APOGEE view of the globular cluster NGC 6544

TL;DR

This study uses high-resolution near-infrared spectroscopy from APOGEE to analyze the chemical composition and multiple stellar populations of the globular cluster NGC 6544, revealing its complex chemical history and signs of tidal disruption.

Contribution

It provides the first detailed chemical characterization of NGC 6544 using APOGEE data, highlighting intra-cluster abundance variations and multiple stellar populations.

Findings

NGC 6544 has [Fe/H] = -1.44 ± 0.04 dex and [$ mf ext{alpha}$/Fe] = 0.20 ± 0.04 dex.

Significant spreads in [Fe/H] and [Al/Fe] indicate multiple populations.

Evidence of Mg-Al anticorrelation and second-generation stars present across evolutionary stages.

Abstract

The second phase of the APOGEE survey is providing near-infrared, high-resolution, high signal-to-noise spectra of stars in the halo, disk, bar and bulge of the Milky Way. The near-infrared spectral window is especially important in the study of the Galactic bulge, where stars are obscured by the dust and gas of the disk in its line-of-sight. We present a chemical characterisation of the globular cluster NGC 6544 with high-resolution spectroscopy. The characterisation of the cluster chemical fingerprint, given its status of "interloper" towards the Galactic bulge and clear signatures of tidal disruption in its core is crucial for future chemical tagging efforts. Cluster members were selected from the DR16 of the APOGEE survey, using chemo-dynamical criteria of individual stars. A sample of 23 members of the cluster was selected. An analysis considering the intra-cluster abundance variations, known anticorrelations is given. According to the RGB content of the cluster, the iron content and $\alpha$-enhancement are [Fe/H] $= -1.44 \pm 0.04$ dex and [$\alpha$/Fe] $= 0.20 \pm 0.04$ dex, respectively. Cluster members show a significant spread in [Fe/H] and [Al/Fe] that is larger than expected based on measurement errors. An [Al/Fe] spread, signal of an Mg-Al anticorrelation is observed and used to constraint the cluster mass budget, along with C, N, Mg, Si, K, Ca, and Ce element variations are discussed. Across all the analysed evolutionary stages (RGB and AGB), about $\sim2/3$ (14 out of 23) show distinct chemical patterns, possibly associated with second-generation stars.