Understanding the origin of stars is one of the greatest challenges. It is important for nearly every aspect of astronomy in studies ranging from galaxy formation and evolution to the origin of the Solar system. Star formation is complex because a wide variety of processes are involved. The density increases by 20 orders of magnitude from that of a molecular cloud core and most of this increase has not been observed yet. Theory predicts a complex interplay between gravity, magnetism, hydrodynamics, radiative transport, and chemistry, all happening in a turbulent medium. Supersonically converging flows make blobs, sheets, and filaments that are continuously deformed by the surrounding flows and further compressed by collisions. Some of them collapse into binary and single stars. Others disperse. All of this is powered on the smallest scales by the gravity itself, and by pre-main sequence winds and expansional motions from nearby HII regions, main sequence stellar winds, and supernovae. Cloud formation itself is highly turbulent, driven in part by large-scale instabilities involving galactic self-gravity, magnetism, rotation and shear, and in part by expansion around OB associations and young stellar complexes.
The activity does not stop once a star forms. Young stars pump energy back into the interstellar medium, restricting or moderating other star formation nearby and triggering new star formation far away in shells and super-shells. Some theories suggest that star formation self-regulates in this way. Other theories suggest it is too chaotic to act like a thermostat with fine-tuned control, but is mostly going as fast as it can until the local ISM is either blown out of the disk or depleted altogether.
The Symposium on ``Triggered Star Formation in a Turbulent Medium'' addresses all of these issues with invited and contributed talks and posters by the world's leading theoreticians and observers. It includes triggering by direct compression from older stars (``sequential star formation''), turbulence compression, and galactic processes such as spiral shocks, and it considers the after-effects of star formation such as heating, shell creation, and disk blowout, which provide some type of feedback.
The detailed star formation processes at high z are addressed. Is star formation in the first galaxies the same as it is locally? What is the evidence for the actual star formation mechanisms in high-redshift galaxies? Can we calibrate local star formation with thresholds, Schmidt laws and efficiencies so that it becomes predictable in diverse environments? Does the role of feedback change as galaxies build up their mass over cosmological time? How important is triggering from galaxy interactions in the modern Universe and over a Hubble time? Note that these questions concern the mechanisms of star formation in each galaxy or proto-galaxy, rather than the average rate among all galaxies in the early Universe.
Other questions to be addressed concern the structure of star forming regions. Is it hierarchical or fractal? Is cluster formation universal? A central issue is to identify the conditions required for the formation of proto-clusters in a small fraction of the volume of GMCs. In particular, are the initial conditions for the clustered mode of SF fundamentally different from the conditions for the distributed mode of SF? Is triggering involved for cluster formation? Answering these questions requires a global view of GMCs at mm/sub-mm wavelengths (in both the dust continuum and appropriate molecular line tracers) with a high spatial dynamic range. What fraction of stars form in dense clusters? What are the mass distributions of clouds, clusters and stars, and why are they so similar? How can the differences be explained?
New instruments like the large-format bolometer arrays SCUBA, MAMBO, and LABOCA will provide wide-field mapping of molecular clouds at millimeter and sub-millimeter wavelength. These maps should show the internal structures of molecular cores and the relations between this structure, the pre-stellar cores, and the pre-main sequence stars. Is there sequential triggering even inside a molecular core? Are there correlations between star mass and position in a core, or between star mass and the morphology of gas structure? Even higher resolution will be achieved with ALMA, which will start its operation with 8 antenna only one year after this conference. ALMA and CARMA should see proto-brown dwarfs and their associated gas in the nearest molecular clouds. Observational studies with the VLT and other ground-based telescopes will provide a nearly complete census of young stars in many nearby clusters. Near-infrared interferometers like the VLTI will study the ionized environment of massive stars, and the Herschel Space Observatory (to be launched in 2007) will provide unique wide-field mapping capabilities (at an angular resolution comparable to or better than SCUBA, MAMBO, and LABOCA depending on wavelength) in the key 75-500 micron range, where embedded GMCs, pre-stellar cores, and proto-stars emit most of their luminosities. Infrared observations with the Spitzer Space Telescope will have contributed greatly to our understanding of star formation by the time of this Symposium, as will X-ray studies with XMM Newton and Chandra satellites.