The double sub-giant branch of NGC 6656 (M 22): a chemical characterization^⋆,⋆⋆

¹ Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str. 1 85741 Garching bei München Germany
e-mail: amarino@MPA-Garching.MPG.DE; mbergema@MPA-Garching.MPG.DE; klind@MPA-Garching.MPG.DE
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain
e-mail: milone@iac.es; aparicio@iac.es
³ Department of Astrophysics, University of La Laguna, 38200 La Laguna, Tenerife, Canary Islands, Spain
⁴ Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA
e-mail: chris@verdi.as.utexas.edu
⁵ UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
e-mail: kraft@ucolick.org
⁶ Department of Astronomy, University of Washington, Seattle, WA 98195, USA
e-mail: wall@astro.washington.edu
⁷ INAF – Osservatorio Astronomico di Teramo, via M. Maggini, 64100 Teramo, Italy
e-mail: cassisi@oa-teramo.inaf.it
⁸ Research School of Astronomy & Astrophysics, Australian National University, Mt Stromlo Observatory, via Cotter Rd, Weston, ACT 2611, Australia
e-mail: martin@mso.anu.edu.au
⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: luigi.bedin@oapd.inaf.it
¹⁰ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: mhilker@eso.org
¹¹ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
e-mail: ymomany@eso.org
¹² Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: giampaolo.piotto@unipd.it
¹³ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
e-mail: iur@obs.carnegiescience.edu
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
¹⁵ Pontificia Universidad Católica de Chile, Departmento de Astronomía y Astrofisíca, Casilla 306, Santiago 22, Chile
e-mail: mzoccali@astro.puc.cl

Received: 2 November 2011
Accepted: 11 February 2012

Abstract

We present an abundance analysis of 101 subgiant branch (SGB) stars in the globular cluster M 22. Using low-resolution FLAMES/GIRAFFE spectra we have determined abundances of the neutron-capture strontium and barium and the light element carbon. With these data we explore relationships between the observed SGB photometric split in this cluster and two stellar groups characterized by different contents of iron, slow neutron-capture process (s-process) elements, and the α element calcium, which we previously discovered in M 22’s red-giant stars. We show that the SGB stars correlate in chemical composition and the color–magnitude diagram position. The stars with higher metallicity and relative s-process abundances define a fainter SGB, while stars with lower metallicity and s-process content reside on a relatively brighter SGB. This result has implications for the relative ages of the two stellar groups of M 22. In particular, it is inconsistent with a broad spread in ages of the two SGBs. By accounting for the chemical content of the two stellar groups, isochrone fitting of the double SGB suggests that their agesare not different by more than  ~300 Myr.