Observing strategy - open clusters

Cluster selection is optimized to fine-sample the age-[Fe/H]-radial distance-mass parameter space. Open clusters (OCs) in all phases of evolution (except embedded), from ~ 106 Myr up to ~ 10 Gyr will be included, sampling different environments and star formation conditions. This will provide suffcient statistics to explore the dynamical evolution of clusters; the same sample will map stellar evolution as a function of metallicity for 0.1 ≤ M/M_⊙ ≤ 100, even for short-lived evolutionary phases, and provide a population large enough to throughly investigate metallicity as a function of Galactocentric radius and age.

The total sample will include ~ 100 clusters. The young cluster (<100 Myr) sample will include:

i) targets closer than ~ 1500 pc, the distance up to which Gaia will provide transverse velocity with precision better than internal velocity dispersion, as will our radial velocities, even for M stars;

ii) massive clusters at larger distances, where only OBA stars will be targeted. The older sample also includes both nearby and very distant clusters.

In the former we survey the whole population, down to the M dwarf regime, while in the latter we will observe RGB and clump giants, and early MS stars. For all clusters we will use GIRAFFE to target faint cluster members (down to V=19), while UVES fibers will be fed with brighter or key objects (down to V=16.5), to be used for accurate abundance determination or for which better precision in radial velocity (RV) is required. Six GIRAFFE set-ups will be employed (HR03/05A/06/14A/15N/21). HR03/05A/06/14A contain a large number of spectral features to be used to derive RVs and characteristics (e.g., temperature, gravity, wind) of early-type stars; HR15N/21 are instead the most commonly used gratings for late-type stars; they access a large enough number of lines to derive RVs, as well as to retrieve key information on the star characteristics (e.g., temperature, Li, accretion rates, chromospheric activity, rotation). As to UVES, CD3 is most suitable both for early-type (520 nm setting) stars and late-type members (580 nm setting). We finally mention that all cluster Fibre Positioner Observer Support Software (FPOSS) configurations will be observed at least twice to identify binaries.