Millimeter-wave interferometric study of the p Ophiuchi A region. I. Small-scale structures of dust continuum sources

We performed 3 mm continuum and (CO)-C-12 (J = 1-0) line observations of th e rho Ophiuchi A region using the Nobeyama Millimeter Array. The high-resol ution dust continuum map reveals that the two submillimeter sources (SM1 an d SM1N), which are considered to be pre-protostellar cores, consist of smal ler scale fragments of 600-1100 AU in size. The small fragments lie along t wo filamentary features that cross each other at the position close to SM1, with a typical projected separation of 1200 AU. This is the first detectio n of such small-scale fragments in pre-protostellar cores from dust continu um observations. The masses and densities of the small fragments are estima ted to be 0.054-0.14 M. and (2.0- 15) x 10(7) cm(-3), respectively, and the y appear to be gravitationally bound. From our CO observations, we have discovered a previously unknown CO outflo w whose axis is almost parallel to that of the nearby outflow associated wi th VLA 1623. However, the center of this outflow is likely to be the near-i nfrared source GY 30, and does not coincide with any of the small fragments . All small fragments except one show no sign of protostellar activities su ch as CO outflow and centimeter radio continuum emission, suggesting that t hey are in the evolutionary stage prior to protostar formation with outflow activities. On the other hand, the small fragment with the largest mass is likely to be associated with 6 cm continuum emission, which is thought to be free-free emission. This implies the possibility that the protostellar f ormation has already occurred in the fragment. The presence of small-scale fragments with and without protostellar activities suggests that star forma tion in this region occur in such a small fragment or through coalescence o f several small fragments.