Session 49 - The Frontiers of Far Ultraviolet Astrophysics - I.
Topical, Oral session, Wednesday, June 10
Sierra/Padre,

[49.07] Far Ultraviolet Observations of the Neutral Interstellar Medium

U. J. Sofia (Villanova University)

The far ultraviolet (FUV) spectral region contains transitions for the dominant ions of many abundant species in the neutral interstellar medium (ISM). These ions include H \sc i, D \sc i, C \sc ii, N \sc i, O \sc i, Mg \sc ii, Si \sc ii, P \sc ii, Ar \sc i and Fe \sc ii. These species are useful for studying diffuse interstellar clouds and the physical and chemical characteristics of the dust which exists in them. The Ar \sc i absorption features at 1048 and 1066Åare the only argon lines available which can be used to determine the neutral argon abundance in the ISM. When compared to the neutral hydrogen abundance along a sightline, the neutral argon abundance may be a useful diagnostic for determining which process, photoionization or collisional ionization, is responsible for partially ionizing any gas in a primarily neutral region. This is possible because of the large photoionization cross section of neutral argon. This suggestion will have to be tested by comparing n(e) to N(Ar \sc i)/N(H \sc i) in interstellar clouds which are abundant in free electrons. This type of analysis may also allow one to place any observed higher ion species in either classical H \sc ii regions or partially ionized neutral interstellar clouds. Results from IMAPS and Copernicus will be discussed.