New cluster members and halo stars of the Galactic globular cluster NGC 1851

TL;DR

This study spectroscopically identifies new and extratidal halo stars of the globular cluster NGC 1851, revealing an extended stellar halo and confirming cluster membership through radial velocities and metallicity estimates.

Contribution

First spectroscopic identification of cluster and halo members of NGC 1851, confirming an extended stellar halo and providing detailed velocity and metallicity data.

Findings

Nine new probable cluster members identified.

Four probable extratidal halo stars found up to 3.1 times the tidal radius.

Halo stars' velocities and metallicities are consistent with NGC 1851.

Abstract

NGC 1851 is an intriguing Galactic globular cluster, with multiple stellar evolutionary sequences, light and heavy element abundance variations and indications of a surrounding stellar halo. We present the first results of a spectroscopic study of red giant stars within and outside of the tidal radius of this cluster. Our results identify nine probable new cluster members (inside the tidal radius) with heliocentric radial velocities consistent with that of NGC 1851. We also identify, based on their radial velocities, four probable extratidal cluster halo stars at distances up to ~3.1 times the tidal radius, which are supportive of previous findings that NGC 1851 is surrounded by an extended stellar halo. Proper motions were available for 12 of these 13 stars and all are consistent with that of NGC 1851. Apart from the cluster members and cluster halo stars, our observed radial velocity distribution agrees with the expected distribution from a Besancon disk/N-body stellar halo Milky Way model generated by the Galaxia code, suggesting that no other structures at different radial velocities are present in our field. The metallicities of these stars are estimated using equivalent width measurements of the near infrared calcium triplet absorption lines and are found, within the limitations of this method, to be consistent with that of NGC 1851. In addition we recover 110 red giant cluster members from previous studies based on their radial velocities and identify three stars with unusually high radial velocities.