Detailed study of the Milky Way globular cluster Laevens 3

TL;DR

This study provides detailed photometric and spectroscopic analysis of Laevens 3, revealing its properties as an old, metal-poor, faint, and compact globular cluster in the Milky Way's outer halo, with signs of mass segregation.

Contribution

It offers the first comprehensive characterization of Laevens 3, including structural, stellar, and kinematic properties, confirming its status as a globular cluster in the outer halo.

Findings

Laevens 3 is an old (13 Gyr), metal-poor ([Fe/H] = -1.8) globular cluster.

It is faint (Mv = -2.8 mag) and compact (radius = 11.4 pc).

The system shows signs of mass segregation.

Abstract

We present a photometric and spectroscopic study of the Milky Way satellite Laevens 3. Using MegaCam/CFHT g and i photometry and Keck II/DEIMOS multi-object spectroscopy, we refine the structural and stellar properties of the system. The Laevens 3 colour-magnitude diagram shows that it is quite metal-poor, old (13.0 +-1.0 Gyr), and at a distance of 61.4+-1.0 kpc, partly based on two RR Lyrae stars. The system is faint (Mv = -2.8+0.2-0.3 mag) and compact (r = 11.4+-1.0 pc). From the spectroscopy, we constrain the systemic metallicity ([Fe/H]spectro = -1.8+-0.1 dex) but the metallicity and velocity dispersions are both unresolved. Using Gaia DR2, we infer a mean proper motion of (mu_alpha, mu_delta) = (0.51+-0.28, -0.83+-0.27) mas yr-1, which, combined with the system's radial velocity (<vr> = -70.2+-0.5 km s-1), translates into a halo orbit with a pericenter and apocenter of 40.7+5.6-14.7 and 85.6+17.2-5.9 kpc, respectively. Overall, Laevens 3 shares the typical properties of the Milky Way's outer halo globular clusters. Furthermore, we find that this system shows signs of mass-segregation which strengthens our conclusion that Laevens 3 is a globular cluster.