Stars and brown dwarfs in theσ Orionis cluster

Abstract

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.

Young Stars Near Earth: The Octans-Near Association and Castor Moving Group

“…Of 26 systems considered by Barrado y Navascues (1998), he concludes that only 16 are physically associated and he expresses doubts as to whether a Castor moving group exists. Notwithstanding this uncertainty, he proposes a UVW for the group of −10.7, −8.0, −9.7 (3.5, 2.4, 3.0) km s −1 which was subsequently adopted by Montes et al (2001) and by Caballero (2010). Anosova & Orlov (1991) include Fomalhaut and its companion GJ 879 as members of their proposed Castor moving group.…”

XMM-Newtonobservations of the young open cluster aroundλ Orionis

“…After the FPS06 paper was published, several new studies of the σ Ori cluster have become available, which have significantly improved the membership information for many stars. Before performing the comparison, we have therefore updated the catalogue of FPS06, using mainly information from the Mayrit catalogue by Caballero (2008b), the spectroscopic studies by Sacco et al (2008) and Maxted et al (2008), the Spitzer study by Hernández et al (2007), the proper motion study by Caballero (2010), and updated photometry from Kenyon et al (2005) and Mayne & Naylor (2008). We rejected ∼20% (40/210) of the late-type stars considered as members by FPS06, most of which were not detected in X-rays, and added 25 new members and candidates with M ∼ 0.1−2 M , nine of which were detected by FPS06; for the others, upper limits were computed as described in FPS06 and in Sect.…”

HRC-I/ChandraX-ray observations towardsσ Orionis

“…They derived nuclear and isochronal ages about 10 Ma older than expected for σ Orionis stars. Mayrit 605079 might belong to a differentiated young stellar population in the Orion Belt Caballero 2007a;Maxted et al 2008) or be instead an active field M-dwarf interloper with CN contamination around the Li i line (Caballero 2010). …”