[52.10] What EUV Spectroscopy can tell us about the Local Interstellar Medium

AAS 200th meeting, Albuquerque, NM, June 2002
Session 52. The Future of Extreme Ultraviolet Astronomy
Topical Session Oral, Wednesday, June 5, 2002, 8:30-10:00am, 10:45am-12:30pm, 2:30-4:00pm, 4:15-6:00pm, Morning in Ballroom A, Afternoon in Ballroom B

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[52.10] What EUV Spectroscopy can tell us about the Local Interstellar Medium

J.L. Linsky (JILA, University of Colorado & NIST)

Fundamental questions concerning the interstellar gas in our local region of the Galactic disk include: (1) the degree of ionization of hydrogen and helium and the source(s) of the ionization, (2) the geometrical structure of the various components of the local interstellar medium, and (3) the physical explanation for the existence and properties of these structures in the dynamic environment of the Galactic disk. The extreme ultraviolet spectrum from 912A to 100A and below contains essential spectroscopic diagnostics for measuring the ionization of hydrogen and helium along lines of sight through the Local Bubble and beyond, out to several hundred parsecs in some directions. Pioneer measurements of the column densities of neutral hydrogen, neutral helium, and ionized helium by EUVE provide essential data for addressing these three questions, but the sensitivity and spectral resolution of EUVE limited its contributions to answering these questions. Measurements of the hydrogen column densities from the Lyman continuum absorption are insensitive to the presence of multiple velocity components along the line of sight, and thus are valuable tests of column densities inferred from analyses of Lyman line absorption by STIS and FUSE.

A future EUV spectrometer with greater sensitivity will be able to measure accurate hydrogen column densities to 10²⁰ cm^-2. A future instrument with higher spectral resolution should also be able to measure column densities from the resonance lines of HeI and HeII. Also, accurate measurements of the relative strengths of stellar coronal emission lines originating from common upper levels should provide accurate measurements of the hydrogen and helium column densities along the lines of sight to late-type stars.

This work is supported by NASA through grant S-56500-D to NIST and the University of Colorado.

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