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3.2. Element Ratios

Element abundance ratios are another important piece of information, since the various elements are synthesized in stars with different masses and lifetimes. The alpha-element/Fe ratio, in particular is a good diagnostic, because the alpha-capture elements are produced mainly in very massive stars and expelled into the ISM by Type II or Ib,c supernovae, while Fe is mainly produced in Type I supernovae by longer-lived stars. Thus the alpha/Fe ratio is an indicator of how rapidly star formation and metal enrichment occurred within a system: high alpha/Fe indicates enrichment over short time scales, possibly in starburst events, while low alpha/Fe may indicate enrichment over much longer time scales, as in systems with roughly constant star formation rate over their lifetimes.

Of current interest is the question of whether the Galactic halo was formed in a monolithic collapse (Eggen, Lynden-Bell & Sandage 1962) or was aggregated from mergers of smaller sub-units (Searle & Zinn 1978). It is speculated that the nearby dE satellites may be representative of those sub-units.

It is clear from their CMDs that many of the Milky Way satellites are not like the halo population, since they contain stars that are much young than those in the halo. At the same time, we know that the Sagittarius dE is being tidally disintegrated by the Galaxy and will become part of the halo. Although systems like Fornax and Carina appear not to be representative of the current halo populations, much less complex systems like Ursa Minor and Draco could be similar to the structures out of which the halo formed.

The test of this possibility is that the ages and compositions of stars in the dEs are similar to those in the halo. It is known that halo stars show elevated [alpha/Fe] compared to disk stars, reflecting a dominant nucleosynthesis contribution from massive stars, while [Ba/Eu] is low in halo stars, indicating that the s-process (which is the main source of Ba) has not had sufficient time to contribute to the abundances in halo stars, while the r-process (the main source of Eu) dominates in metal-poor stars. Indeed, very metal-poor halo giants show evidence for a purely r-process contribution to the abundances of heavy neutron capture elements (Sneden et al. 2000).

Little high-resolution spectroscopy of giants in even the nearest dEs have been carried out because of the faintness of the stars (16th-20th magnitude). However, the first such studies have become available due to the availability of the 10-m Keck telescopes (Shetrone, Coté, & Sargent 2001). Shetrone et al. have obtained high-resolution spectra for 5-6 stars in each of the Draco, Ursa Minor, and Sextans galaxies, deriving abundances for a variety of elements. The comparison of element abundance ratios in these stars show a puzzling mixed bag of results. Although Shetrone et al. argue that [alpha/Fe] is low in the dEs compared to halo stars, closer examination shows that [Ca/Fe] and [Ti/Fe] do appear to be lower, but [Mg/Fe] and [Si/Fe] appear to agree with halo ratios. Meanwhile, [Ba/Eu] values in the dEs appear to be in good agreement with those in halo stars. The comparison between dEs and halo stars seems to be inconclusive at the present time, perhaps not surprising given the small samples of stars at present. The samples of stars for each galaxy certainly need to be enlarged to determine the evolution of element ratios in these galaxies. Nevertheless, the Shetrone et al. (2001) study illustrates the power of high-resolution spectroscopy with the new large telescopes. More work of this nature is highly encouraged.

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