  We recently proposed that molecular cloud dense cores undergo a prolonged
period of quasi-static contraction prior to true collapse. This theory could
explain the observation that many starless cores exhibit, through their
spectral line profiles, signs of inward motion. We now use our model, together
with a publicly available radiative transfer code, to determine the emission
from three commonly used species - N2H+, CS, and HCN. A representative dense
core of 3 Msun that has been contracting for 1 Myr has line profiles that
qualitatively match the observed ones. In particular, optically thick lines
have about the right degree of blue-red asymmetry, the empirical hallmark of
contraction. The J=2-1 rotational transition of CS only attains the correct
type of profile if the species is centrally depleted, as has been suggested by
previous studies. These results support the idea that a slow, but accelerating,
contraction leads to protostellar collapse. In the future, the kind of analysis
presented here can be used to assign ages to individual starless cores.
