Protostellar collapse in dense cores in Taurus

We present the results of a survey for dense molecular condensations in Taurus and the succeeding detection of a high-density condensation that is very close to the moment of the formation of a protostellar core within a time scale of similar to10(4) yr. We have carried out a survey for dense molecular condensations with the 45-m telescope at Nobeyama in (HCO+)-C-13 (J = 1-0) on the basis of a complete (CO)-O-18 survey made with the 4-m telescope at Nagoya. We detected similar to55 dense condensations, of which similar to45 are starless. These starless condensations are compact (R less than or similar to 0.1 pc) and of high-density (similar to10(5) cm(-3)) and thus are highly probable candidates for protostellar condensations. One of the starless condensations, named MC 27 in our catalog, is the densest, similar to10(6) cm(-3), based on the observation in (HCO+)-C-13 (J = 1-0, 3-2). A statistical analysis indicates a very short time scale of similar to10(4) yr. These properties strongly suggest that MC 27 is in a very early stage of star formation. The HCO+ (J = 3-2, 4-3) profiles show the existence of an infalling envelope. The 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 to10(3-4) yr.