25 ORIONIS: A KINEMATICALLY DISTINCT 10 Myr OLD GROUP IN ORION OB 1a

摘要

We report here on the photometric and kinematic properties of a well-defined group of nearly 200 low-mass premain-sequence stars, concentrated within ～ 1° of the early-B star 25 Ori, in the Orion OB 1 a 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, parallaxes, 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~(-1), offset ～—10 km s~(-1) from the velocity characterizing the younger stars of the Ori OB1b subassociation, and —4 km s~(-1) from the velocity of widely spread young stars of the Ori OBla 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 OB 1a subassociation. The low-mass members follow a well-defined band in the color-magnitude diagram, consistent with an isochronal age of ～7— 10 Myr. The ～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 σ Ori cluster. The 25 Ori stellar aggregate is the most populous ～ 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.