2015A&A...581A..39S

The global Milky Way Star Clusters (MWSC) survey provided new cluster membership lists and mean cluster parameters for nearly 80% of all previously known Galactic clusters. The MWSC data reduction pipeline involved the catalogue of positions and proper motions (PPMXL) on the International Celestial Reference System (ICRS) and near-infrared photometry from the Two Micron All Sky Survey (2MASS). In the first extension to the MWSC, photometric filters were applied to the 2MASS catalogue to find new cluster candidates that were subsequently confirmed or rejected by the MWSC pipeline. To further extend the MWSC census, particularly of nearby clusters, we aimed at discovering new clusters by conducting an almost global search in proper motion catalogues as a starting point. We first selected high-quality samples from the PPMXL and the Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4) for comparison and verification of the proper motions. For 441 circular proper motion bins (radius 15mas/yr) within ±50mas/yr, the sky outside a thin Galactic plane zone (|b|<5°) was binned in small areas (``sky pixels'') of 0.25x0.25 deg². Sky pixels with enhanced numbers of stars with a certain common proper motion in both catalogues were considered as cluster candidates. After visual inspection of the sky images, we built an automated procedure that combined these representations of the sky for neighbouring proper motion subsamples after a background correction. The 692 compact cluster candidates detected above a threshold that was equivalent to a minimum of 12 to 130 cluster stars in dependence on the Galactic latitude were then cross-checked with known star clusters and clusters of galaxies. New candidates served as input for the MWSC pipeline. About half of our candidates overlapped with known clusters (46 globular and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or the Magellanic Clouds. About 10% of our candidates are 63 new open clusters confirmed by the MWSC pipeline. They predominantly occupy the two inner Galactic quadrants and have apparent sizes and numbers of highly probable members slightly larger than those of the typically small MWSC clusters, whereas their other parameters (ages, distances, tidal radii) fall in the typical ranges. As we searched for compact clusters, we did not find new very nearby (extended) clusters, and the mean total proper motion of the new 63 clusters is with 6.3mas/yr similar to the MWSC average (5.5mas/yr). Only four new clusters have mean proper motions between 10mas/yr and our observed maximum of about 13mas/yr.