Evidence for Pressure‐driven Flows and Turbulent Dissipation in the Serpens NW Cluster

Abstract

We have mapped the dense gas distribution and dynamics in the northwest region of the Serpens molecular cloud in the CS (2-1) and N2H+ (1-0) lines and 3 mm continuum using the FCRAO telescope and the Berkeley-Illinois-Maryland Association (BIMA) interferometer. Seven continuum sources are found. The N2H+ spectra are optically thin, and fits to the seven hyperfine components are used to determine the distribution of velocity dispersion. Eight cores, two with continuum sources, six without, lie at a local line width minimum and optical depth maximum. The CS spectra are optically thick and generally self-absorbed over the full 0.2 pc extent of the map. We use the line wings to trace outflows around at least three, and possibly four, of the continuum sources and the asymmetry in the self-absorption as a diagnostic of relative motions between core centers and envelopes. The quiescent regions with low N2H+ line width tend to have more asymmetric CS spectra than the spectra around the continuum sources, indicating higher infall speeds. These regions have typical sizes ~5000 AU, line widths ~0.5 km s-1, and infall speeds ~0.05 km s-1. The correlation of CS asymmetry with N2H+ velocity dispersion suggests that the inward flows of material that build up pre-protostellar cores are driven at least partly by a pressure gradient rather than by gravity alone. We discuss a scenario for core formation and eventual star-forming collapse through the dissipation of turbulence.