A SEARCH FOR INFALL MOTIONS TOWARD NEARBY YOUNG STELLAR OBJECTS

We report observations of 47 candidate protostars in two optically thi ck lines [H2CO (2(12)-1(11)) and CS (2-1)] and one optically thin line [N2H+ (1-0)] using the IRAM 30 m, SEST 15 m, and Haystack 37 m radio telescopes. The sources were selected for the redness of their spectra (T-bol < 200 K) and their near distance (d < 400 pc). Most of the sou rces have asymmetric, optically thick lines. The observed distribution of velocity differences, delta V = (V-thick - V-thin)/Delta V-thin, i s skewed toward negative (blueshifted) velocities for both the H2CO an d CS samples. This excess is much more significant for class 0 than fo r class I sources, suggesting that we detect infall motions toward cla ss 0 and not toward class I sources. This indicates a difference in th e physical conditions in the circumstellar envelopes around class I an d class 0 sources, but it does not rule out the presence of infall ont o class I sources by, for example, lower opacity gas. Bipolar outflows alone, or rotation alone, cannot reproduce these statistics if the sa mple of sources has randomly oriented symmetry axes. We identify 15 sp ectroscopic infall candidates, six of which are new. Most of these inf all candidates have primarily turbulent rather than thermal motions an d are associated with clusters rather than being isolated.