Detailed study of the Milky Way globular cluster Laevens 3

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.