Millimeter‐Wave Interferometric Study of the ρ Ophiuchi A Region. I. Small‐Scale Structures of Dust Continuum Sources

Abstract

We performed 3 mm continuum and 12CO (J = 1-0) line observations of the ρ Ophiuchi A region using the Nobeyama Millimeter Array. The high-resolution dust continuum map reveals that the two submillimeter sources (SM1 and SM1N), which are considered to be pre-protostellar cores, consist of smaller scale fragments of 600-1100 AU in size. The small fragments lie along two filamentary features that cross each other at the position close to SM1, with a typical projected separation of 1200 AU. This is the first detection of such small-scale fragments in pre-protostellar cores from dust continuum observations. The masses and densities of the small fragments are estimated to be 0.054-0.14 M☉ and (2.0-15) × 107 cm-3, respectively, and they appear to be gravitationally bound. From our CO observations, we have discovered a previously unknown CO outflow whose axis is almost parallel to that of the nearby outflow associated with VLA 1623. However, the center of this outflow is likely to be the near-infrared source GY 30, and does not coincide with any of the small fragments. All small fragments except one show no sign of protostellar activities such as CO outflow and centimeter radio continuum emission, suggesting that they 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 formation has already occurred in the fragment. The presence of small-scale fragments with and without protostellar activities suggests that star formation in this region occur in such a small fragment or through coalescence of several small fragments.