The two metallicity groups of the globular cluster M 22: a chemical perspective^⋆,⋆⋆

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany
e-mail: amarino@MPA-Garching.MPG.DE
² Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: anna.marino@unipd.it; giampaolo.piotto@unipd.it
³ Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA
e-mail: chris@verdi.as.utexas.edu
⁴ UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
e-mail: kraft@ucolick.org
⁵ Department of Astronomy, University of Washington, Seattle, WA 98195, USA
e-mail: wall@astro.washington.edu
⁶ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston, ACT 2611, Australia
e-mail: jen@mso.anu.edu.au; gdc@mso.anu.edu.au
⁷ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
e-mail: milone@iac.es
⁸ Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain
⁹ Department of Physics and Astronomy, The University of Utah, Salt Lake City, UT 84112, USA
e-mail: iii@physics.utah.edu
¹⁰ Physics Department, Grove City College, 100 Campus Drive, Grove City, 16127 Pennsylvania, USA
e-mail: gonzalezg@gcc.edu
¹¹ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
e-mail: jfulb@skysrv.pha.jhu.edu
¹² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: mhilker@eso.org
¹³ Pontificia Universidad Católica de Chile, Departmento de Astronomía y Astrofisíca, Casilla 306, Santiago 22, Chile
e-mail: mzoccali@astro.puc.cl
¹⁴ Herzberg Institute of Astrophysics, National Research Council Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
e-mail: Peter.Stetson@nrc-cnrc.gc.ca

Received: 19 January 2011
Accepted: 26 April 2011

Abstract

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M 22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and α element Ca. The principal results of this study are: (i) substantial star-to-star metallicity scatter (− 2.0  ≲  [Fe/H]  ≲   −1.6); (ii) enhancement of s-process/r-process neutron-capture abundance ratios in a fraction of giants, positively correlated with metallicity; (iii) sharp separation between the s-process-rich and s-process-poor groups by [La/Eu] ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H], suggesting that this element also has a significant s-process component; and (v)presence of Na-O and C-N anticorrelations in both the stellar groups.