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We report here on the photometric and kinematic properties of a well definedgroup of nearly 200 low-mass pre-main sequence stars, concentrated within ~ 1deg of the early-B star 25 Ori, in the Orion OB1a sub-association. We refer tothis stellar aggregate as the 25 Orionis group. The group also harbors theHerbig Ae/Be star V346 Ori and a dozen other early type stars with photometry,parallaxes, and some with IR excess emission, consistent with group membership.The number of high and low-mass stars is in agreement with expectations from astandard Initial Mass Function. The velocity distribution for the young starsin 25 Ori shows a narrow peak centered at 19.7 km/s, very close to the velocityof the star 25 Ori. Our results provide new and compelling evidence that the 25Ori group is a distinct kinematic entity, and that considerable space andvelocity structure is present in the Ori OB1a sub-association. The low-massmembers follow a well defined band in the color-magnitude diagram, consistentwith an isochronal age of ~ 7-10 Myr, depending on the assumed evolutionarymodel. The highest density of members is located near the star 25 Ori, but theactual extent of the cluster cannot be well constrained with our present data.In a simple-minded kinematic evolution scenario, the 25 Ori group may representthe evolved counterpart of a younger aggregate like the sigma Ori cluster. The25 Ori stellar aggregate is the most populous ~ 10 Myr sample yet known within500 pc, setting it as an excellent laboratory to study the evolution ofsolar-like stars and protoplanetary disks.Comment: 28 pages, 5 figures. Astrophysical Journal, in press. Abridged  abstrac

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