Emission line profiles of tracer molecule H _ { 2 } CO 140 GHz transition from gravitational core collapsing clouds in the dynamic process of forming protostars are calculated , using a simple ray-tracing radiative transfer model .
Three self-similar dynamic inside-out core collapse models – the conventional polytropic model , the empirical hybrid model and the isothermal model – for star-forming molecular clouds are examined and compared .
The isothermal model can not produce observed asymmetric double-peak molecular line profiles .
The conventional polytropic model , which gives flow velocity , mass density and temperature profiles self-consistently , can produce asymmetric double-peak line profiles for a core collapsing cloud .
In particular , the blue peak is stronger than the red peak , consistent with a broad class of molecular line profile observations .
We find that line profiles are robust against variations in the polytropic index \gamma once the effective line-centre opacity \kappa _ { 0 } is specified .
The relative strengths of the blue and red peaks within a molecular line profile are determined by the cloud temperature gradient , but the emission at frequencies between the two line peaks is determined by detailed density and velocity profiles in the cloud core .
In the presence of a static dense kernel at the centre of a collapsing cloud , strong internal absorption along the line-of-sight may occur , causing a suppression to the red wing of the blue line peak .
If reliably resolved in frequency by observations , this signature may be potentially useful for probing the environs of an infant protostar .
The conventional polytropic model can be utilized to produce molecular line-profile templates , for extracting dynamical information from line spectra of molecular globules undergoing a gravitational core collapse .
We show a sample fit using the 140 GHz H _ { 2 } CO emission line from the central region of the molecular globule B335 by our model with \gamma = 1.2 .
The calculation of line profiles and fitting processes also offer a scenario to estimate the protostellar mass , the kernel mass accretion rate , and the evolution time scale of a core collapsing cloud .
Our model can be readily adapted to other tracer molecules with more or less constant abundances in star-forming clouds .