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

TL;DR

This study measures proper motions of cool and ultracool candidate members in Upper Scorpius to confirm membership, identify run-away objects, and analyze their kinematic properties, revealing some objects with discrepant motions and similar velocities across mass ranges.

Contribution

It provides the first proper motion analysis of cool and ultracool members in Upper Scorpius, highlighting potential non-members and dynamical interactions.

Findings

7.6% of confirmed young objects show discrepant proper motions.

Discrepant objects may belong to other associations or have experienced dynamical interactions.

Very low mass members have similar transverse velocities to higher mass members.

Abstract

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. 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. 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 4km/s