Stars and brown dwarfs in the σ Orionis cluster II. A proper motion study

Aims. I attempt to fully understand the origin of the stellar and substellar populations in the young sigma 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 sigma 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 sigma Ori. Primarily because of the long time baseline of more than half a century, the errors in the measured proper motions are lower than 2mas 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 sigma 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 sigma Orionis, they can be instead used to discard a number of cluster member candidates without spectroscopy.