The s-Process Enrichment of the Globular Clusters M4 and M22

Abstract

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 $s$-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 ${}^{17}$O($\alpha$,$\gamma$)${}^{21}$Ne rate produce insufficient heavy $s$-elements and Pb to match the empirical distributions. For models that incorporate AGB yields, we find that intermediate-mass yields (with a ${}^{22}$Ne 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 ${}^{13}$C pocket is required. With our assumption that ${}^{13}$C pockets form for initial masses below a transition range between 3.0 and 3.5 M${}_{\odot }$, 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.

arXiv comment13 pages. Accepted for publication in ApJ

References in corpus (87)

• The chemical composition of the Sun
• Pre-Supernova Evolution of Massive Single and Binary Stars
• Neutron capture in low mass Asymptotic Giant Branch stars: cross sections and abundance signatures
• The evolution of rotating stars
• Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra
• Updated stellar yields from Asymptotic Giant Branch models
• Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters
• A Triple Main Sequence in the Globular Cluster NGC 2808
• Intrinsic iron spread and a new metallicity scale for Globular Clusters
• The O-Na and Mg-Al Anticorrelations in Turn-Off and early Subgiants in Globular Clusters
• The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon
• Evolution, nucleosynthesis and yields of low mass AGB stars
• Evolution, nucleosynthesis and yields of low mass AGB stars at different metallicities (II): the FRUITY database
• The early star generations: the dominant effect of rotation on the CNO yields
• The Rise of the s-Process in the Galaxy
• Spectroscopic and Photometric evidence of two stellar populations in the Galactic Globular Cluster NGC6121 (M4)
• Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139)
• The Abundances of Neutron Capture Species in the Very Metal-Poor Globular Cluster M15: An Uniform Analysis of RGB and RHB Stars
• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
• Stellar Turbulent Convection: A Selfconsistent Model
• Super-Massive Stars as a Source of Abundance Anomalies of Proton-Capture Elements in Globular Clusters
• Yields of AGB and SAGB models with chemistry of low- and high-metallicity Globular Clusters
• Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-Poor Globular Cluster M4
• The s process in asymptotic giant branch stars of low metallicity and the composition of carbon-enhanced metal-poor stars
• Very low metallicity massive star models: Pre-SN evolution and primary nitrogen production
• A strong case for fast stellar rotation at very low metallicities
• Non-standard s process in low metallicity massive rotating stars
• The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars
• The two metallicity groups of the globular cluster M22: a chemical perspective
• Advanced burning stages and fate of 8-10 Mo stars
• New Analyses of Star-to-Star Abundance Variations Among Bright Giants in the Mildly Metal-Poor Globular Cluster M5
• Asymptotic Giant Branch evolution at varying surface C/O ratio: effects of changes in molecular opacities
• Multiple stellar populations in the globular cluster NGC 1851
• An abundance analysis of bright giants in the globular cluster NGC 1851
• Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
• Light nuclei in galactic globular clusters : constraints on the self-enrichment scenario from nucleosynthesis
• s-Process Nucleosynthesis in Advanced Burning Phases of Massive Stars
• Iron and neutron-capture element abundance variations in the globular cluster M2 (NGC 7089)
• The Bizarre Chemical Inventory of NGC 2419, An Extreme Outer Halo Globular Cluster
• Modelling self-pollution of globular clusters from AGB stars
• s-Process in Low Metallicity Stars. III. Individual analysis of CEMP-s and CEMP-s/r with AGB models
• The effects of rotation on the s-process nucleosynthesys in Asymptotic Giant Branch stars
• Sodium-Oxygen anticorrelation among Horizontal Branch Stars in the Globular Cluster M4
• A Lower Limit on the Age of the Universe
• More lead stars
• Presupernova evolution and explosive nucleosynthesis of zero metal massive stars
• The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars
• Lead: Asymptotic Giant Branch production and Galactic Chemical Evolution
• The Uncertainties in the 22Ne + alpha-capture Reaction Rates and the Production of the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars of Intermediate Mass
• Heavy Element Nucleosynthesis in the Brightest Galactic Asymptotic Giant Branch stars
• Detailed abundances of Red Giants in the Globular Cluster NGC~1851: C+N+O and the Origin of Multiple Populations
• A new imprint of fast rotators: low 12C/13C ratios in extremely metal-poor halo stars
• Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars
• Dredge-up and envelope burning in intermediate mass giants of very low metallicity
• 3D hydrodynamical simulations of a proton ingestion episode in a low-metallicity asymptotic giant branch star
• Hot Stars in Globular Cluster - A Spectroscopist's View
• Reaction rates for the s-process neutron source 22Ne+α
• The double sub-giant branch of NGC 6656 (M22): a chemical characterization
• Heavy element abundances in giant stars of the globular clusters M4 and M5
• Ba stars and other binaries in first and second generation stars in globular clusters
• Star Formation Histories of Globular Clusters with Multiple Populations. I. ω Cen, M22, and NGC 1851
• Post-AGB stars in the SMC as tracers of stellar evolution: the extreme s-process enrichment of the 21 micrometer star J004441.04-732136.4
• The self-enrichment scenario in intermediate metallicity globular clusters
• Constraints on the ${}^{22}Ne(\alpha ,n{)}^{25}$Mg s-process neutron source from analysis of ${}^{nat}Mg+n$ total and ${}^{25}Mg(n,\gamma )$ cross sections
• A Comparison of Copper Abundances in Globular Cluster and Halo Field Giant Stars
• Formation of Multiple Populations in Globular Clusters: constraints on the dilution by pristine gas
• Heavy elements in Globular Clusters: the role of AGB stars
• Dependence of S-Process Nucleosynthesis in Massive Stars on Triple-Alpha and 12C(a,g)16O Reaction Rate Uncertainties
• Rubidium and lead abundances in giant stars of the globular clusters M4 and M5
• Characterizing the Heavy Elements in Globular Cluster M22 and an Empirical s-process Abundance Distribution Derived from the Two Stellar Groups
• O and Na abundance patterns in open clusters of the Galactic disk
• Rubidium and lead abundances in giant stars of the globular clusters M 13 and NGC 6752
• Towards a working model for the abundance variations within Globular Clusters stars
• Lithium abundances in red giants of M4: evidence for asymptotic giant branch star pollution in globular clusters?
• CNO and F abundances in the globular cluster M22 (NGC 6656)
• SALT reveals the barium central star of the planetary nebula Hen 2-39
• Fluorine variations in the globular cluster NGC 6656 (M22): implications for internal enrichment timescales
• Circumstellar effects on the Rb abundances in O-rich AGB stars
• The lead discrepancy in intrinsically s-process enriched post-AGB stars in the Magellanic Clouds
• Is the Sulphur Anomaly in Planetary Nebulae Caused by the s-Process?
• The AGB stars of the intermediate-age LMC cluster NGC 1846 Variability and age determination
• Nucleosynthesis in the Stellar Systems omega Cen and M22
• On the necessity of composition-dependent low-temperature opacity in metal-poor AGB stars
• On the internal pollution mechanisms in the globular cluster NGC 6121 (M4): heavy-element abundances and AGB models
• Stellar population synthesis of post-AGB stars: the s-process in MACHO47.2496.8
• Revisited fluorine abundances in the globular cluster M22 (NGC 6656)
• The effect of including molecular opacities of variable composition on the evolution of intermediate-mass AGB stars

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