Rowan J. Smith, Rahul Shetty, Henrik Beuther, Ralf S. Klessen, Ian A. Bonnell
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. When sightlines pass through filaments or the central protostar of MSFRs optically thick line profiles generally portray the blue asymmetry associated with the large scale collapse motions. Low mass star forming regions do not always show a blue asymmetry, as the surrounding medium may or may not be collapsing. 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+, 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.
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http://arxiv.org/abs/1304.4950
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