A very dense low-mass molecular condensation in taurus: Evidence for the moment of protostellar core formation

We present evidence for a protostellar condensation that is very close to the moment of the formation of a protostellar core within a time scale of similar to 10(4) yr. This starless condensation, named MC 27 in complete surveyed molecular condensations in Taurus, is similar to 0.1 pc in size and similar to 3 M. in mass. It exhibits fairly strong and narrow (HCO+)-C-13 emission of the J = 4-3, 3-2, and 1-0 transitions, as well as self-reversed profiles of HCO+ J = 4-3 and 3-2. MC 27 has a density of similar to 10(6) cm(-3) within less than or similar to 1000 AU at the center, which is the highest value among the similar to 40 starless condensations in Taurus. MC 27 shows a sharply peaked density distribution; the molecular intensity is well fitted by a power-law density distribution of r(similar to-2) over 0.02 pc < r < 0.2 pc. A statistical analysis indicates a very short time scale of similar to 10(4) yr, which is consistent with a free-fall time scale for a density of similar to 10(6) cm(-3). These properties strongly suggest that MC 27 is in a very early stage of star formation. A Monte Carlo simulation of the present profiles indicates that the infall velocity at 2000-3000 AU should be 0.2-0.3 km s(-1) while it is less than 0.3 km s(-1) at less than or similar to 1000 AU. This derived infall velocity profile can be explained by a dynamical-collapse model of supercritical condensation prior to formation of the first protostellar core by similar to 10(3-4) yr.