Dynamical collapse in star forming cores

The observational evidence of molecular cloud collapse is rare partly due to confusion of coexisting processes such as molecular outflows. In this thesis, we studied a number of dense molecular cloud cores with centimeter- and millimeter-wave interferometers. Toward two infall candidates in the W51 region, we find consistent spectroscopic signatures of cloud collapse in molecular lines of NH{dollar}sb3{dollar} CS, CH{dollar}sb3{dollar}CN, CH{dollar}sb3{dollar}OCH{dollar}sb3{dollar} and HCOOCH{dollar}sb3{dollar}. The optically thick lines show the inverse P-Cygni or double-peaked profiles with the blue-shifted components stronger than the red-shifted ones. The line asymmetries increase systematically toward transitions of larger optical depths or toward a transition synthesized with higher angular resolutions. These characteristics are in excellent agreement with the expected signatures from a centrally condensed infalling cloud.; The infall motion at scales from 0.04 pc to 0.2 pc appears to be radial and remains nearly constant. The mass densities in the infall region increase inward and scale with radius as r{dollar}sp{lcub}-2{rcub}{dollar}. The infall at the inner part ({dollar}sbsp{lcub}sim{rcub}{lcub}<{rcub}{dollar}0.04 pc) of the dense cores is accompanied by rotational motions which spin up as {dollar}rmomegapropto rsp{lcub}-1.2{rcub}{dollar}. The two infalling cores are embedded in an extended cloud which rotates in a different direction than the dense cores. It is likely that these dense cores are formed through fragmentation of the extended cloud. In this case, the rotational axis of the parental cloud may not be preserved during this process. Although the fragmentation may play important roles in core formation, it seems to be the collapse of individual cores that leads to the formation of OB stars.; We also studied two low mass regions, GSS30 and L1157, in which the dominant phenomenon turns out to be outflow motions. In the case of GSS30, we found an expanding core which is likely disrupted by the formation of stars. In L1157, we observed a powerful outflow from a very low luminosity (11 L{dollar}odot{dollar}) source which gives rise to heating and shocked emission in the cloud core.