Subproject B5: An Astrometric, Photometric, and Spectroscopic Census of Galactic Open Clusters

Stars always seem to form in groups, most of which do not survive the early rapid gas expulsion phase. The remaining ones usually disperse later due to internal and external effects, also contributing their stellar content to the field. At the beginning of the first funding period, about 2000 open clusters and associations were known, of which 60% did not have their basic properties determined, while the properties of the other 40% were often only poorly or heterogeneously measured. Using 2MASS and PPMXL, the work in B5 resulted in a catalog of 3,000 clusters with proper motions, distances, reddenings, ages, tidal radii, and masses, all homogeneously and uniformly derived. The publications in B5 are primary references for open cluster work world-wide. We noticed a curious lack of open clusters within 1 kpc. This paucity may be in part due to the difficulty of recognizing nearby stars as members of clusters, but nonetheless be reflecting a real deficit. Work is currently continuing with clustering analysis techniques to identify the missing objects.

In the second funding period, we plan to continue this successful work taking advantage of Gaia astrometry as well as spectrophotometry also from other surveys to confirm and extend the membership lists of known open clusters and to determine accurate cluster parameters. We will also continue to search for still undetected clusters, both within 1 kpc as well as further away. Moreover, a new aspect will be added to B5, focusing on spectroscopic abundance determination. Upon dissolution, open clusters contribute their stars to the field population of the Galactic disk. We will use spectroscopic surveys such as the SDSS and the Gaia-ESO Survey as well as a proprietary stellar spectroscopic survey of open clusters obtained at the Asiago Observatory for abundance analyses of stars in open clusters. The chemistry of Galactic open clusters, selected in bins of age, Galactocentric distance, and height above or below the Galactic plane, will allow us to address properties of the Galactic disk on a global scale by studying radial/vertical gradients (as traced by star clusters), which are linked to gas inflows, gas accretion, and Galactic fountains, as well as to the disk’s dynamical evolution. On small scales we will measure local (in-)homogeneities in the star formation and enrichment history of the disk, which may reveal themselves as different chemical patterns of open clusters at similar Galactocentric distances and ages. Subproject B5 is closely linked to B2, A5, and A6.