Molecular Line Profiles of Collapsing Gas Clouds

TL;DR

This paper models molecular line profiles from collapsing gas clouds using self-similar models, successfully reproducing observed asymmetric double-peak profiles and providing a tool to infer physical parameters of star-forming regions.

Contribution

It introduces a self-consistent polytropic collapse model that accurately reproduces observed line profiles and offers a method to extract physical parameters from molecular line spectra.

Findings

The isothermal model cannot produce observed asymmetric line profiles.

The polytropic model reproduces asymmetric double-peak profiles with a stronger blue peak.

The model can estimate protostellar mass, accretion rate, and collapse timescale.

Abstract

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 cannot 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. The relative strengths of the blue and red peaks within a molecular line profile are determined by the cloud temperature gradient. 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.