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 class 0 protostar GF9-2 inthe filamentary dark cloud GF 9 (d = 200 pc). GF9-2 stands unique in the sensethat it shows H2O maser emission, a clear signpost of protostar formation,whereas it does not have a high-velocity large-scale molecular outflowevidenced by our deep search for CO wing emission. These facts indicate thatGF9-2 core is early enough after star formation so that it still retains someinformation of initial conditions for collapse. Our 350 um dust continuumemission image revealed the presence of a protostellar envelope in the centerof a molecular core. The mass of the envelope is ~0.6 Msun from the 350 um fluxdensity, while LTE mass of the core is ~3 Msun from moleuclar lineobservations. Combining visibility data from the OVRO mm-array and the 45mtelescope, we found that the core has a radial density profile of$\rho(r)\propto r^{-2}$ for 0.003 < r/pc < 0.08 region. Molecular line dataanalysis revealed that the velocity width of the core gas increasesinward,while the outermost region maintains a velocity dispersion of a fewtimes of the ambient sound speed. The broadened velocity width can beinterpreted as infall. Thus, the collapse in GF9-2 is likely to be described byan extension of the Larson-Penston solution for the period after formation of acentral star. We derived the current mass accretion rate of ~3E-05 Msun/yearfrom infall velocity of ~ 0.3 km/s at r~ 7000 AU. All results suggest thatGF9-2 core has been undergoing gravitational collapse for ~ 5000 years sincethe formation of central protostar(s), and that the unstable state initiatedthe collapse ~2E+05 years (the free-fall time) ago.