How to Identify Pre‐Protostellar Cores

We have observed the HCO+ J=3-2 line toward 17 starless cores selected fromthe list of Ward-Thompson et al. (1994). Six of these cores have lineasymmetries indicative of collapse. The excess of blue-skewed profiles overred-skewed profiles is at least as large as that found in samples of Class 0and early Class I sources. The observed line profiles have the same narrowlinewidths and small peak temperatures predicted for young sources inevolutionary models, but the blue/red ratios, like those of older sources, arehigher than models predict. The infall signature also occurs over large scales,suggesting that these cores have overall inward motions. We have divided thesestarless cores into two groups based on the continuum photometry ofWard-Thompson et al. and our HCO+ data. We find stronger HCO+ emission amongthe cores detected in the submillimeter and all the blue-skewed line profilesare in this group, supporting the suggestion of Ward-Thompson et al. that theseare the pre-protostellar cores.Comment: 22 pages, 6 figures, to be published in The Astrophysical Journa