Greaves, J. S.; White, Glenn J.; Ohishi, M.; Hasegawa, T. and Sunada, K.
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Seven diffuse molecular clouds have been detected in absorption, using the Sgr B2 star-formation region was used as a source of background continuum emission. Transitions were observed at frequencies around 49, 85 and 98 GHz, from CS, C34S, H13CN, H13CO+, SiO and C3H2. Clouds detected in absorption include the "nuclear disk", the 3 kpc expanding arm, spiral arms in the Galactic Plane, and two unidentified regions. The nuclear disk line profile was found to be inconsistent with homogeneous disk or bar models, instead suggesting irregular perturbations of the gas within a few hundred pc of the Galactic Centre. Absorption in CS was detected in two different rotational transitions, leading to reliable estimates of the physical parameters of the clouds. In particular, exitation temperaturers could be estimated, instead of assumed values being used, as was the case in previous studies. Results from an LTE analysis and from LVG modelling show that the absorption lines are mostly optically thin, with molecular column densities ~1012-14cm-2 per cloud. Excitation temperatures as high as 5K were found, inconsistent with heating by the 2.7K cosmic background radiation alone. Cloud densities were estimated at nH2~104cm-3, or less if the gas is highly subthermalised.
|Item Type:||Journal Article|
|Copyright Holders:||1992 European Southern Observatory|
|Keywords:||galactic structure; interstellar matter; massive stars; molecular clouds; star formation; continuous radiation; relic radiation; rotational spectra|
|Academic Unit/Department:||Science > Physical Sciences
|Interdisciplinary Research Centre:||Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)|
|Depositing User:||G. J. White|
|Date Deposited:||25 May 2012 09:54|
|Last Modified:||25 Jan 2016 21:55|
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