Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.273..918g&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 273, Issue 4, pp. 918-922.
Astronomy and Astrophysics
Astronomy
11
Ism: Clouds, Ism: Molecules, Ism: Structure, Radio Lines: Ism
Scientific paper
First detections are presented of the rotational transition HCN J=3-2 in absorption towards background continuum sources. The lines of sight studied were towards Sgr B2 and W49 A,and approximately eleven foreground clouds were detected, at velocities associated with spiral arms and the large-scale bar. In contrast to lower frequency absorption lines previously observed, HCN J=3-2 is a good tracer of dense molecular gas, and LVG radiative transfer modelling has been used to estimate cloud densities. The mean n(H_2) of four clouds seen towards Sgr B2 is 8x10^3 cm^-3 (at a kinetic temperature of 20 K), with an upper limit of a few 10^4 cm^-3. However, modelling of CS J=2-1 and J=3-2 absorption lines indicates n(H_2)<~600 cm^-3 for three of these clouds, suggesting that a range of densities is present. The cloud-averaged n(H_2) is found to be close to 200 cm^-3, in which case cloud masses are a few 100 M_. The n(H2) range found in the absorption clouds covers a regime from the average in GMCs to somewhat less than in star-forming cores, which suggests that these clouds represent an intermediate stage of fragmentation.
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