[LMS2018] J238.42835417+64.57927222 , the SIMBAD biblio

We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broad-band g and i photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modelling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour-magnitude space implies Dra II is old (13.5 ± 0.5 Gyr), very metal-poor, very faint (L_V=₁₈₀⁺¹²⁴_–72 L_☉), and at a distance d = 21.5 ±0.4 kpc. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H] = -2.7 ± 0.1 dex). Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved (σ_vr< 5.9 km s^–1 at the 95 per cent confidence level) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity (<v_r>= -342.5^+1.1_–1.2 km s^–1). We further show that the spectroscopically confirmed members of Dra II have a mean proper motion of (µ_α^*,µ^_δ)=(1.26±_0.27,0.94±_0.28) mas/yr_ in the Gaia DR2 data, which translates to an orbit with a pericentre and an apocentre of 21.3 ^+0.7_–1.0 and 153.8 ^+56.7_–34.7 kpc, respectively. Taken altogether, these properties favour the scenario of Dra II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario.