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TwitterDOI: 10.1051/0004-6361/200811226
Physical properties of Southern infrared dark clouds
T. Vasyunina1, H. Linz1, Th. Henning1, B. Stecklum2, S. Klose2, and L.-Å. Nyman31 Max Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
e-mail: [vasyunina,linz,henning]@mpia.de
2 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: [stecklum;klose]@tls-tautenburg.de
3 ESO, Santiago 19, 19001 Casilla, Chile
e-mail: lnyman@eso.org
Received 24 October 2008 / Accepted 6 February 2009
Abstract
Context. What are the mechanisms by which massive stars form? What are the initial conditions
for these processes?
It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) represent the birth-sites
of massive stars. Therefore, these clouds have been receiving an
increasing amount of attention, and their analysis offers the opportunity to tackle the afore mentioned questions.
Aims. To enlarge the sample of well-characterised IRDCs in the
southern hemisphere, where ALMA will play a major role in the near future,
we have developed a program to study the gas and dust of southern
infrared dark clouds.
The present paper attempts to characterize the continuum properties of this
sample of IRDCs.
Methods. We cross-correlated 1.2 mm continuum data from SIMBA bolometer array
mounted on SEST telescope with Spitzer/GLIMPSE images to
establish the connection between emission sources at millimeter wavelengths and the IRDCs that
we observe at 8 
m in absorption against the bright PAH background. Analysing the dust emission and extinction
enables us to determine the masses and column densities, which are important quantities in
characterizing the initial conditions of massive star formation. We also evaluated
the limitations of the emission and extinction methods.
Results. The morphology of the 1.2 mm continuum emission is in all cases in close agreement with the mid-infrared
extinction. The total masses of the IRDCs were found to range from 150 to
1150 
(emission data) and from 300 to 1750 (extinction data). We derived peak column
densities of between 0.9 and
cm-2 (emission data)
and 2.1 and
cm-2 (extinction data). We demonstrate that the extinction method is unreliable at
very high extinction values (and column densities) beyond 
values of roughly 75 mag according to
the Weingartner & Draine (2001) extinction relation
model B (around 200 mag when following the common
Mathis (1990, ApJ, 548, 296) extinction calibration). By taking the spatial resolution effects into account and restoring
the column densities derived from the dust emission to a linear resolution of 0.01 pc, peak column densities
of 3–
cm-2 are obtained, which are much higher than typical values for low-mass cores.
Conclusions. Taking into account the spatial resolution effects, the derived column densities are beyond the column density
threshold of
cm-2 required by theoretical considerations for massive star formation.
We conclude that the values of column densities derived for
the selected IRDC sample imply that these objects are excellent candidates for objects in the
earliest stages of massive star formation.
Key words: ISM: dust, extinction -- ISM: clouds -- infrared: ISM -- radio continuum: ISM -- stars: formation
© ESO 2009