Chemical abundances for the outer halo cluster Pal 4 from co-added high-resolution spectroscopy

Abstract

Context. Chemical element abundances for distant Galactic globular clusters (GCs) hold important clues to the origin of the Milky Way halo and its substructures. Aims. We study the chemical composition of red giant stars in Pal 4 -one of the most remote GCs in the Milky Way -and compare our abundance measurements to those for both low surface brightness dwarf galaxies, and GCs in the inner and the outer halo. Methods. By co-adding high-resolution, low-S/N Keck/HIRES spectra of 19 stars along the red giant branch, we estimate chemical abundance ratios of 20 α-, iron peak-, and neutron-capture elements. Our method gives total uncertainties on most element-to-iron ratios of typically 0.2 dex. Results. We measure [Fe/H] = −1.41 ± 0.04 (statistical) ± 0.17 (systematic) and an α-enhancement of [α/Fe] = +0.38 ± 0.11 dex, which is consistent with the canonical value of ∼+0.4 dex found for Galactic halo field stars and most halo GCs at this metallicity. Although Pal 4 has higher enhancements in the heavier elements with respect to the halo, the majority of the element ratios are, within the measurement errors, consistent with those for local halo field stars. We find, however, evidence for a lower [Mg/Ca] ratio than in other halo clusters. Conclusions. Based on the available evidence, we conclude that the material from which Pal 4 and the Galactic halo formed experienced similar enrichment processes, despite the apparently younger age of this cluster. Within the limitations of our methodology we find no significant indication of an iron spread, as is typical of genuine GCs of the Milky Way. However, abundance ratios for individual stars in Pal 4 and other distant satellites are urgently needed to understand the relationship, if any, between remote GCs and other halo substructures (i.e., luminous and ultra-faint dwarf spheroidal galaxies).

The chemical composition of a regular halo globular cluster: NGC 5897

“…At a mean of −0.43 dex and −0.70 dex, the [Mn/Fe] and [Cu/Fe] ratios are very low, yet typical of metal-poor halo field and GC stars. Overall, the Fe-peak elements are compatible with other GCs at similar metallicities and fall within the halo trends of, e.g., Cayrel et al (2004) and Ishigaki et al (2013); see also the discussions in Koch & Côté (2010). Nissen & Schuster (2011) reported on systematic differences in the abundance distribution of halo stars between −1.6 < [Fe/H] < −0.4 in the solar neighborhood, which are most pronounced in Na, Cu, and Zn.…”

A comprehensive chemical abundance study of the outer halo globular cluster M 75

“…We used an absolute abundance analysis method, which closely follows the procedures described in Koch et al (2009); Koch & Côté (2010). The line list was assembled from various sources (Koch & Côté 2010, and references therein), and complemented with atomic data from the Kurucz data base 2 .…”

“…The sample of dSph stars also comes from Venn et al (2004) and includes the abundances of individual stars from Carina, Fornax, Leo, Scl, UMi, Sex, and Draco dSphs. The mean abundances of various GCs are taken from Pritzl et al (2005) and complemented with the more recent results for NGC 1851 (Carretta et al 2011b), M 5 (Yong et al 2008), and the outermost halo clusters Pal 3 (Koch et al 2009), and Pal 4 (Koch & Côté 2010). We chose to plot in Fig.…”

The velocity dispersion and mass function of the outer halo globular cluster Palomar 4

“…The most recent determination of the chemical composition of Pal 4 was presented by Koch & Côté (2010). According to their abundance analysis of the same spectra that we use for our kinematical study, Pal 4 has a metallicity of [Fe/H] =−1.41 ± 0.17 dex and an α‐element enhancement of [α/Fe] = 0.38 ± 0.11 dex.…”