Nine new open clusters within 500 pc from the Sun

¹ Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12−14, 69120 Heidelberg, Germany
e-mail: roeser@ari.uni-heidelberg.de;elena@ari.uni-heidelberg.de
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Observatoire astronomique de Strasbourg, Université de Strasbourg − CNRS UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France

Received: 21 June 2016
Accepted: 6 August 2016

Abstract

Aims. One of the results of the Milky Way Star Clusters (MWSC) survey was the detection of a slight under-density of old (ca. 1 Gyr) clusters within the nearest kilo-parsec from the Sun. This under-density may be due to an ineffectiveness in the detection of larger structures with lower surface brightness. We report on our attempts to reveal such clusters.

Methods. We derived proper motions from a combination of Tycho-2 with URAT1, and obtained a mean precision of about 1.4 mas/yr per co-ordinate for 1.3 million stars north of −20° declination. We cut the sky into narrow proper motion slices and searched for spatial over-densities of stars in each slice. We then examined stars from over-densities in optical and near-infrared colour-magnitude diagrams to determine if they are compatible with isochrones of a cluster. We estimated the field star contamination using our data and the Besançon Galactic model.

Results. We detected nine hitherto unknown open clusters in the vicinity of the Sun with ages between 70 Myr and 1 Gyr, and distances between 200 and 500 pc.