A survey for infall motions toward starless cores.: II.: CS (2-1) and N2H+ (1-0) mapping observations

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 N2H+ (1-0) and (CO)-O-18 (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 N2H+ and thirty seven well-defined N2H+ cores have been identified. The N2H+ emission is generally compact enough to find a peak, while the CS and (CO)-O-18 emissions are more diffuse. For each core, we have derived the normalized velocity difference (deltaV(CS)) between the thick CS and thin N2H+ peak velocities. We define 10 strong and nine probable infall candidates, based on deltaV(CS) analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the deltaV(CS) 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(-1). 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.