2001ApJS..136..703L

We present the results of an extensive mapping survey of ``starless'' cores in the optically thick line of CS (2-1) and the optically thin lines of N<sub>2</sub>H<sup>+</sup> (1-0) and C<sup>18</sup>O (1-0). The purpose of this survey was to search for signatures of extended inward motions. A total of 53 targets were observed in the three lines with the FCRAO 14 m telescope. Thirty-three regions were mapped in both CS and N<sub>2</sub>H<sup>+</sup>, and thirty seven well-defined N<sub>2</sub>H<sup>+</sup> cores have been identified. The N<sub>2</sub>H<sup>+</sup> emission is generally compact enough to find a peak, while the CS and C<sup>18</sup>O emissions are more diffuse. For each core, we have derived the normalized velocity difference (δV<sub>CS</sub>) between the thick CS and thin N<sub>2</sub>H<sup>+</sup> peak velocities. We define 10 ``strong'' and nine ``probable'' infall candidates, based on δV<sub>CS</sub> analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the δV<sub>CS</sub> parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple two-layer radiative transfer model to fit the profiles, we derive one-dimensional infall speeds, the values of half of which lie in the range of 0.05-0.09 km.s<sup>–1</sup>. These values are similar to those found in L1544 by Tafalla et al., and this result confirms that infall speeds in starless cores are generally faster than expected from ambipolar diffusion in a strongly subcritical core. In addition, the observed infall regions are too extended to be consistent with the ``inside-out'' collapse model applied to a very low mass star. In the largest cores, the spatial extent of the CS spectra with infall asymmetry is larger than the extent of the N₂H⁺ core by a factor of 2-3. All these results suggest that extended inward motions are a common feature in starless cores, and that they could represent a necessary stage in the condensation of a star-forming dense core.