Detailed Chemical Abundances of Four Stars in the Unusual Globular Cluster, Palomar 1

TL;DR

This study presents detailed chemical abundances for four stars in the unusual globular cluster Palomar 1, revealing its unique chemical signature that supports its extragalactic origin and distinct evolutionary history.

Contribution

It provides high-resolution spectroscopic data for Palomar 1's stars, highlighting its chemical peculiarities and potential extragalactic associations, which were previously unconfirmed.

Findings

Palomar 1 has a high metallicity for its location.

Its alpha-element ratios resemble those in dwarf galaxies.

Unusual neutron capture element ratios suggest a different enrichment history.

Abstract

Detailed chemical abundances for twenty one elements are presented for four red giants in the anomalous outer halo globular cluster Palomar 1 ($R_{\rm{GC}} = 17.2$ kpc, $Z=3.6$ kpc) using high-resolution (R=36000) spectra from the High Dispersion Spectrograph (HDS) on the Subaru Telescope. Pal 1 has long been considered unusual because of its low surface brightness, sparse red giant branch, young age, and its possible association with two extragalactic streams of stars---this paper shows that its chemistry further confirms its unusual nature. The mean metallicity of the four stars, $[\rm{Fe/H}] = -0.60 \pm 0.01$, is high for a globular cluster so far from the Galactic center, but is low for a typical open cluster. The [$\alpha$/Fe] ratios, though in agreement with the Galactic stars within the $1\sigma$ errors, agree best with the lower values in dwarf galaxies. No signs of the Na/O anticorrelation are detected in Pal 1, though Na appears to be marginally high in all four stars. Pal 1's neutron capture elements are also unusual: its high [Ba/Y] ratio agrees best with dwarf galaxies, implying an excess of second-peak over first-peak s-process elements, while its [Eu/$\alpha$] and [Ba/Eu] ratios show that Pal 1's contributions from the r-process must have differed in some way from normal Galactic stars. Therefore, Pal 1 is chemically unusual, as well in its other properties. Pal 1 shares some of its unusual abundance characteristics with the young clusters associated with the Sagittarius dwarf galaxy remnant and the intermediate-age LMC clusters, and could be chemically associated with the Canis Majoris overdensity; however it does not seem to be similar to the Monoceros/Galactic Anticenter Stellar Stream.