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Session 15 - Russell Prize Lecture.
Oral session, Monday, June 09
North Main Hall A,
One of the most active fields of investigation at the present time is the series of events that starts with the formation of cores in molecular clouds, follows their collapse to form stars with accompanying disks, and then follows the set of processes that can occur in such disks to form planets. The initial part of the story is investigated by astronomers and the latter part by geoscientists, who rarely communicate with one another because of the jargon barrier. Here I shall discuss some of the interdisciplinary connections. Molecular cloud cores form as a result of ambipolar diffusion which carries neutral molecules across magnetic field lines. A simple expectation would then be that the gas would flow along the field lines to form a pancake configuration, but observations show that the cores are strongly elongated along the field lines instead. Since the radial velocities of the molecules are also nonthermal, it follows that the energy content of the dense interstellar medium is dominated by MHD waves, except at the centers of cores where the velocities are thermal. If no other major processes enter, then continued shrinkage of the cores accelerates the collapse toward free fall conditions, but the total time interval well exceeds a million years. However, in the case of the solar system, we find primitive meteoritic materials containing the decay products of extinct radioactivities with mean lives as short as 150,000 years. In our case at least, core collapse was quicker. I shall discuss star formation triggered by shocks, probably primarily from supernova explosions a few parsecs away, and give other evidence that such shocks may dominate the total star formation process. This should also produce great diversity in planetary systems that form in such events.
