Molecular Cloud Structure: The VLT View

João Alves, Charles Lada, Elizabeth Lada, Marco Lombardi, Edwin A. Bergin

Despite 30 years of molecular spectroscopy of dark clouds little is

understood about the internal structure of these objects and

consequently about the initial conditions that give rise to star and

planet formation. This is largely due to the fact that molecular clouds

are primarily composed of molecular hydrogen, which is virtually

inaccessible to direct observation. The traditional methods used to

derive the basic physical properties of these clouds therefore make use

of observations of trace H2 surrogates, namely those rare

molecules with sufficient dipole moments to be easily detected by radio

spectroscopic techniques (e.g., Lada 1996, Myers 1999), and interstellar

dust, whose thermal emission can be detected by radio continuum

techniques (e.g., André et al. 2000). However, as discussed in

the previous article in this book by M. Walmsley and collaborators, the

interpretation of results derived from these methods is not always

straightforward (see also Alves, Lada, & Lada 1999 and Zucconi,

Walmsley, & Galli 2001). Several poorly constrained effects inherent

in these techniques (e.g., deviations from local thermodynamic

equilibrium, opacity variations, chemical evolution, small-scale

structure, depletion of molecules, unknown emissivity properties of the

dust, unknown dust temperature) make the construction of an unambiguous

picture of the physical structure of these objects a very difficult

task. There is a clear need for a less complicate and more robust tracer

of H2 to access not only the physical structure of these

objects but also to accurately calibrate molecular abundances and dust

emissivity inside these clouds. The deployment of sensitive, large

format infrared array cameras on large telescopes however, has fulfilled

this need by enabling the direct measurement of the dust extinction

toward thousands of individual background stars observed through the

densest regions of a molecular cloud. Such measurements are free from

the complications that plague molecular-line or dust emission data and

enable detailed maps of cloud density structure to be constructed.

Department of Astrophysics
External organisation(s)
University of Florida, Gainesville, Harvard-Smithsonian Center for Astrophysics, European Southern Observatory (Germany)
Publication date
Austrian Fields of Science 2012
103004 Astrophysics
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