NH₃ (1,1) hyperfine intensity anomalies in infall sources

Abstract

Identifying infall motions is crucial for our understanding of accretion processes in regions of star formation. The NH$_ $ $(1,1)$ hyperfine intensity anomaly (HIA) has been proposed to be a readily usable tracer for such infall motions in star-forming regions harboring young stellar objects at very early evolutionary stages. In this paper, we seek to study the HIA toward 15 infall candidate regions in order to assess its reliability as an infall tracer. Using deep observations of the NH$_ $ $(1,1)$ transition with the Effelsberg 100\,m telescope, we identified HIAs toward all 15 targets. Of the 15 sources, 14 exhibit anomalous intensities in either the inner or outer satellite lines. All the derived HIAs conform to the framework of the existing two models, namely hyperfine selective trapping (HST) and systematic contraction or expansion motion (CE) models. In our sample of infall candidates, the majority of the HIAs remain consistent with the HST model. Only in three targets are the HIAs consistent with infall motions under the CE model. Thus, the HIA could indeed be used as an infall tracer, but does not appear to be highly sensitive to infall motions in our single-dish data. Nevertheless, the emission could be blended with emission from outflow activities. HIAs consistent with the HST model show stronger anomalies with increasing kinetic temperatures K $), which is expected based on the HST model. On the other hand, HIAs consistent with infall motions show little dependence on $T_ K $. Therefore, HIAs may preferably trace the infall of cold gas.