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Дата изменения: Fri May 5 20:20:19 2000 Дата индексирования: Tue Oct 2 04:04:42 2012 Кодировка: |
A collapse or implosion of a cloud of gas containing molecules and dust may lead to a low-mass star like our Sun. High-mass stars could form through the accretion of smaller protostars. By studying the physics of molecular clouds, we hope to learn if and why the collapse occurs.
We have investigated the nature of the molecular clouds by performing computer simulations. These new simulations account for many, but not all, realistic cloud properties. In three dimensions, with supersonic turbulence and magnetohydrodynamics, we have analysed the fields of shock waves produced. In work carried out together with Mac Low (New York), Zuev (Colorado), Heitsch (Heidelberg) and Klessen (Leiden), we have been able to distinguish between decaying turbulence, producing an exponential spectrum of shock speeds, and driven turbulence, producing a specific power-law spectrum.
A Unification Scheme for protostars was developed and extended in several directions. Clear predictions were made which have proven invaluable in stimulating and motivating new research projects. The scheme adopts the hypothesis that the mass of cloud gas is redistributed through several components. The core, envelope, accretion disc, jets and outflow all evolve on the same time-scales, passing through distinct stages from birth (Class 0 protostar), toddler (Class 1), childhood (Class 2), puberty (Class 3) to adolescence (Pre-Main Sequence).
The simple rules yield evolutionary tracks on diagrams relating any two of the above components. In a collaboration with Stanke (Bonn) and Zinnecker (Potsdam), work is now underway to test these predictions against statistics obtained from wide-field infrared surveys. An extension of the Unification Scheme to lower mass proto-brown-dwarfs suggests that they too will possess detectable outflows.
The interaction of young stars with their surroundings are accentuated within Herbig-Haro Objects, where streams and `bullets' of ejected gas impact on the ambient molecular clouds. Infrared observations penetrate the clouds and reveal the processes occurring deep within. We are progressing with a programme of infrared observations and interpretations of the outflows. This includes a large-scale imaging study of bow shocks in the Orion Molecular Clouds (with Ka Chun Yu and Bally et al., Colorado) and spatially-resolved excitation and kinematic studies of well-known outflows (with Eislöffel, Tautenburg, Germany; and Davis, UKIRT, Hawaii) such as HH1/2 and HH46/47.
The question how objects can be accelerated to and maintain high speeds is important not only to astronomers. Projectiles -- be they torpedoes, rocks, missiles or people -- are slowed down by friction as they try to penetrate their environments, creating bow waves, sonic booms and turbulent wakes. In our Galaxy, in regions associated with young stars and star formation, we now realize that there are swarms of projectiles called Herbig-Haro Objects, named after the two scientists who first researched them. They appear to move ballistically -- like cannonballs. But what are they doing there, where are the cannons and why were they fired?
We have now detected many jets of these interstellar bullets amidst narrow streams of gas. The jets are sometimes found in symmetric pairs, at the centre of which we can often only just see some highly obscured star-like bodies. These bodies turn out to be protostars: the stars-to-be. The moment a star is born is signalled by a round of cannon-fire!
The astrophysical jets are spectacular. But why should star birth be accompanied by such violent events? Recent work has shown that they are essential to the whole star-formation process. A star cannot easily form in isolation because of angular momentum conservation: it would wind up spinning so fast that it would be unable to collapse. It appears that the jets and bullets carry away the angular momentum during the final collapse. Michael Smith and collaborators have searched for, and found, the first evidence of rotation within the jets.
Observed over several years, the associated interstellar bullets are found to move at hundreds of kilometres per second. This leaves us with new questions: how can molecules survive such shattering speeds, and how do the cloud and protostar contrive to transfer angular momentum from the cloud to the bullets? Michael Smith hopes that finding the answers to these problems will ultimately contribute to our ability to control energy resources and to fly faster, on top of the direct goal of understanding our Universe. To this end, he is now undertaking a new programme of supercomputer simulations in which models for the interstellar bullets can be tested.
In addition to this work on star-forming regions in our own Galaxy, Michael Smith has also begun a new study of star formation in external galaxies, particularly the very energetic `starburst' galaxies often associated with distant interacting, or colliding stellar systems. In these objects new stars, especially massive stars, are being formed at rates, and under extreme conditions, not matched anywhere within our own Galaxy.
During the year, Michael Smith delivered a number of scientific and popular talks to groups both locally and abroad, and supervised two QUB undergraduates on summer project work. Towards the end of the financial year he obtained a research grant through the Joint Research Equipment Initiative to purchase a Silicon Graphics Origin2000 supercomputer. Under this scheme, the cost of the supercomputer, approximately £250k, is funded partly by the PPARC (£ 125k), with the balance being provided by the industrial partner, in this case Silicon Graphics Inc. (SGI). Michael Smith also successfully obtained a PPARC research grant to support a postdoctoral research assistant for three years to work on the project `The Origin and Evolution of Protostars: Tracking with Magnetohydrodynamic Numerical Simulations'.