The giant molecular cloud environments of infrared dark clouds

Journal Article · Thu Aug 20 04:00:00 EDT 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/809/2/154· OSTI ID:22882748

Hernandez, Audra K. ^[1]

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1. Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706 (United States)

We study giant molecular cloud (GMC) environments surrounding 10 infrared dark clouds (IRDCs), using {sup 13}CO(1−0) emission from the Galactic Ring Survey. We measure physical properties of these IRDCs/GMCs on a range of scales extending to radii, R, of 30 pc. By comparing different methods for defining cloud boundaries and for deriving mass surface densities and velocity dispersions, we settle on a preferred “CE,τ,G” method of “Connected Extraction” in position–velocity space plus Gaussian fitting to opacity-corrected line profiles for velocity dispersion and mass estimation. We examine how cloud definition affects measurements of the magnitude and direction of line-of-sight velocity gradients and velocity dispersions, including associated dependencies on size scale. CE,τ,G-defined GMCs show velocity dispersion versus size relations σ ∝ s{sup 1/2}, which are consistent with the large-scale gradients being caused by turbulence. However, IRDCs have velocity dispersions that are moderately enhanced above those predicted by this scaling relation. We examine the dynamical state of the clouds, finding mean virial parameters α-bar {sub vir}≃1.0 for GMCs and 1.6 for IRDCs, broadly consistent with models of magnetized virialized pressure-confined polytropic clouds, but potentially indicating that IRDCs have more disturbed kinematics. CE,τ, G-defined clouds exhibit a tight correlation of σ/R{sup 1/2}∝Σ{sup n}, with n ≃ 0.7 for GMCs and 1.3 for IRDCs (cf. a value of 0.5 expected for a population of virialized clouds). We conclude that while GMCs show evidence for virialization over a range of scales, IRDCs may be moderately supervirial. Alternatively, IRDCs could be virialized but have systematically different {sup 13}CO gas-phase abundances, i.e., owing to freeze-out, affecting mass estimations.

• OSTI ID: 22882748
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 809; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CARBON 13
CARBON MONOXIDE
COMPARATIVE EVALUATIONS
COSMIC GASES
DISPERSIONS
EMISSION
EXTRACTION
FREEZING OUT
MASS
OPACITY
SPACE
STARS
TURBULENCE
VELOCITY