The Gaia-ESO Survey: A globular cluster escapee in the Galactic halo ^⋆,⋆⋆

¹ Department of Physics and AstronomyUppsala University, Box 516, 75120, Uppsala Sweden
e-mail: karin.lind@physics.uu.se
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
³ Moscow MV Lomonosov State University, Sternberg Astronomical Institute, 119992 Moscow, Russia
⁴ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
⁵ Research School of Astronomy & Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
⁶ Institute of Space Sciences (IEEC-CSIC), Campus UAB, Fac. Ciéncies, Torre C5 parell 2, 08193 Bellaterra, Spain
⁷ Lab. d’Astrophysique de Bordeaux, CNRS, Université Bordeaux, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
⁸ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁹ Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
¹⁰ Universidad de La Laguna, Dept. Astrofísica , 38206, La Laguna, Tenerife, Spain
¹¹ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹² ASI Science Data Center, via del Politecnico SNC, 00133 Roma, Italy
¹³ Lab. Lagrange, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
¹⁴ Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
¹⁵ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁶ INAF–Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁷ Astrophysics Group, Keele University, Keele, Staffordshire ST5 5BG, UK
¹⁸ Institute of Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁹ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Received: 19 December 2014
Accepted: 27 January 2015

Abstract

A small fraction of the halo field is made up of stars that share the light element (Z ≤ 13) anomalies characteristic of second generation globular cluster (GC) stars. The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters, which are believed to have once been more massive. Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg, but until now no such star has been found. We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eighteen months of the Gaia-ESO public spectroscopic survey. One halo star at the base of the red giant branch, here referred to as 22593757-4648029 is found to have [ Mg/Fe ] = −0.36 ± 0.04 and [ Al/Fe ] = 0.99 ± 0.08, which is compatible with the most extreme ratios detected in GCs so far. We compare the orbit of 22593757-4648029 to GCs of similar metallicity andfind it unlikely that this star has been tidally stripped with low ejection velocity from any of the clusters. However, both chemical and kinematic arguments render it plausible that the star has been ejected at high velocity from the anomalous GC ω Centauri within the last few billion years. We cannot rule out other progenitor GCs, because some may have disrupted fully, and the abundance and orbital data are inadequate for many of those that are still intact.