Global survey of star clusters in the Milky Way

III. 139 new open clusters at high Galactic latitudes^⋆

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: sschmeja@ari.uni-heidelberg.de
² Main Astronomical Observatory, 27 Academica Zabolotnogo Str., 03680 Kiev, Ukraine
³ Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, Russia
⁴ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 20 September 2013
Accepted: 8 June 2014

Abstract

Context. An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (t ≳ 1 Gyr) open clusters in the solar neighbourhood (d ≲ 1 kpc).

Aims. To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes | b | > 20°.

Methods. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour–magnitude space according to typical colour–magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters.

Results. In total we discovered 782 density enhancements, 524 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [ yr ] ) < 9.7, distances d< 3 kpc, and distances from the Galactic plane 0.3 <Z< 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size.