2007ApJ...670.1225M

A sample of 58 probable members of the association of pre-main-sequence stars around the filamentary ρ Ophiuchi cloud is investigated. Using astrometric proper motions from the UCAC-2 catalog and the convergent point method, the total heliocentric systemic velocity is estimated at 19 km/s, and the mean distance at 145 pc. A small, statistically insignificant difference between the geometric convergent point and the actual direction of velocity defined by the observed radial velocity implies a small, if any, rate of expansion of the association. The Ophiuchus association appears to have a considerable depth, with half of the members lying within 15 pc of the center. When the moving cluster distances are taken into account, the near-infrared M_Ks versus (J-K_s) H-R diagram reveals an apparent large spread of ages between 14 Myr and younger than 1 Myr. Most of the youngest stars are located along a slightly curved strip just south of the densest cloud, extending in the east-west direction roughly aligned with the central streamer of warm dust. The intersection of this strip with a thin segment of shocked dust visible in the IRAS 12 µm map at (α,δ)=(16^h31^m,-24.5°) is marked with a small cluster of probably very young stars around ROXs 43A. The large extent and depth of the association, the moderate rate of expansion, the spread in ages of about 14 Myr, and the alignment of very young stars with the dusty streamer point at a dynamical mode of star formation in this region, scattered far and wide around the main core.