Molecular line emission from turbulent clouds
V. Ossenkopf;
AaA, 2002, 391, 295
ABSTRACT:In the last years substantial progress has been made in modelling turbulent clouds and describing their structure by characteristic
parameters.
The missing link for a systematic comparison between models and
observations is the lack of efficient radiative transfer algorithms to generate
molecular line maps from the models comparable to the observed maps.
A fully
self-consistent solution of the radiative transfer problem is computationally very
demanding and hardly suited to evaluate a large set of cloud models with regard to
their agreement with observed molecular cloud structures.
We introduce a
new, computationally efficient code to calculate the line profiles based
on two reasonable approximations.
It is able to compute the molecular
line maps in turbulent cloud models with an accuracy of about 20% fast enough
to be run on large sets of model clouds.
Applying the code to
hydrodynamic, and magnetohydrodynamic cloud models we study how their structure
would appear in molecular line observations.
We show that no single
molecular line provides a good measure for the density structure in the
models.
The X factor, translating the integrated line intensities into column
densities, can be approximately constant within a density range covering up to a
factor 100 in few transitions but for each line this behaviour breaks down
outside of a limited range of densities.
Optical depth effects and subthermal
excitation result in a significant modification of the distribution of line
intensities relative to the column density distribution.
All lower transitions
of CO isotopes only trace gas at low and intermediate densities which is
distributed over all scales in molecular clouds.
Turbulence models driven on the
largest scales reproduce the observed scaling behaviour.
Higher CO
transitions are only excited in dense cores resulting from shocks or gravitational
collapse.
The existence of massive dense cores resulting from collapse can only be
inferred when comparing observations in different transitions taken with an
excellent signal-to-noise ratio or from dedicated high-density tracers.
The
line profiles obtained from turbulence models driven on large scales break
up into several fragments in contrast to observations of molecular
clouds without heavy star-formation which show typically smooth profiles
with close-to-Gaussian shape.
None of the turbulence simulations
provides a good match of all observed properties for this type of clouds.
The
velocity scaling behaviour of all observations and turbulence models is
consistent with the interpretation of a molecular cloud as shock-dominated
medium.
More observational data are needed to provide a reliable conclusion on the
degree of intermittency.
As molecular lines fail to reflect the density
structure of an interstellar cloud line observations should be combined with
dust continuum observations to deduce column densities.
On the other hand
we need the velocity information contained in line observations to
discriminate between different turbulence models.
KEYWORDS: radiative transfer, ism: clouds, ism: structure, radio lines: ism
PERSOKEY:turbulence, simulation, co, ,
CODE: ossenkopf2002a