The Initial Conditions for Gravitational Collapse of a Core: An Extremely Young Low-Mass Class 0 Protostar GF 9-2

We present a study of the natal core harboring the low-mass Class 0 protostar GF 9-2 in the filamentary dark cloud GF 9 using the Nobeyama 45 m and CSO 10.4 m telescopes and the OVRO millimeter array. GF 9-2 is unique in the sense that it shows H₂O masers, a clear signpost of protostar formation, but does not have a high-velocity large-scale outflow. These facts indicate that the GF 9-2 core is early enough after star formation that it still retains some information of initial conditions for collapse. Our 350 μm dust continuum image identified a protostellar envelope with an extent of ≃5400 AU in the center of the molecular core ≃0.08 pc in size. The envelope mass is estimated to be ≃0.6 M_☉ from the 350 μm flux density, while the LTE mass of the core is ≃3 M_☉ from molecular lines. We found that the core has a radial density profile of ρ(r) ∝ r^-2 for the 0.003 ≲ r/pc ≲ 0.08 region and that the velocity width of the core gas increases inward, while the outermost region maintains a velocity dispersion of a few times the ambient sound speed. If we interpret the broadened velocity width as infall, the core collapse can be well described by an extension of the Larson-Penston solution for the period after formation of a central star. We derived the mass accretion rate of ≃3 × 10^-5 M_☉ yr^-1 from infall velocity of ≃0.3 km s^-1 at r ≃ 7000 AU. Furthermore, we found evidence that a protobinary is being formed at the core center. All of the results suggest that the GF 9-2 core has been undergoing gravitational collapse for ≲5000 yr since the protostar formation and that the unstable state initiated the collapse ≃2 × 10⁵ yr (the free-fall time) ago.