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Session 51 - Interstellar Medium II.
Display session, Thursday, January 08
Exhibit Hall,

[51.01] Limits on column density variations of the Local Interstellar Medium on AU size spatial scales

J. Vallerga (UC, Berkeley), J. Bloch (LANL)

Using the Extreme Ultraviolet Explorer (EUVE), we can detect a change in the hydrogen column density of the Local Cloud on the order of 1scale of 2 A.U. by observing the spectrum of the same star twice, separated in time by 5 to 6 months. We will report on the results of two such observations of two stars, the B2III star Adhara (\epsilon CMa) and the hot white dwarf G191 B2b. Adhara is the brightest EUV source at wavelengths longer than 504Å\ where the H cross section is at its greatest, while G191B2b has the strongest flux over the neutral He Lyman continuum for a sensitive neutral He column measurment. The key to achieving this sensitivity is the differential nature of the measurement, which requires a source with constant flux and two identical instrument configurations for both observations.Other than the assumption of constant flux, a detailed understanding of the spectral shape of the EUV source is not required. Either a detection of variable absorption or an upper limit at these previously unexplored scale lengths will place important constaints on the coherent spatial structure of warm interstellar "clouds".

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