A Survey of Infall Motions toward Starless Cores. I. CS (2–1) and N2H+(1–0) Observations

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Abstract
We present the first results of a survey of 220 starless cores selected primarily by their optical obscuration and observed in CS (2-1), N2H+ (1-0), and C18O (1-0) using the Northeast Radio Observatory Corporation (NEROC) Haystack 37 m telescope. We have detected 163 out of 196 sources observed in CS, 72 out of 142 in N2H+, and 30 out of 30 in C18O. In total, 69 sources were detected in both CS and N2H+. The isolated component of the N2H+ (1-0) spectrum (F1F = 0,1-1,2) usually shows a weak symmetric profile that is optically thin. In contrast, a significant fraction of the CS spectra show non-Gaussian shapes, which we interpret as arising from a combination of self-absorption due to lower excitation gas in the core front and kinematics in the core. The distribution of the normalized velocity difference (δVCS) between the CS and N2H+ peaks appears significantly skewed to the blue (δVCS < 0), as was found in a similar study of dense cores with embedded young stellar objects (YSOs). The incidence of sources with blue asymmetry tends to increase as the total optical depth or the integrated intensity of the N2H+ line increases. This overabundance of "blue" over "red" sources suggests that inward motions are a significant feature of starless cores. We identify seven strong infall candidates and 10 probable infall candidates. Their typical inward speeds are subsonic, approximately 0.04-0.1 km s-1, so they contain "thermal" infall motions, unlike the faster inward speeds associated with most YSOs. We discuss the importance of the choice of a consistent set of line frequencies when using the velocity shift between an optically thick and a thin line as a tracer of infall, and show how the results of the survey depend on that frequency choice.
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