Validity of the uniform infall velocity assumption in collapsing regions

Determine whether the infall velocity field is uniform across collapsing star-forming regions, as assumed by the two-layer model and the Hill model used to interpret blue-asymmetric molecular line profiles, or whether the infall speed varies with radius; quantify the implications of any non-uniformity for inferring infall from HCO+ J=1–0 and related tracers.

Background

Parametric line-profile models such as the two-layer model and the Hill model are widely used to infer infall velocities from blue-asymmetric profiles in molecular lines. A core assumption of these models is that the infall velocity is spatially uniform across the region being modeled.

The paper highlights that whether this uniform-infall assumption holds in collapsing regions remains uncertain. This uncertainty motivates the authors’ spatially resolved modeling of SDC335 to probe radial variations in infall velocity and assess how departures from uniformity affect line profiles and inferred kinematics.