AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 63. Evolution of Chemical Abundances over Cosmic Time
Topical, Oral, Wednesday, June 2, 1999, 8:30-10:00am, 10:45am-12:30pm, 2:00-3:30pm, 3:45-5:30pm, Continental Ballroom A

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[63.05] Relative Abundances in the Milky Way and the Magellanic Clouds

D. E. Welty (U. Chicago)

Gas phase abundances for many atoms and ions may be obtained for interstellar and intergalactic media ranging from nearby IS clouds to QSO absorption-line systems (ALS) at z > 4. In principle, we may thus trace the evolution of the abundances of many elements, due to different nucleosynthetic processes, over a fair fraction of the history of the universe --- if we can account for the effects of ionization and dust depletion.

Recent observational and atomic data are providing more detailed, accurate, and extensive abundances for IS clouds in different environments in the Galactic disk, in the halo, and in (or associated with) the Magellanic Clouds. At metallicities below solar by about 0.3 and 0.6 dex, respectively, the LMC and SMC provide key intermediate steps between our Galaxy and the even lower metallicity QSOALS. For the Milky Way ISM, these new data suggest that 1) the total (gas + dust) abundances of at least some elements may be non-solar, and 2) the element-to-element pattern and the overall level of the depletions appear to be related. Corrections to inferred relative abundances (e.g., [X/Zn]) for unobserved ions are relatively minor (or can be estimated) in many cases. Analyses of stellar and nebular spectra suggest that the relative abundances of many elements are similar for comparable objects in the MW, LMC, and SMC (though there can be differences for N and some r- and s-process elements). There is no significant enhancement of the \alpha-elements, relative to the ``Fe-peak'' elements, for the MC stars. While the IS extinction curves and dust-to-gas ratios found in the MC are generally different from those typically seen in our Galaxy, several recent studies of the MC ISM indicate that the patterns of relative gas phase IS abundances (and thus the depletion patterns) are similar to those found for clouds in the disk or halo of the MW. If these depletion patterns do not depend on metallicity (as appears to be the case locally), then they may be used to estimate total elemental abundances for individual QSOALS.

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