Abundance variations in the globular cluster M71 (NGC 6838)^*

¹ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil e-mail: [abrito;barbuy]@astro.iag.usp.br
² Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, VA 22903-0818, USA e-mail: rps7v@mail.astro.virginia.edu
³ Laboratório Nacional de Astrofísica/MCT, CP 21, Itajubá, MG, 37500-000, Brazil e-mail: bruno@lna.br

Received: 16 January 2008
Accepted: 22 April 2008

Abstract

Context. Abundance variations in moderately metal-rich globular clusters can give clues about the formation and chemical enrichment of globular clusters.

Aims. CN, CH, Na, Mg and Al indices in spectra of 89 stars of the template metal-rich globular cluster M71 are measured and implications on internal mixing are discussed.

Methods. Stars from the turn-off up to the Red Giant Branch (0.87 < log g < 4.65) observed with the GMOS multi-object spectrograph at the Gemini-North telescope are analyzed. Radial velocities, colours, effective temperatures, gravities and spectral indices are determined for the sample.

Results. Previous findings related to the CN bimodality and CN-CH anticorrelation in stars of M71 are confirmed. We also find a CN-Na correlation, and Al-Na, as well as an Mg₂-Al anticorrelation.

Conclusions. A combination of convective mixing and a primordial pollution by AGB or massive stars in the early stages of globular cluster formation is required to explain the observations.