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Session 84 - IGM.
Display session, Friday, January 09
We have found an isolated cloud in a z=1.94 C \sc iv quasar absorption line system whose aluminum to silicon abundance ratio is larger than solar. Such a pattern, which is difficult to understand in current scenarios of chemical evolution, has never been detected in any interstellar cloud in our Galaxy. Since Al depletes onto dust grains more severely than Si, this result is accentuated when dust depletion is taken into account. Our findings are based upon CLOUDY photoionization modeling of the Si \sc iv, Al \sc ii, and Al \sc iii column densities, and of a limit on the Si \sc ii column density. We compare this unusually Al--rich cloud to another cloud in the same system that is more typical of Galactic abundance patterns. We present comparisons to other high redshift systems with detected Al, both from our HIRES/Keck data and from a damped system observed by Lu et al. (1996). The latter also has a single Al--enhanced cloud in the midst of a larger multiple cloud system. We speculate as to the processes/conditions that could give rise to such an Al--rich abundance pattern. At high redshifts (z \sim 2) there may be Al--rich clouds to which there is no counterpart at low redshift.
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