Proper motions of cool and ultracool candidate members in the Upper Scorpius OB association ^*

¹ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 – La Laguna, Tenerife, Spain e-mail: bouy@iac.es
² CAB-CSIC/INTA, Ctra de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
³ University of Central Florida, Department of Physics, PO Box 162385, Orlando, FL 32816-2385, USA

Received: 4 October 2008
Accepted: 1 July 2009

Abstract

Aims. Proper motion measurements of the cool and ultracool populations in the Upper Scorpius OB association are crucial to confirm membership and to identify possible run-away objects.

Methods. We cross-match samples of photometrically selected and spectroscopically confirmed cool and ultracool (K5 < SpT < M8.5) candidate members in the Upper Scorpius OB association using the literature and the USNO-B and the UCAC2 catalogues. 251 of these objects have a USNO-B and/or UCAC2 counterpart with proper motion measurements.

Results. A significant fraction (19 objects, 7.6 ± 1.8%) of spectroscopically confirmed young objects show discrepant proper motion. They must either belong to unidentified coincident foreground associations, or originate from neighboring star forming regions or have recently experienced dynamical interactions within the association. The observed accretor and disc frequencies are lower among outliers, but with only 19 objects it is unreliable to draw firm statistical conclusions. Finally, we note that transverse velocities of very low mass members are indistinguishable from those of low mass members within ≈4 km s^-1.