71 0 v 1 2 4 Ja n 20 07 25 Orionis : A kinematically distinct 10 Myr old group in Orion OB 1 a 1

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 ∼ 1 of the early-B star 25 Ori, in the Orion OB1a sub-association. 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, consistent with 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 young stars in 25 Ori shows a narrow peak centered at 19.7 km s, very close to the velocity of the star 25 Ori. This velocity peak is offset ∼ −10 km s from the velocity characterizing the younger stars of the Ori OB1b sub-association, and −4 km s from the velocity of more widely spread young stars of the Ori OB1a population near the σ Ori cluster; 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 sub-association. The low-mass Based on observations obtained at the Llano del Hato National Astronomical Observatory of Venezuela, operated by CIDA for the Ministerio de Ciencia y Tecnoloǵıa; the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona; and the Fred Lawrence Whipple Observatory of the Smithsonian Institution, USA Centro de Investigaciones de Astronomı́a (CIDA), Apartado Postal 264, Mérida, 5101-A, Venezuela. E-mails: [email protected], [email protected] Department of Astronomy, University of Michigan, 500 Church St. 830 Dennison, Ann Arbor, MI 48109, USA. E-mails: [email protected], [email protected], [email protected] Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA. E-mails: [email protected], [email protected] Associate Researcher, Smithsonian Astrophysical Observatory