Excitations and line profiles of CO in clumpy molecular clouds: radiative interactions of clump and interclump media.

Details Excitations and line profiles of CO in clumpy molecular clouds: radiative interactions of clump and interclump media. Excitations and line profiles of CO in clumpy molecular clouds: radiative interactions of clump and interclump media.

Aug 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...312..981p&link_type=abstract

Astronomy and Astrophysics, v.312, p.981-988

Astronomy and Astrophysics

Astrophysics

15

Line: Profiles, Radiative Transfer, Ism: Clouds, Radio Lines: Ism

Scientific paper

With a three dimensional Monte Carlo code of radiative transfer, we have investigated how the clump and interclump media interact with each other radiatively in exciting CO molecules in clumpy interstellar clouds. A spherical model cloud is divided into ~(4π/3)x15^3^ cubic cells of equal size. Some of the cells filled with cold (T_k,c_=10K) and dense gas are considered to be the clumps; while the rest with hot (T_k,ic_=40K) rarefied gas the interclump medium. The clump density of n_c_(H_2_)=(4~8)x10^3^cm^-3^ is kept an order of magnitude higher than the interclump density of n_ic_(H_2_)=(2~4)x10^2^cm^-3^. For the optically thick ^12^CO transitions, the excitation conditions in one medium are significantly affected by the presence of the other. As the volume filling factor of the clumps increases, the excitation temperatures in both media approach each other. This trend is more apparent for the high J transitions than for the low J ones. The change in the excitation conditions causes the ^12^CO line intensities to increase by a factor of up to 3, as the volume filling factor increases from 0.0 to 0.3. An inclusion of the interclump gas with density as low as 2x10^2^cm^-3^ makes the whole cloud optically so thick that the ^12^CO line emission is mostly emerging from the cloud surface and exhibits flat top features or self-absorption dips. For the optically thin transitions of ^13^CO, on the other hand, the excitation conditions in the interclump medium are rather insensitive to the clump volume filling factor. Syntheses of line profiles show that the bumpy structures of the ^13^CO lines are originated mainly from the dense clumps. In this case the interclump gas makes the line intensities increase slightly. Taking MBM12 as an example of clumpy clouds, we observed its core part with the 14m telescope of TRAO. We apply our model calculations to the CO J=1-0 spectra observed with a high S/N ratio and a fine velocity resolution to reveal the physical conditions inside. In order for the clumpy cloud model to be compatible with the observations, either CO molecules should be depleted in the interclump gas or the interclump space should not be completely filled with molecular gas.

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