The Properties and Formation of Infrared Dark Clouds

Type Conference Paper
Names Audra K. Hernandez, J. C. Tan
Proceedings Title Bulletin of the American Astronomical Society
Conference Name American Astronomical Society, AAS Meeting #217, #216.04
Volume 43
Pages 21604
Date January 1, 2011
Library Catalog
Abstract Stars are born within and from the densest regions of Giant Molecular Clouds (GMCs). When seen in absorption against the diffuse mid-infrared Galactic background, these regions are known as Infrared Dark Clouds (IRDCs) and are likely to be at an early evolutionary stage in the star formation process, thus yielding information on the initial conditions of massive star and star cluster formation. I present two studies using CO line emission to gain insight on the physical properties and formation of IRDCs and their surrounding GMCs. First, for one highly filamentary IRDC, I compare mass surface densities derived from CO emission, observed with the IRAM 30m telescope, with those derived using mid-IR extinction mapping. I find agreement between the two methods at the factor of two level, over a range of mass surface densities of 0.01 to 0.09 g/cm2. As surface densities increase, there is a trend for the CO derived estimates to decrease relative to mid-IR extinction mapping, which may indicate effects of depletion or systematic temperature gradients. Ellipsoidal and filamentary virial analyses suggest that surface pressure terms are dynamically important and that the IRDC may not yet have reached virial equilibrium. Second, using a large sample of IRDCs, I analyze the properties of their surrounding GMCs traced by 13CO from the Galactic Ring Survey of Jackson et al., especially their rotation and degree of virialization. A substantial fraction of the clouds rotate in a retrograde sense compared to their Galactic orbits, supporting models of IRDC formation via converging molecular flows driven by GMC collisions.
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