CHARACTERIZING THE HEAVY ELEMENTS IN GLOBULAR CLUSTER M22 AND AN EMPIRICALs-PROCESS ABUNDANCE DISTRIBUTION DERIVED FROM THE TWO STELLAR GROUPS

We present an empirical s -process abundance distribution derived with explicit knowledge of the r -process component in the low-metallicity globular cluster M22. We have obtained high-resolution, high signal-to-noise spectra for six red giants in M22 using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory. In each star we derive abundances for 44 species of 40 elements, including 24 elements heavier than zinc ( Z = 30) produced by neutron-capture reactions. Previous studies determined that three of these stars (the " r + s group") have an enhancement of s -process material relative to the other three stars (the " r -only group"). We confirm that the r + s group is moderately enriched in Pb relative to the r -only group. Both groups of stars were born with the same amount of r -process material, but s -process material was also present in the gas from which the r + s group formed. The s -process abundances are inconsistent with predictions for asymptotic giant branch (AGB) stars with M ⩽ 3 M ☉ and suggest an origin in more massive AGB stars capable of activating the 22 Ne(α, n ) 25 Mg reaction. We calculate the s -process "residual" by subtracting the r -process pattern in the r -only group from the abundances in the r + s group. In contrast to previous r - and s -process decompositions, this approach makes no assumptions about the r - and s -process distributions in the solar system and provides a unique opportunity to explore s -process yields in a metal-poor environment.