Line Profiles of Cores within Clusters: II Signatures of Dynamical
  Collapse during High Mass Star Formation
  Observations of atomic or molecular lines can provide important information
about the physical state of star forming regions. In order to investigate the
line profiles from dynamical collapsing massive star forming regions (MSFRs),
we model the emission from hydrodynamic simulations of a collapsing cloud in
the absence of outflows. By performing radiative transfer calculations, we
compute the optically thick HCO+ and optically thin N2H+ line profiles from two
collapsing regions at different epochs. Due to large-scale collapse, the MSFRs
have large velocity gradients, reaching up to 20 km/s/pc across the central
core. The optically thin lines typically contain multiple velocity components
resulting from the superposition of numerous density peaks along the
line-of-sight. The optically thick lines are only marginally shifted to the
blue side of the optically thin line profiles, and frequently do not have a
central depression in their profiles due to self-absorption. As the regions
evolve the lines become brighter and the optically thick lines become broader.
The lower order HCO+ (1-0) transitions are better indicators of collapse than
the higher order (4-3) transitions. We also investigate how the beam sizes
affect profile shapes. Smaller beams lead to brighter and narrower lines that
are more skewed to the blue in HCO+ relative to the true core velocity, but
show multiple components in N2H+. High resolution observations (e.g. with ALMA)
can test these predictions and provide insights into the nature of MSFRs.
