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The s-process enrichment of the globular clusters M4 and M22

Shingles, Luke; Karakas, Amanda; Hirschi, Raphael; Fishlock, Cherie; Yong, David; Da Costa, Gary; Marino, Anna

Description

We investigate the enrichment in elements produced by the slow neutron-capture process (s-process) in the globular clusters M4 (NGC 6121) and M22 (NGC 6656). Stars in M4 have homogeneous abundances of Fe and neutron-capture elements, but the entire cluster is enhanced in s-process elements (Sr, Y, Ba, Pb) relative to other clusters with a similar metallicity. In M22, two stellar groups exhibit different abundances of Fe and s-process elements. By subtracting the mean abundances of s-poor from...[Show more]

dc.contributor.authorShingles, Luke
dc.contributor.authorKarakas, Amanda
dc.contributor.authorHirschi, Raphael
dc.contributor.authorFishlock, Cherie
dc.contributor.authorYong, David
dc.contributor.authorDa Costa, Gary
dc.contributor.authorMarino, Anna
dc.date.accessioned2015-12-13T22:32:46Z
dc.identifier.issn0004-637X
dc.identifier.urihttp://hdl.handle.net/1885/75723
dc.description.abstractWe investigate the enrichment in elements produced by the slow neutron-capture process (s-process) in the globular clusters M4 (NGC 6121) and M22 (NGC 6656). Stars in M4 have homogeneous abundances of Fe and neutron-capture elements, but the entire cluster is enhanced in s-process elements (Sr, Y, Ba, Pb) relative to other clusters with a similar metallicity. In M22, two stellar groups exhibit different abundances of Fe and s-process elements. By subtracting the mean abundances of s-poor from s-rich stars, we derive s-process residuals or empirical s-process distributions for M4 and M22. We find that the s-process distribution in M22 is more weighted toward the heavy s-peak (Ba, La, Ce) and Pb than M4, which has been enriched mostly with light s-peak elements (Sr, Y, Zr). We construct simple chemical evolution models using yields from massive star models that include rotation, which dramatically increases s-process production at low metallicity. We show that our massive star models with rotation rates of up to 50% of the critical (break-up) velocity and changes to the preferred 17O(α, γ)21Ne rate produce insufficient heavy s-elements and Pb to match the empirical distributions. For models that incorporate asymptotic giant branch yields, we find that intermediate-mass yields (with a 22Ne neutron source) alone do not reproduce the light-to-heavy s-element ratios for M4 and M22, and that a small contribution from models with a 13C pocket is required. With our assumption that 13C pockets form for initial masses below a transition range between 3.0 and 3.5 M ⊙, we match the light-to-heavy s-element ratio in the s-process residual of M22 and predict a minimum enrichment timescale of between 240 and 360 Myr. Our predicted value is consistent with the 300 Myr upper limit age difference between the two groups derived from isochrone fitting.
dc.publisherIOP Publishing
dc.rightsAuthor/s retain copyright
dc.sourceAstrophysical Journal, The
dc.titleThe s-process enrichment of the globular clusters M4 and M22
dc.typeJournal article
local.description.notesImported from ARIES
local.identifier.citationvolume795
dc.date.issued2014
local.identifier.absfor030600 - PHYSICAL CHEMISTRY (INCL. STRUCTURAL)
local.identifier.ariespublicationU3488905xPUB4763
local.type.statusPublished Version
local.contributor.affiliationShingles, Luke, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationKarakas, Amanda, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationHirschi, Raphael, University of Tokyo
local.contributor.affiliationFishlock, Cherie, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationYong, David, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationDa Costa, Gary, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationMarino, Anna, College of Physical and Mathematical Sciences, ANU
local.bibliographicCitation.issue1
local.bibliographicCitation.startpage34/1
local.bibliographicCitation.lastpage12
local.identifier.doi10.1088/0004-637X/795/1/34
dc.date.updated2015-12-11T09:10:25Z
local.identifier.scopusID2-s2.0-84908025330
local.identifier.thomsonID000343857300034
dcterms.accessRightsOpen Access
CollectionsANU Research Publications

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