An outer shade of Pal: Abundance analysis of the outer halo globular cluster Palomar 13

TL;DR

This study analyzes the chemical composition of the outer halo globular cluster Palomar 13 using high-resolution spectra, revealing its metal-poor nature and chemical similarities to certain accreted halo objects, but with ambiguous origins.

Contribution

First detailed chemical abundance analysis of Palomar 13 from high-resolution spectra, providing insights into its origin and relation to halo accretion events.

Findings

Palomar 13 is metal-poor with [Fe/H] ≈ -1.91.

Mean alpha-element abundance is consistent with halo stars.

Chemical similarities to Gaia-Enceladus and NGC 5466, but kinematic data suggest complex origins.

Abstract

At a Galactocentric distance of 27 kpc, Pal 13 is an old globular cluster (GC) in the outer halo. We present a chemical abundance analysis of this remote system from high-resolution spectra obtained with Keck/HIRES. Owing to the low signal-to-noise ratio of the data, our analysis is based on a coaddition of the spectra of 18 member stars. We are able to determine integrated abundance ratios for 16 species of 14 elements, of $\alpha$-elements (Mg,Si,Ca,Ti), Fe-peak (Sc,Mn,Cr,Ni,Cu,and Zn), and n-capture elements (Y,Ba). While the mean Na abundance is found to be slightly enhanced and halo-like, our method does not allow us to probe an abundance spread that would be expected in this light element if multiple populations are present in Pal 13. We find a metal-poor mean metallicity of $-1.91\pm0.05$ (statistical) $\pm$ 0.22 (systematic), confirming that Pal 13 is a typical metal-poor representative of the outer halo. While there are some differences between individual $\alpha$-elements, such as halo-like Mg and Si versus the mildly lower Ca and Ti abundances, the mean [$\alpha$/Fe] of 0.34$\pm$0.06 is consistent with the marginally lower $\alpha$ component of the halo field and GC stars. We discuss our results in the context of other objects in the outer halo and consider which of these objects were likely accreted. We also discuss the properties of their progenitors. While chemically, Pal 13 is similar to Gaia-Enceladus and some of its GCs, this is not supported by its kinematic properties. Moreover, its chemodynamical similarity with NGC 5466, a progeny of the Sequoia accretion event, might indicate a common origin in this progenitor. However, the ambiguities in the full abundance space of this comparison emphasize the difficulties in unequivocally labeling a single GC as an accreted object, let alone assigning it to a single progenitor. (Abridged)