A Search for Infall Motions toward Nearby Young Stellar Objects

We report observations of 47 candidate protostars in two optically thicklines (H2CO 212-111 and CS 2-1) and one optically thin line (N2H+ 1-0) usingthe IRAM 30-m, SEST 15-m, and Haystack 37-m radio telescopes. The sources wereselected from the redness of their spectra (Tbol < 200 K) and their neardistance (d < 400 pc). Most of the sources have asymmetric optically thicklines. The observed distribution of velocity differences, deltaV =(Vthick-Vthin)/FWHMthin, is skewed toward negative (blue-shifted) velocitiesfor both the H2CO and CS samples. This excess is much more significant forClass 0 than for Class I sources, suggesting that we detect infall motionstoward Class 0 and not toward Class I sources. This indicates a difference inthe physical conditions in the circumstellar envelopes around Class I and Class0 sources, but does not rule out the presence of infall onto Class I sources bye.g. lower opacity gas. Bipolar outflows alone, or rotation alone, cannotreproduce these statistics if the sample of sources has randomly orientedsymmetry axes. We identify 15 spectroscopic infall candidates, of which 6 arenew. Most of these infall candidates have primarily turbulent rather thanthermal motions, and are associated with clusters rather than being isolated.Comment: 33 pages, AAS LaTeX, includes 5 postscript figures, to appear in the ApJ, vol 489, November 10, 1997 issu