The Milky Way Bulge Extra-Tidal Star Survey: NGC 6569

TL;DR

This study provides spectroscopic evidence of ongoing tidal stripping of the globular cluster NGC 6569 in the Milky Way bulge, identifying candidate debris stars and quantifying the cluster's mass loss rate.

Contribution

First detailed debris analysis of a massive bulge globular cluster, combining spectroscopy, photometry, and orbit modeling to characterize tidal debris and mass loss.

Findings

Identification of 40 candidate tidal debris stars.

Mean metallicity of debris stars matches cluster core.

Estimated mass-loss rate of 1.0-1.6 solar masses per Myr.

Abstract

We present spectroscopic evidence for tidal debris associated with the bulge globular cluster NGC 6569, based on medium-resolution (R ~ 11,000) Anglo-Australian Telescope spectra of 303 stars. Targets were selected using Blanco DECam Bulge Survey (BDBS) photometry and Gaia DR3 astrometry, spanning 7-30 arcmin (~1-5 rt, where rt is the King-model tidal radius) from the cluster center. Orbit-based modeling predicts a strongly time-variable Jacobi radius, with rJ ~ 8-11 arcmin near pericenter and ~18-22 arcmin near apocenter, so stars just outside rt can be unbound and feeding leading and lagging tidal tails. We identify 40 stars with kinematics and abundances consistent with previous, or borderline, cluster membership. The seven highest-quality candidates (S/N > 30) have mean [Fe/H] = -0.83 +/- 0.14 and [alpha/Fe] = +0.38 +/- 0.06 dex, matching the bound population. Interpreting these stars as recently stripped debris implies a present-day mass-loss rate of 1.0-1.6 solar masses per Myr, or 5.6 +/- 1.3% of the current cluster mass per Gyr. These results indicate ongoing tidal stripping of NGC 6569 and quantify its contribution to the bulge field. This paper is part of the Milky Way Bulge Extra-Tidal Star Survey (MWBest) and is our first detailed debris study of a massive bulge globular cluster.