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تسجيل مستخدم جديدMolecular Line Observations of Infrared Dark Clouds II: Physical Conditions
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Using a source selection biased towards high mass star forming regions, we used a Large Velocity Gradient (LVG) code to calculate the H2 densities and CS column densities for a sample of Midcourse Space Experiment (MSX) 8 micron infrared dark cores. Our average H2 density and CS column density were 1.14 x 10e6 cm-3 and 1.21 x 10e13 cm-2 respectively. In addition, we have calculated the Jeans mass and Virial mass for each core to get a better understanding of their gravitational stability. We found that core masses calculated from observations of N2H+ J = 1-0 and C18O J = 1-0 by Ragan et al. 2006 (Paper 1) were sufficient for collapse, though most regions are likely to form protoclusters. We have explored the star-forming properties of the molecular gas within our sample and find some diversity which extends the range of infrared dark clouds from very the massive clouds that will create large clusters, to clouds that are similar to some of our local counterparts (e.g. Serpens, Ophiuchus).
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It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) likely represent the birth sites massive stars. Therefore, this class of objects gets increasing attention. To enlarge the sample of well-characterised IRDCs in the southern hemi
sphere, we have set up a program to study the gas and dust of southern IRDCs. The present paper aims at characterizing the continuuum properties of this sample of objects. We cross-correlated 1.2 mm continuum data from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths and the IRDCs we see at 8 $mu$m in absorption against the bright PAH background. Analysing the dust emission and extinction leads to a determination of masses and column densities, which are important quantities in characterizing the initial conditions of massive star formation. The total masses of the IRDCs were found to range from 150 to 1150 $rm M_odot$ (emission data) and from 300 to 1750 $rm M_odot$ (extinction data). We derived peak column densities between 0.9 and 4.6 $times 10^{22}$ cm$^{-2}$ (emission data) and 2.1 and 5.4 $times 10^{22}$ cm$^{-2}$ (extinction data). We demonstrate that the extinction method fails for very high extinction values (and column densities) beyond A$_{rm V}$ values of roughly 75 mag according to the Weingartner & Draine (2001) extinction relation $R_{rm V} = 5.5$ model B. The derived column densities, taking into account the spatial resolution effects, are beyond the column density threshold of 3.0 $times 10^{23}$ cm$^{-2}$ required by theoretical considerations for massive star formation. We conclude that the values for column densities derived for the selected IRDC sample make these objects excellent candidates for objects in the earliest stages of massive star formation.
We study Giant Molecular Cloud (GMC) environments surrounding 10 Infrared Dark Clouds (IRDCs), using $^{13}$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,$tau$,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,$tau$,G-defined GMCs show velocity dispersion versus size relations $sigmapropto{s}^{1/2}$, 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 $bar{alpha}_{rm{vir}}simeq 1.0$ 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,$tau$,G-defined clouds exhibit a tight correlation of $sigma/R^{1/2}proptoSigma^n$, with $nsimeq0.7$ for GMCs and 1.3 for IRDCs (c.f., 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 super virial. Alternatively, IRDCs could be virialized but have systematically different $^{13}$CO gas phase abundances, i.e., due to freeze-out, affecting mass estimations.
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