2014A&A...570A..61F

Discovered in the last decade as overdensities of resolved stars, the ultra-faint dwarfs (UFDs) are among the least luminous, most dark-matter dominated, and most metal-poor galaxies known today. They appear as sparse, loose objects with high mass-to-light ratios. Hercules is the prototype of the UFD galaxies. To date, there are still no firm constraints on its total luminosity due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination. To better constrain Hercules properties, we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour-colour diagram. We have obtained images of Hercules in the r_Sloan, B_Bessel and U_spec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The r_Sloan new dataset combined with data from the LBT archive span a time baseline of about 5yr, allowing us to measure proper motions of stars in the Hercules direction for the first time. The U_spec data along with existing LBT photometry allowed us to use colour-colour diagram to further remove the field contamination. Thanks to a highly-accurate procedure to derive the r_Sloan-filter geometric distortion solution for the LBC-red, we were able to measure stellar relative proper motions to a precision of better than 5mas/yr down to r_Sloan≃22mag and disentangle a significant fraction (>90%) of Milky Way contaminants. We ended up with a sample of 528 sources distributed over a large portion of the galaxy body (∼0.12deg²). Of these sources, 171 turned out to be background galaxies and additional foreground stars from the analysis of the U_spec-B_Bessel vs. B_Bessel-r_Sloan colour-colour diagram. This leaves us with a sample of 357 likely members of the Hercules UFD. We compared the cleaned colour-magnitude diagram (CMD) with evolutionary models and synthetic CMDs, confirming the presence in Hercules of an old population (t=12±2Gyr) with wide spread metallicity (-3.3<[Fe/H]< -1.8). Our procedure to estimate star proper motions proved to be a very effective way to identify likely members for stellar systems as far as 130kpc. The present selection, based on both proper motions and colour-colour diagram, provides a robust identification of Hercules members and a new target list for more spectroscopic investigations.