Space Velocities of Southern Globular Clusters VII. NGC 6397, NGC 6626 (M 28) and NGC 6656 (M 22)

TL;DR

This study measures the proper motions of three southern globular clusters, determines their orbits, and discusses implications for their formation and the assembly history of the Milky Way halo.

Contribution

It provides new absolute proper motion measurements for NGC 6397, NGC 6626, and NGC 6656, and analyzes their orbits to infer their origins and the galaxy's formation history.

Findings

NGC 6397's proper motion differs from HST measurements by 2.5 mas/yr.

Orbits of clusters with iron abundance spread are unrelated, indicating multiple progenitors.

Globular clusters with abundance spreads suggest formation in massive progenitor systems.

Abstract

We have measured the absolute proper motions of globular clusters NGC 6397, NGC 6626 (M 22) and NGC 6656 (M 28) as part of our ongoing Southern Proper-Motion Program. The reference system is the ICRS via Hipparcos stars for these three low galactic latitude clusters. Formal errors range between ~0.3 and 0.7 mas/yr. Notable is the result for NGC 6397 which differs by 2.5 mas/yr from two HST determinations, while agreeing with previous ground-based ones. We determine orbits for all three clusters in an axisymmetric and barred model of the Galaxy and discuss these in the context of globular-cluster formation. M 22 is a well-known cluster with an iron abundance spread; such clusters are now believed to have formed in massive parent systems that can retain ejecta of core-collapsed SNe. We find that the five currently-accepted globular clusters with iron/calcium abundance spread show orbits unrelated to each other, thus suggesting at least five independent, massive progenitors that have contributed to the build-up of the Milky-Way halo.