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Molecules, grains, and shocks - A comparison of CO, H I, and IRAS data
Heiles, Carl; Reach, William T.; Koo, Bon-Chul
AA(California, University, Berkeley), AB(California, University, Berkeley), AC(California, University, Berkeley)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 332, Sept. 1, 1988, p. 313-327. NSF-NASA-supported research. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Carbon Monoxide, H I Regions, Infrared Astronomy, Interstellar Matter, Molecular Clouds, Particle Size Distribution, Radio Emission, Shock Waves, Brightness, Infrared Astronomy Satellite, Interstellar Gas, Interstellar Radiation, Normal Density Functions
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The authors have compared the IR and H I properties, and CO content, of a set of 26 isolated, degree-sized interstellar clouds. The comparisons offer some conclusions concerning the effects of kinematics on molecular content and grain size distribution. The departure of S100/NHI, where S100 is the 100 mum surface brightness, from the theoretically predicted value is a measure of the H2 content of clouds. Even clouds with low column density, ≡2.4×1020H-nuclei cm-2 or less, may contain more H2 than H I, in contrast to results obtained from UV absorption line studies. The dependence of S60/S100 on velocity implies that fast shocks preferentially destroy large grains or produce small grains, or both. The data imply that very small grains (VSGs) are formed in shocks in the 10 - 20 km s-1 velocity range, and destroyed at slightly higher velocities. Nearly all clouds, independently of kinematics, appear to contain VSGs. Some clouds must have very large fractions of their total carbon in the form of polycyclic aromatic hydrocarbons (PAHs), if VSGs are exclusively PAHs.

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