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Session 49 - The Frontiers of Far Ultraviolet Astrophysics - I.
Topical, Oral session, Wednesday, June 10
Atoms that are collisionally ionized to high charge levels can serve as important tracers of very hot, diffuse gases in space. The element oxygen is particularly useful because it has a large cosmic abundance, and at a charge of +5 it becomes a lithium-like ion with a very strong resonance doublet in the far uv (\lambda\lambda 1032, 1038ÅIn collisional ionization equilibrium O VI exhibits a peak fractional abundance of 0.2 at T\approx 300,000K, and under most circumstances it is unlikely to be created by photoionization. Fundamental properties of hot gases in the disk and lower halo of our Galaxy have been derived from surveys of O VI absorption in the spectra of early-type stars, providing a useful supplement to information derived from studies of the diffuse, soft x-ray emission. Moreover, absorption by O VI occasionally has been identified with distant gas systems in front of quasars. Unfortunately, we have not yet performed a large-scale, systematic survey of the radiation emitted by O VI, which turns out to be an important mode for radiative cooling at the relevant temperatures.
A study of O VI absorption planned for FUSE will extend to stars out to a distance of about 4 kpc in the Galactic plane and even farther in the Galactic halo. Compared with earlier results from the Copernicus satellite that were limited to stars often nearer than 1 kpc, new conclusions on the distribution and properties of the hot gas will be less strongly influenced by the persistent contribution from the local material surrounding us, or by gases heated by the interaction of the target stars' stellar winds with their surroundings. In addition, the new survey should provide complete coverage over a broad range of radial velocities, and special attempts will be made to register very shallow absorption features with extraordinarily large velocity dispersions.
Program listing for Wednesday