All quiet in the outer halo : chemical abundances in the globular cluster Pal 3

Koch, Andreas and Côté, P. and McWilliam, A. (2009) All quiet in the outer halo : chemical abundances in the globular cluster Pal 3. Astronomy and Astrophysics, 506 (2). pp. 729-743. ISSN 1432-0746

Abstract

Context. Globular clusters (GCs) in the outer halo are important probes of the composition and origin of the Galactic stellar halo. Aims. We derive chemical element abundance ratios in red giants belonging to the remote (R ~ 90 kpc) GC Pal 3 and compare our measurements to those for red giant stars in both inner and outer halo GCs. Methods. From high-resolution spectroscopy of four red giants, obtained with the Magellan/MIKE spectrograph at moderately high S/N, we derive chemical abundances for 25 $\alpha$-, iron peak-, and neutron-capture elements. These abundance ratios are confirmed by co-adding low S/N HIRES spectra of 19 stars along the red giant branch. Results. Pal 3 shows $\alpha$-enhanced abundance patterns, and also its Fe-peak and neutron-capture element ratios, are fully compatible with those found in halo field stars and representative inner halo GCs of the same metallicity (such as M 13). The heavy elements in Pal 3 appear to be governed by r-process nucleosynthesis. Our limited sample does not show any significant star-to-star abundance variations in this cluster, although a weak Na-O anti-correlation cannot be ruled out by the present data. Conclusions. Pal 3 thus appears as an archetypical GC with abundance ratios dissimilar to dwarf spheroidal stars, ruling out a direct connection to such external systems. This conclusion is underscored by the lack of significant abundance spreads in this GC, in contrast to the broad abundance distributions seen in the dwarf galaxies. Pal 3 appears to have evolved chemically in analogy to the majority of GCs belonging to the Galactic inner and outer halo, experiencing a similar enrichment history.