How to identify pre-protostellar cores

We have observed the HCO+ J = 3-2 line toward 17 starless cores selected fr om the list of Ward-Thompson and coworkers. Six of these cores have line as ymmetries indicative of collapse. The excess of blue-skewed profiles over r ed-skewed profiles is at least as large as that found in samples of Class 0 and early Class I sources. The observed line profiles have the same narrow line widths and small peak temperatures predicted for young sources in evo lutionary models, but the blue/red ratios, like those of older sources, are higher than models predict. The infall signature also occurs over large sc ales, suggesting that these cores have overall inward motions. We have divi ded these starless cores into two groups based on the continuum photometry of Ward-Thompson and coworkers and our HCO+ data. We find stronger HCO+ emi ssion among the cores detected in the submillimeter, and all the blue-skewe d line profiles are in this group, supporting the suggestion of Ward-Thomps on and coworkers that these are the preprotostellar cores.