A double stellar generation in the globular cluster NGC 6656 (M 22)^*

Two stellar groups with different iron and s-process element abundances

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy e-mail: [anna.marino;antonino.milone;giampaolo.piotto;andrea.bellini]@unipd.it
² P. Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile e-mail: fmarino@astro.puc.cl
³ Departamento de Astronomia, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile e-mail: svillanova@astro-udec.cl
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: bedin,bellini@stsci.edu
⁵ Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: alvio.renzini@oapd.inaf.it

Received: 11 February 2009
Accepted: 17 May 2009

Abstract

We present a chemical abundance analysis based on high resolution UVES spectra of seventeen bright giant stars in the Globular Cluster (GC) M 22. We obtained an average iron abundance of [Fe/H] = -1.76±0.02 (internal errors only) and an α enhancement of 0.36±0.04 (internal errors only). Na and O, and Al and O follow the well known anticorrelations found in many other GCs. We identified two groups of stars with significantly different abundances of the s-process elements Y, Zr, and Ba. The relative numbers of the two group members are very similar to the ratio of the number of stars in the two sub giant branches (SGB) of M 22. Y and Ba abundances do not correlate with Na, O, and Al. The s-element rich stars are also richer in iron and have higher Ca abundances. The results from high resolution spectra were confirmed by analyses of lower resolution GIRAFFE spectra of fourteen additional M 22 stars. The analyses of the GIRAFFE spectra also show that the Eu – a pure r-process element – abundance is not related to the iron content. We discuss the chemical abundance pattern of M 22 stars in the context of GC multiple stellar populations phenomenon.