A Dynamically Collapsing Core and a Precursor of a Core in a Filament Supported by Turbulent and Magnetic Pressures

Abstract

To study physical properties of the natal filament gas around the cloud core harboring an exceptionally young low-mass protostar GF9-2, we carried out J=1-0 line observations of 12CO, 13CO, and C18O molecules using the Nobeyama 45m telescope. The mapping area covers ~1/5 of the whole filament. Our 13CO and C18O maps clearly demonstrate that the core formed at the local density maxima of the filament, and the internal motions of the filament gas are totally governed by turbulence with Mach number of ~2. We estimated the scale height of the filament to be H = 0.3 ~ 0.7 pc, yielding the central density of n_c = 700 ~4200 cm^-3. Our analysis adopting an isothermal cylinder model shows that the filament is supported by the turbulent and magnetic pressures against the radial and axial collapse due to self-gravity. Since both the dissipation time scales of the turbulence and the transverse magnetic fields can be comparable to the free-fall time of the filament gas of 10^6 years, we conclude that the local decay of the supersonic turbulence made the filament gas locally unstable, hence making the core collapse. Furthermore, we newly detected a gas condensation with velocity width enhancement to ~0.3 pc south-west of the GF9-2 core. The condensation has a radius of ~0.15 pc and an LTE mass of ~5 Msun. Its internal motion is turbulent with Mach number of ~3, suggestive of a gravitationally unbound state. Considering the uncertainties in our estimates, however, we propose that the condensation is a precursor of a cloud core which would have been produced by the collision of the two gas components identified in the filament.