2004A&A...421.1113B

Evidence for a large scale flow of low density gas onto the Cepheus A young stellar cluster is presented. Observations of K-band near-infrared and multi-transition CS and N₂H⁺ millimeter line emission are shown in relation to a sub-millimeter map of the cool dust around the most embedded stars. The near-infrared emission is offset from the dust peak suggesting a shift in the location of star formation over the history of the core. The CS emission is concentrated toward the core center but N₂H⁺ peaks in two main cores offset from the center, opposite to the chemistry observed in low mass cores. A starless core with strong CS but weak N₂H⁺ emission is found toward the western edge of the region. The average CS(2-1) spectrum over the cluster forming core is asymmetrically self-absorbed suggesting infall. We analyze the large scale dynamics by applying a one-dimensional radiative transfer code to a model spherical core with constant temperature and linewidth, and a density profile measured from an archival 850µm map of the region. The best fit model that matches the three CS profiles requires a low CS abundance in the core and an outer, infalling envelope with a low density and undepleted CS abundance. The integrated intensities of the two N₂H⁺ lines is well matched with a constant N₂H⁺ abundance. The envelope infall velocity is tightly constrained by the CS(2-1) asymmetry and is sub-sonic but the size of the infalling region is poorly determined. The picture of a high density center with depleted CS slowly accreting a low density outer envelope with normal CS abundance suggests that core growth occurs at least partially by the dissipation of turbulent support on large scales.