Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sptz.prop30922g&link_type=abstract
Spitzer Proposal ID #30922
Physics
Scientific paper
Recent results from the Canadian Galactic Plane Survey show that cold diffuse clouds are an important component of the Galactic interstellar medium. These clouds, detected as self-absorption features at 21 cm, have temperatures and densities that bridge the gap between the ambient atomic ISM and the colder, denser molecular medium. Consequently, they may be the long-sought sites where atomic gas condenses into the molecular phase required for star formation. Although critical to the evolution of matter in the Galaxy, molecular condensation is poorly understood, in large part because its quiescent nature renders it less detectable than more energetic phase changes like dissociation, ionization, or recombination. CDCs offer a unique opportunity to study this evolutionary ``missing link''. The question of the origin and stability of these structures is raised. Without molecular gas, the low temperatures of these cold diffuse clouds are difficult to explain, unless the standard heating and cooling mechanisms are not in the expected balance. As dust grains are directly involved in the heating (photo-electric effect on grains) and cooling (via the formation of molecules), infrared dust emission is an ideal tracer of the physical conditions of these structures. Here we propose to use Spitzer observations to put some strong constraints on the origin of these cold diffuse clouds. We wish use IRAC and MIPS to map a Galactic region well known for its abundant cold diffuse cloud features. The Spitzer data will allow us to fully characterize the dust size distribution, and to estimate the thermal balance of these structures. With its high angular resolution, the Spitzer data will also give us the opportunity to study in detail the small-scale structure of objects undergoing of one of the key transitions of the interstellar medium.
Brunt Christopher
Carey Sean
Francois Francois
Gibson Steven
Marc-Antoine Marc-Antoine
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