A TGAS/Gaia DR1 parallactic distance to the σ Orionis cluster

Abstract

With the new Tycho-Gaia Astrometric Solution (TGAS), I derive a new parallactic distance to the young σ Orionis open cluster, which is a cornerstone region for studying the formation and evolution of stars and substellar objects from tens of solar masses to a few Jupiter masses. I started with the list of the 46 brightest cluster stars of Caballero (2007). After identifying the 24 TGAS stars in the 30 arcmin-radius survey area and accounting for 11 FGKM-type dwarfs and giants in the foreground and background, I got a list of 13 cluster members and candidates with new parallaxes. Of them, I discarded five cluster member candidates with questionable features of youth and/or discordant parallaxes and proper motions, including a distant Herbig Ae/Be star, and proceeded with the remaining eight stars. The σ Orionis mean heliocentric distance is d=360_-35⁺⁴⁴ pc, which is consistent with a contemporaneous interferometric determination that concludes a two-decade dispute on the cluster distance. As a by-product, the reclassification of those five cluster member candidates, now interlopers, shows a manifest deficit of cluster stars between 1.2 and 2.1 M_⊙, which leaves the door open to new astrometric membership analyses with the next Gaia data releases.