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I.I. Ivans (UT-Austin & McDonald Obs.), R.P. Kraft (UCSC & Lick Obs.), C. Sneden (UT-Austin & McDonald Obs.), G.H. Smith (UCSC & Lick Obs.)
Among the bright giants of a number of globular clusters, large star-to-star abundance variations are observed in elements C, N, O, Na, Mg, and Al. Some of these variations have been attributed to proton-capture nucleosynthesis in the stars themselves during the giant phase of their evolution. How far down the giant branch can the variations of different elements be observed? To explore this issue, we have employed the Keck HIRES to obtain high signal-to-noise data for over 20 giant stars of fainter magnitudes than have been previously observed at high resolution in M5. Indeed, the infamous anti-correlation of oxygen and sodium abundances persists in this cluster, down to at least MV < -0.3.
In addition to star-to-star abundance variations, cluster-to-cluster variations in the overall means and ranges of abundances are also observed. M5 has often been considered a ``twin'' to globular cluster M4, in age, in metallicity, and in chemical composition. However, while we do find similar [Fe/H] ratios in these clusters (M5 ~ -1.3 and M4 ~ -1.2), we also find that the [Al/Fe], [Si/Fe], [Ba/Fe], and [La/Fe] ratios in M5 appear to be a factor of 2 (or more) smaller than they are in M4. This is not a small sample effect; dozens of stars have now been observed at high resolution in each of these clusters. We claim that these abundance differences imply a primordial or multi-generational effect: M5 and M4 have undergone very different nucleosynthetic histories, prior to and/or subsequent to cluster formation. This poster discusses the new M5 abundance results in the context of both primordial and intrinsic nucleosynthetic sites.
This research is supported by NSF grants AST-9618351 and AST-9618364.
The author(s) of this abstract have provided an email address for comments about the abstract: iivans@astro.as.utexas.edu