Protostars and Planets VI, Heidelberg, July 15-20, 2013
High-fidelity view of the structure and fragmentation of the high-mass, filamentary IRDC G11.11-0.12
Kainulainen, Jouni (Max-Planck-Institute for Astronomy)
Ragan, Sarah (Max-Planck-Institute for Astronomy)
Henning, Thomas (Max-Planck-Institute for Astronomy)
Stutz, Amelia (Max-Planck-Institute for Astronomy)
Star formation in molecular clouds is intimately linked to their internal mass distribution. We have developed a novel dust extinction mapping technique to derive high-dynamic-range, arcsecond-scale resolution column density data for infrared dark clouds (IRDCs). The technique uses both the near-infrared colors of stars that shine through IRDCs and the mid-infrared surface-brightness features that the IRDCs cause. Using the new technique, we present an unprecedentedly detailed analysis of the column density structure of a high-mass, filamentary IRDC G11.11-0.12 (G11). We also probe the mass distribution of the cloud using dust emission data gathered with Herschel. These two independent techniques yield a strikingly good agreement, highlighting their complementarity and robustness. Using the data, we show that the linear mass density of G11 (Ml ~= 600 Msun/pc) greatly exceeds the critical value of a self-gravitating, non-turbulent cylinder. We further show that G11 harbors a relatively large amount of high-column density gas compared to nearby molecular clouds. By its mass distribution, G11 is analogous to the Orion A cloud, despite its low star-forming activity. This suggests that the amount of dense gas in molecular clouds is more closely connected to environmental properties or galaxy-scale processes than to the star-forming efficiencies of the clouds themselves. We then examine hierarchical fragmentation in G11 over wide ranges of size-scales and densities. We show that at scales 0.5 pc > l > 8 pc, the fragmentation of G11 is in agreement with that of a self-gravitating cylinder. At scales smaller than 0.5 pc, the results agree better with spherical Jeans\' fragmentation. One possible explanation for the change in fragmentation characteristics is the size-scale-dependent collapse time-scale that results from the finite size of real molecular clouds: at scales l < 0.5 pc, fragmentation becomes sufficiently rapid to be unaffected by global instabilities.
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