A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)

TL;DR

This study provides a detailed chemical and kinematic analysis of over 300 stars in the globular cluster NGC 6273, revealing multiple stellar populations with varying metallicities and complex enrichment histories, characteristic of 'iron-complex' clusters.

Contribution

It is the first comprehensive high-resolution spectroscopic survey of NGC 6273, identifying multiple populations and their chemical signatures, and comparing its properties to other complex globular clusters.

Findings

NGC 6273 has an intrinsic metallicity spread from [Fe/H] = -2 to -1.

At least three distinct stellar populations with different [Fe/H] values were identified.

A possible Mg-Al anti-correlation exists only in stars with [Fe/H] > -1.65.

Abstract

Recent observations have shown that a growing number of the most massive Galactic globular clusters contain multiple populations of stars with different [Fe/H] and neutron-capture element abundances. NGC 6273 has only recently been recognized as a member of this "iron-complex" cluster class, and we provide here a chemical and kinematic analysis of > 300 red giant branch (RGB) and asymptotic giant branch (AGB) member stars using high resolution spectra obtained with the Magellan-M2FS and VLT-FLAMES instruments. Multiple lines of evidence indicate that NGC 6273 possesses an intrinsic metallicity spread that ranges from about [Fe/H] = -2 to -1 dex, and may include at least three populations with different [Fe/H] values. The three populations identified here contain separate first (Na/Al-poor) and second (Na/Al-rich) generation stars, but a Mg-Al anti-correlation may only be present in stars with [Fe/H] > -1.65. The strong correlation between [La/Eu] and [Fe/H] suggests that the s-process must have dominated the heavy element enrichment at higher metallicities. A small group of stars with low [alpha/Fe] is identified and may have been accreted from a former surrounding field star population. The cluster's large abundance variations are coupled with a complex, extended, and multimodal blue horizontal branch (HB). The HB morphology and chemical abundances suggest that NGC 6273 may have an origin that is similar to omega Cen and M 54.