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Session 3 - Clusters.
Display session, Monday, June 10
Tripp Commons,
Sodium and magnesium abundances have been obtained from a sample of over 100 giant stars in the globular cluster M13 from spectra obtained using the KPNO 4-m Hydra fiber positioner and bench spectrograph. The stars observed in M13 range from the faintest at M_V \simeq +1.0 and log g \simeq 2.6 up to the tip of the giant branch, and include 18 stars on the asymptotic giant branch. Among the lower luminosity giants, the sodium abundances have a large star-to-star range, approximately from --0.3 \leq [Na/Fe] \leq +0.5. However, the sodium abundances of the most luminous giants (M_V < --1.7) are usually high; typically, [Na/Fe] \geq +0.3, with a much smaller star-to-star scatter. The asymptotic giant branch stars have smaller sodium abundances on average than do the red giant branch tip stars. Magnesium is uniformly overabundant ([Mg/Fe] \simeq +0.3) in all stars with a low sodium abundance, but the [Mg/Fe] ratio ranges from approximately --0.3 to +0.3 in stars with a high sodium abundance. These sodium and magnesium abundance variations in M13 are discussed in the context of proton capture and deep mixing hypotheses. In addition to the CN and ON hydrogen burning chains previously discussed in the literature, the NeNa and MgAl burning chains have also contributed to the abundance mixture observed in M13 giants. At least some of the products of proton capture chains have been produced in situ in the giants, and brought to the surface, most probably via deep mixing. Evidence in support of the occurrence of proton capture nucleosynthesis and deep mixing among M13 stars includes 1) the absence of sodium-poor stars at the red giant tip, 2) the fact that asymptotic branch stars have lower sodium abundances on average than do stars near the red giant tip, and 3) the existence of a positive correlation between sodium and nitrogen abundances as well as a partial anti-correlation of sodium and magnesium abundances.
