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Session 37 - Pierce Prize Lecture.
Invited session, Tuesday, January 16
1st Floor, La Villita Assembly Building

[37.01] Chemistry of the Galactic Bulge and Halo

A. McWilliam (Carnegie Observatories)

The first detailed abundance analysis of a sample of Galactic bulge K giant stars in Baade's Window was performed, based on high S/N echelle spectra. Contrary to the previously accepted picture, that the bulge is super-metal-rich (SMR), we find that the mean bulge [Fe/H] is almost identical to the mean [Fe/H] of solar neighborhood red giant stars. The most metal-rich bulge giant has the same [Fe/H] as \mu Leo, a well known SMR disk giant.

Bulge giants possess an unusual \alpha-element abundance pattern: enhancements of Ti and Mg, but normal Si and Ca. This abundance pattern identifies the bulge as a chemically distinct component of the Galaxy, and indicates that nucleosynthesis of the bulge differed from the solar neighborhood; probably due to a dispersion in supernova (SN) \alpha-element yields and a different mixture of SN flavors in the bulge than the disk. The enhanced Ti abundances qualitatively explain the observation that bulge M giants have later spectral types than solar neighborhood M giants of the same temperature. The enhanced Mg abundances are reminiscent of the Mg enhancements found in elliptical galaxies.

We have also performed abundance analysis of 33 extremely metal poor halo stars (-4.0 \le[Fe/H]\le -2.0), utilizing echelle spectra acquired at Las Campanas Observatory. We find previously unnoticed trends of [Cr/Fe], [Mn/Fe], and [Co/Fe] with [Fe/H]. The trends indicate that the yields of these elements are not the same for all type II SN and that there was a metallicity-dependence of the mixture of SN flavors at low metallicity. We also find a huge intrinsic dispersion of heavy-element abundances relative to iron, and heavy-element abundance patterns consistent with r-process nucleosynthesis; similar to the results of Gilroy et al. (1988, ApJ, 327, 298). We attribute the large spread of heavy-element to iron ratios to a dispersion of type II SN flavors, with vastly different heavy-element to iron yield ratios, and the absence of efficient mixing when the Galaxy was very metal-poor. The heavy-element abundance dispersion provides information about the mass of the chemically evolving Galactic sub-units and the number of SN which participated in individual chemical enrichment events at low metallicity.

We identify one star, CS 22892--052, with 40 times the solar Eu/Fe ratio, and conclude that the heavy elements in this star are dominated by a single SN event. Another interesting star is CS 22949--037, with an abundance pattern which appears to be depleted in iron-peak elements, by \sim 1 dex, relative to elements of lower and higher atomic number.

Program listing for Tuesday