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/Canada-France-Hawaii Telescope 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 (M_V = -2.8^{+0.2}_{-0.3} mag) and compact (r_h = 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 (μ _α ^*,μ _δ)=(0.51 ± 0.28,-0.83 ± 0.27) mas yr^-1, which, combined with the system's radial velocity (< v_r> = -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 that strengthens our conclusion that Laevens 3 is a globular cluster.