THE SOUTHERN FLANKING FIELDS OF THE 25 ORIONIS GROUP

The stellar group surrounding the Be (B1 Vpe) star 25 Ori was discovered to be a pre-main-sequence (PMS) population by the CIDA Variability Survey of Orion and subsequent spectroscopy. We analyze Sloan Digital Sky Survey multiepoch photometry to map the southern extent of the 25 Ori group and characterize its PMS population. We compare this group to the neighboring Orion OB1a and OB1b subassociations and to active star formation sites (NGC 2068/NGC 2071) within the LDN 1630 dark cloud. We find that the 25 Ori group has a radius of 14, corresponding to 8–11 pc at the distances of Orion OB1a and OB1b. Given that the characteristic sizes of young open clusters are a few parsecs or less, this suggests that 25 Ori is an unbound association rather than an open cluster. Due to its PMS population having a low classical T Tauri star fraction (~10%), we conclude that the 25 Ori group is of comparable age to the 11 Myr Orion OB1a subassociation.