25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a

We report here on the photometric and kinematic properties of a well-defined group of nearly 200 low-mass pre-main-sequence stars, concentrated within similar to 1 degree of the early-B star 25 Ori, in the Orion OB1a subassociation. We refer to this stellar aggregate as the 25 Orionis group. The group also harbors the Herbig Ae/Be star V346 Ori and a dozen other early-type stars with photometry, paraliaxes, and some with IR excess emission, indicative of group membership. The number of high- and low-mass stars is in agreement with expectations from a standard initial mass function. The velocity distribution for the low-mass stars shows a narrow peak at 19.7 km s super(-1), offset similar to -10 km s super(-1) from the velocity characterizing the younger stars of the Ori OB1b subassociation, and -4 km s super(-1) from the velocity of widely spread young stars of the Ori OB1a population; this result provides new and compelling evidence that the 25 Ori group is a distinct kinematic entity, and that considerable space and velocity structure is present in the Ori OB1a subassociation. The low-mass members follow a well-defined band in the color-magnitude diagram, consistent with an isochronal age of similar to 7-10 Myr. The similar to 2 time drop in the overall Li I equivalent widths and accretion fraction between the younger Ori OB1b and the 25 Ori group is consistent with the latter being significantly older. In a simple-minded kinematic evolution scenario, the 25 Ori group may represent the evolved counterpart of the younger sigma Ori cluster. The 25 Ori stellar aggregate is the most populous similar to 10 Myr sample yet known within 500 pc, setting it as an excellent laboratory to study the evolution of solar-like stars and protoplanetary disks.