Infall/expansion velocities in the low-mass dense cores L492, L694-2, and L1521F: dependence on position and molecular tracer

Although surveys of infall motions in dense cores have been carried out for years, few surveys have focused on mapping infall across cores using multiple spectral-line observations. To fill this gap, we present IRAM 30 m telescope maps of N₂H⁺(1–0), DCO⁺(2–1), DCO⁺(3–2), and HCO⁺(3–2) emission toward two prestellar cores (L492 and L694-2) and one protostellar core (L1521F). We find that the measured infall velocity varies with position across each core and choice of molecular line, likely as a result of radial variations in core chemistry and dynamics. Line-of-sight infall speeds estimated from DCO⁺(2–1) line profiles can decrease by 40–50 m s⁻¹ when observing at a radial offset $⩾0.04$ pc from the core's dust continuum emission peak. Median infall speeds calculated from all observed positions across a core can also vary by as much as 65 m s⁻¹, depending on the transition. These results show that while single-pointing, single-transition surveys of core infall velocities may be good indicators of whether a core is either contracting or expanding, the magnitude of the velocities they measure are significantly impacted by the choice of molecular line, proximity to the core center, and core evolutionary state.