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Session 47 - The Diffuse Interstellar Medium, Progress and Puzzles - II.
Oral session, Tuesday, June 11
Historical Society,
Observations of highly ionized gas in the interstellar medium have much to tell us about the structure of the ISM, its thermal properties, and the interactions between massive stars and gas in the Galaxy. Triply ionized atoms such as SI IV and C IV and higher ionization Li-like species such as N V and O VI can only be observed from space since the protective blanket of the Earth's atmosphere absorbs the far-ultraviolet light that contains information about these ions. In the 1970s, the Copernicus satellite provided our first glimpse into the nature of the highly ionized gas traced by O VI and opened a new window for exploration of the properties of the ionized ISM through absorption line spectroscopy. Since then the International Ultraviolet Explorer, Hubble Space Telescope, and a host of smaller successful space missions have continued to improve our understanding of the complex nature of highly ionized gas in galactic environments. The high ions trace gases having temperatures from ~10,000 K to ~200,000 K. They provide a link between the diffuse H-alpha emitting gas of the warm ionized medium and the X-ray producing gas of the very hot (10^6 K) ISM. These "transition temperature" ions also play a key role in understanding the similarities and differences between the extended gaseous halos of the Milky Way and distant galaxies since they can be viewed in absorption in the directions of background quasars. In this talk I will discuss the observational signature of the high ions, their distribution within the disk and halo of the Galaxy, the basic properties of the highly ionized gases they trace, and their significance.
