M.D. Smith, Research Astronomer

The evolution of stars after leaving both the main sequence and the asymptotic giant branch involves the transition into a proto-planetary nebula (PPN) before developing into a planetary nebula (PN). Whilst the central stars are still cool, winds are driven which impinge and shape the envelope. Michael Smith has been analysing new infrared data collected from 11 PPN and PN by Chris Davis (Joint Astronomy Centre, Hawaii) to search for clues as to how the objects evolve. The infrared spectra display emission lines from excited molecules of hydrogen which can be separated into components excited by ultraviolet fluorescence and collisional shock waves. We conclude that shock excitation is dominant in the early PPN phase and the late phase of the PN, with fluorescence critical at intermediate phases. However, molecular hydrogen emission is recorded at all stages and represents a useful probe of the evolutionary status of PPN and PN alike.

Theoretical analysis of shocks in different astronomical environments has continued in collaboration with both Georgi Pavlovski (PhD student) and Alex Rosen (PDRA). Recent advances in submillimetre astronomy and molecular astrophysics prompted Rosen and Smith to simulate fast shocks in which molecules are destroyed. This was intended to be a test problem for a new version of the MHD ZEUS code. However, in most cases comparison with steady-state models was found not to be possible because the reformation of molecules in the post-shock cooling flow causes the shock to become unstable, leading to high-amplitude oscillations (i.e. an overstability). The major cause of this phenomenon, however, was the assumed equilibrium oxygen and carbon chemistry. A subsequent non-equilibrium analysis displayed a much narrower instability regime. Therefore, we require new techniques to simulate flows containing such shocks.

Georgi Pavlovski studied the properties of supersonic turbulence in molecular clouds and its effect on the molecular cloud properties and their chemical evolution. In recent years the traditional picture of molecular clouds as quasi-static objects which form stars slowly over a long lifetime has begun to give way to a new picture of clouds as dynamical entities whose formation and evolution are dominated by the effects of supersonic turbulence. In his thesis, he explored the significant consequences of turbulence both for our understanding of molecular cloud chemistry and for our interpretation of molecular line observations. In particular, he aimed to test the extent to which isothermal simulations correctly model the behaviour of molecular turbulence and to investigate the impact of turbulence on molecular dynamics.

**Figure 8:** The appearance of a variable nebula at various epochs in the optical I-band, atomic [SII] and near-infrared K-band emission, probably illuminated by radiation from an outbursting protostar. A jet has probably cleared away the channel, allowing radiation to reach the cavity walls and to produce a curving structure reminiscent of a helix projected on to the sky plane.
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In order to achieve these objectives, a numerical scheme which accounts for the most important shock-induced chemical reactions and the resulting cooling functions has been coupled with the general purpose hydrodynamical code ZEUS-3D. A number of simulations of molecular turbulence, in which collisional dissociation and reformation on dust grains, were carried out. The results of this work provide insight into how molecular chemistry and gas dynamics combine. The main conclusion is that isothermal simulations adequately model molecular turbulence. We have discovered, however, that the molecular chemistry can be significantly accelerated due to the strong compression and advection associated with supersonic turbulence. Therefore, through the chemistry, turbulence can enhance the rate at which stars form. Thus, molecular clouds may be undergoing rapid dynamical and chemical changes, driven by sources of supersonic turbulence, yet they can appear to be ``chemically old'' due to the increased rate of chemical reactions.

Computer simulations of jets were executed and analysed by Alex Rosen (in Armagh until July 2003) and Michael Smith. These three-dimensional hydrodynamic calculations apply to protostellar jets dominated by flows of molecules. The series of papers exploring the influence of the density, evolution in injection energy, and fast precession, were accepted for publication and a web page constructed to present all the relevant movies (see star.arm.ac.uk/ $^\sim$ mds/Jets/jets.html). In particular, predicted images and spectroscopy for infrared and submillimetre molecular emission lines were presented, in anticipation of upcoming telescope projects such as Herschel, ALMA and JWST.

Anthony Moraghan (TCD) joined the Armagh student population for three months to investigate the spinning of young stars. Under the guidance of Michael Smith, he gathered data on the variation of the brightness of stars and, independently, their speed of rotation (the Doppler shift giving the component of velocity along our line of sight). On comparison, systematic variations were shown to yield anomalies which can be resolved if the standard distances to the star-forming regions are erroneous.

Outflows in star formation regions were explored through infrared and optical observations by Michael Smith and many co-workers. Projects completed included detailed descriptions of individual outflows such as Cepheus E, a study of the driving protostars with the ISO satellite, and the exploration of a new star formation field uncovered by the HELIX group. Details of the latter collaboration can be found at the webpage: star.arm.ac.uk/ $^\sim$ tig/helix/details.html. In these papers, many new methods of interpreting and modelling the data are introduced, as well as the discovery of new objects and structures. In particular, one conspicuously brightened object, which illuminates a variable reflection nebula (see Figure 8), is examined. Follow-up observations are planned which it is believed will confirm that the driving source is one of a special class of outbursting protostars called FuOrs, after the prototype FUOrionis.

Finally, a manuscript entitled ``The Origin of Stars'' was submitted for publication. It is the first monograph on star formation, apart from collective works based on conference proceedings, for fifteen years. It illustrates the recent revolution in our knowledge associated with star birth on many scales.