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

[51.13] Hubble Space Telescope Observations of an Intermediate-High Velocity Cloud in the Low Galactic Halo

K. R. Sembach (Johns Hopkins University), F. Keenan, R. Ryans (Queen's Univ. - Belfast)

For many years it has been realized that the existence of intermediate and high velocity clouds in the Milky Way can provide information about the nature of the gaseous halo that surrounds our Galaxy. Despite this realization, there is still very little known about the chemical compositions, physical properties, and origins of most interstellar clouds that fall into these categories. Furthermore, the relationships (or lack thereof) between intermediate velocity and high velocity clouds themselves are poorly defined, though at least in some cases it does appear that the intermediate velocity clouds may be more confined to the Galactic plane than their higher velocity counterparts. Those clouds that have velocities near the arbitrary velocity cutoff to be considered high velocity (|v| \sim 100 km/s) are particularly worthy of study, since they may contain clues to help distinguish between the competing physical processes responsible for separating (or linking) clouds in these two velocity regimes.

Using the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope, we have obtained high quality spectra (S/N > 50, R \sim 20,000-85,000) of a large number of atomic transitions observable in the intermediate-high velocity cloud in the direction of HD 203664 (l = 61.9, b = -27.5). This cloud has a velocity (LSR) of about +80 km/s and displays absorption in a wide range of ionization stages. Much of the observed absorption may arise in an envelope of gas that extends across the face of the globular cluster M 15, located \sim3 degrees away from the sight line. In this poster, we provide comparisons of the various ionization stages observed and their velocity distributions, information about the chemical and physical properties of the cloud, and some comments about the possible origins of such clouds.

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