Tuesday, June 11, 8:30-12:30, 2:00-5:30
The interstellar medium (ISM) plays a fundamental role in Galactic evolutionary processes. The energetics of the ISM are mostly controlled by the flow of energy from stars in the form of radiation, winds, violent explosions and cosmic rays. The organization of the ISM is strongly influenced by this interaction and the observed structures therefore provide important clues about Galactic evolutionary processes. This session will survey the current state of understanding of the structure of the diffuse ISM (outside of molecular clouds) and current thoughts on important physical processes controlling that structure.
The ISM contains cold, warm, and hot gas components believed, on average, to be in rough pressure equilibrium. What are the processes controlling their physical conditions? How do the components interact with each other? How much space is filled by each, and how does that distribution depend on position in the Galaxy? Is there a Galactic fountain spewing gas into the halo, and if so, what kind--driven by a pervasive hot phase, flowing out of superbubbles and chimneys, or carried by escaping cosmic rays? What roles do magnetic fields and cosmic rays play in controlling ISM structures? These are some of the questions that need answers.
As the observational and theoretical tools have improved, however, it has become clear that many of the answers to the fundamental questions are still beyond our grasp. As a result, this session will concentrate heavily on what is known from observations. Examples of questions that will be dealt with directly include: What is the significance of the cloud shadowing of X-rays found with ROSAT, or of EUV by EUVE? How is the H I arranged in the solar neighborhood? How pervasive are bubbles and superbubbles? What is known about the origin of the ionization leading to the diffuse H alpha emission? What have all the high stage ions seen in absorption told us about the spatial distribution of hot gas in the Galaxy? What diagnostic tool is provided by measurements of component pressures that are significantly different from the norm? How can the very dense features seen in the time varying 21 cm column densities toward pulsars possibly be rationalized with the other things we think we know about the ISM? The observational talks will mention theories that have been proposed as explanations for what is seen, but often highlighting their difficulties and the corresponding puzzles. One talk will review a broad set of theories to provide a consistent context, another will hammer at things that we may have overlooked, and finally, one brave soul will try to tell us what he or she thinks it all means.
Each talk will consist of a broad review of its subject, aimed at nonspecialists. It will include well known aspects, puzzles, anticipated forthcoming enlightenment on the puzzles, and suggestions for further observational or theoretical work. Links will also be made to relevant posters, to be shown the following day.
Introduction:
There will be an accompanying poster session on Wednesday. Persons submitting related poster abstracts to the AAS are invited to request association with this session. Speakers will be encouraged to point out posters related to their topic.
This session's organizing committee is chaired by Don Cox (608-262-5916, cox@wisp.physics.wisc.edu). Other members are Ed Jenkins, Jay Lockman, Colin Norman, Dave Hollenbach, Rich Rand, Blair Savage, John Mathis, Linda Sparke, Robin Shelton, and Dan McCammon.