A combined HST and JWST analysis of the stellar population in Draco II

Draco II (Dra II) is one of the faintest and most metal-poor Milky Way satellites known, yet its nature as either an ultra-faint dwarf galaxy or an extended globular cluster remains debated. We perform a photometric analysis of the stellar population of Dra II reducing deep multi-band observations from the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). We select Dra II member stars based on their proper motions and photometric quality criteria. For the first time, optical and infrared photometry are combined to produce a stellar catalogue reaching 50 percent completeness at F814W = 26.5 mag (corresponding to the absolute magnitude MF814W = 10.1 mag). The colour-magnitude diagram confirms that Dra II hosts an old stellar population with an age of 13.5-1.5+1.7 Gyr at a distance of 19.2 ± 0.9 kpc. We measure an α-enhanced stellar population with a low mean metallicity of [ Fe/H]CaHK = -2.38 ± 0.17 dex and confirm the presence of an intrinsic metallicity spread of σ [ Fe/H] = 0.85 ± 0.13 dex. We identify 10 unresolved main-sequence binary stars in the HST catalogue with mass ratios q > 0.5 in the mass range 0.43 < M1/M⊙ < 0.71, corresponding to a binary fraction of fbinq>0.5 = 0.21 ± 0.07. This value is higher than those typically observed in globular clusters and lies near the upper range measured in open clusters. The stellar mass function is best modelled by a single power law with a slope of 1.53 ± 0.26 over the mass range 0.13–0.77 M⊙. We find no evidence of mass segregation. Consistent results are obtained independently from the JWST catalogue. Combining these findings strongly suggests that Draco II is best classified as an ultra-faint dwarf galaxy rather than a globular cluster.