Volume 514, May 2010
|Number of page(s)||10|
|Section||Stellar structure and evolution|
|Published online||04 May 2010|
Stars and brown dwarfs in the σ Orionis cluster
II. A proper motion study
Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de
Física, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: email@example.com
Accepted: 20 January 2010
Aims. I attempt to fully understand the origin of the stellar and substellar populations in the young σ Orionis open cluster, which is a benchmark for star-forming studies. Because of the very low proper motion of the cluster, late-type dwarfs with appreciable proper motion in the foreground of σ Orionis can be easily discarded as targets from expensive spectroscopic follow-up studies.
Methods. I use the Aladin sky atlas, USNO-B1, public astrometric catalogues, and photographic plate digitisations to identify stars with proper motions that are inconsistent with cluster membership in a circular area of radius 30 arcmin centred on the early-type multiple system σ Ori. Primarily because of the long time baseline of more than half a century, the errors in the measured proper motions are lower than 2 mas a-1.
Results. Of the 42 stars selected for astrometric follow-up, 37 of them are proper-motion cluster interlopers. Some USNO-B1 measurements were affected by partially resolved (visual) multiplicity and target faintness. Because of their late spectral types and, hence, red colours, 24 contaminants had been considered at some point as σ Orionis members. I discuss how contamination may have affected previous work (especially related to disc frequencies) and the curious presence of lithium absorption in three M-dwarf proper motion contaminants. Finally, I classify the bright star HD 294297 as a late-F field dwarf unrelated to the cluster based on a new proper motion measurement.
Conclusions. Although proper motions cannot be used to confirm membership in σ Orionis, they can be instead used to discard a number of cluster member candidates without spectroscopy.
Key words: astronomical databases: miscellaneous / stars: late-type / open clusters and associations: individual: σ Orionis / proper motions
© ESO, 2010
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.