), we have employed numerical simulations to show that all forms of turbulence decay extremely fast. A range of Mach numbers and magnetic energies has been explored, from which the need for the turbulence to be constantly regenerated has been concluded.

In contrast to our everyday experiences with gases and fluids, magnetic fields are almost frozen into the star-forming material. The pressure of the magnetic field must eventually be relieved if the embryonic star, or `protostar', is to arise. Michael Smith is studying how this occurs. It turns out that the field can gradually slip out of the denser clumps of molecules by a process called ambipolar diffusion (a decoupling process: charged ions become rare, and the neutral molecules then slip between them). Alternatively, the magnetic lines of force can reconnect into new configurations which allow the magnetic pressure to escape or to change form.

At first, a protostar is extremely difficult to detect. It is deeply embedded in its parent molecular cloud, totally obscured at infrared wavelengths as well as in the visible. Spectacular jets of molecules, however, betray its presence. Such jets have been observed emanating from the very youngest protostars. We have been engaged in discovering the cause of the jets and their effects on the parental clouds. Michael Smith has undertaken numerical experiments, using a molecular hydrodynamics code, to try to understand some of the observed structures. The project involves Yorke (Jet Propulsion Laboratory, California), Zinnecker (Potsdam) and students Völker and Suttner (Würzburg). A major aim is to disentangle environmental and protostellar factors. Three-dimensional simulations demonstrate the consequences of jet dynamics: precession, pulsation, shear and spray.

It has recently been recognised that the respective evolution of the jets emitted from the protostar and the reservoirs of accumulated ejected gas (termed the `bipolar outflows') could evolve synchronously with that of the protostar, the surrounding accretion disc, and the residual cloud (termed the `envelope'). An attempt has been made to unify these basic components within a simple model framework. The model makes the least controversial assumptions to derive the relationships between outflow mechanics, jet dynamics and protostellar luminosity. Stages in jet behaviour are thus recognisable, as well as diagnostics for the age of an outflow. The model also emphasizes our lack of understanding: the high rate at which energy is liberated and momentum is transferred into the bipolar outflow requires a remarkably efficient mechanism for jet formation.

Other activities and results during 1998 are summarised as follows:

1.

detection of hot molecules with the Infrared Space Observatory; distinguishing shock and fluorescent excitation in DR21; and constructing infrared and submillimetre maps of the high-mass outflow W75N, a gigantic outflow generated in the wakes of bow shocks.

2.

Michael Smith has also been involved in preliminary discussions concerning the formation of a European-wide Star Formation Network, and has given seminars on the subject of stellar jets at the University of Newcastle and at the Armagh meeting of the Astronomical Science Group of Ireland.

3.

Future research in this area will continue with an intensified observational programme employing ISO data and new infrared data to understand individual outflows, and with a new focus on star formation on larger scales, namely the galactic-size `Starbursts' seen in external galaxies. In addition, Michael Smith and Mac Low (New York) will study ambipolar diffusion employing new numerical and analytical techniques, and will examine in detail the formation of bow shock waves and the fragmentation caused by unstable behaviour which leads to current sheets.