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: http://www.stsci.edu/~kgordon/papers/2014ApJ...797...86R.html
Дата изменения: Thu Jan 21 19:33:42 2016 Дата индексирования: Sun Apr 10 16:25:10 2016 Кодировка: ISO8859-5 Поисковые слова: molecular cloud |
The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and lifecycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and HЮБ observations. In the diffuse atomic interstellar medium (ISM), we derive the GDR as the slope of the dust-gas relation and find GDRs of 380+250-130\+/- 3 in the LMC, and 1200+1600-420\+/- 120 in the SMC, not including helium. The atomic-to-molecular transition is located at dust surface densities of 0.05 M т?? pc-2 in the LMC and 0.03 M т?? pc-2 in the SMC, corresponding to A V ~ 0.4 and 0.2, respectively. We investigate the range of CO-to-H2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on X CO to be 6 У? 1020 cm-2 K-1 km-1 s in the LMC (Z = 0.5 Z т??) at 15 pc resolution, and 4 У? 1021 cm-2 K-1 km-1 s in the SMC (Z = 0.2 Z т??) at 45 pc resolution. In the LMC, the slope of the dust-gas relation in the dense ISM is lower than in the diffuse ISM by a factor ~2, even after accounting for the effects of CO-dark H2 in the translucent envelopes of molecular clouds. Coagulation of dust grains and the subsequent dust emissivity increase in molecular clouds, and/or accretion of gas-phase metals onto dust grains, and the subsequent dust abundance (dust-to-gas ratio) increase in molecular clouds could explain the observations. In the SMC, variations in the dust-gas slope caused by coagulation or accretion are degenerate with the effects of CO-dark H2. Within the expected 5-20 times Galactic X CO range, the dust-gas slope can be either constant or decrease by a factor of several across ISM phases. Further modeling and observations are required to break the degeneracy between dust grain coagulation, accretion, and CO-dark H2. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.
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