A Survey for Infall Motions toward Starless Cores. II. CS (2-1) and N2H+ (1-0) Mapping Observations

By C. Lee, P. Myers, M. Tafalla. 2001

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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 C18O (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 C18O emissions are more diffuse. For each core, we have derived the normalized velocity difference (δVCS) between the thick CS and thin N2H+ peak velocities. We define 10 "strong" and nine "probable" infall candidates, based on δVCS analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the δVCS 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.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/09a2fc27d0d096479175c9e014bfcd7f2ee51256

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Citations in the corpus

On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B (2004, unknown): “Additional References: (1) C. Gottlieb, priv. comm., (2) Müller et al. (2001), (3) Cazzoli et al. (2002), (4) Gottlieb et al. (2003), (5) Kukolich (1967), (6) Lee et al. (2001), astronomical, (7) Caselli et al. (2002c), astronomical.”
Systematic Molecular Differentiation in Starless Cores (2001, unknown): “Following Lee et al. (2001), we use these frequencies: 93176.258 MHz for N2H + (JF1F = 101–012), 97980.953 MHz for CS (J = 2–1), 109782.172 for C18O (J = 1–0), and 23694.4949 MHz for NH3 (J,K = 1, 1).”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “The first two of these properties are often interpreted as evidence of quasi-static contraction in the cores, as proposed by Shu (1977, herafter S77), although it has been difficult to reconcile them with the third property (e.g., Lee et al. 2001; Bergin & Tafalla 2007).”
On the internal structure of starless cores II. A molecular survey of L1498 and L1517B (2006, methodology): “Recent infall searches in starless cores (Lee et al. 2004; Sohn et al. 2004), do use this property of DCO+ and HCN to penetrate deeper than using traditional tracers like CS or HCO+ (e.g., Lee et al. 2001).”
Probing the Evolutionary Status of Starless Cores through N2H+ and N2D+ Observations (2004, methodology): “Deuterium Fractionation and Extended Infall Lee et al. (1999, 2001, 2004) in a series of papers conducted a search for infall motions toward a sample of 70 starless cores.”
Dense Cores in Dark Clouds. XIV. N2H+ (1-0) Maps of Dense Cloud Cores (2002, background): “Large values of s VLSR Vfith i=Gr ( 1) are also present in TMC-2, an infall candidate with CS asymmetry (Lee et al. 2001).”
From molecular cores to planet-forming disks: An SIRTF legacy program (2003, methodology): “The initial criteria for selection of the isolated dense core sample were that the distance is less than 400 pc, that the size is not too large (typically less than 5 ), and that the core has been mapped in a dense gas tracer, typically NH3 (Jijina, Myers, & Adams 1999), CS (Lee, Myers, & Tafalla 20”
Control of star formation by supersonic turbulence (2000, background): “Similar conclusions can be derived for a variety of other sources (see the review of Myers, Evans, & Ohashi 2000; or the extended survey for infall motions in prestellar cores by Lee, Myers, and Tafalla 1999, 2001).”