A large area CO (J = 2 goes to 1) mapping of the giant molecular clouds in Orion

Details A large area CO (J = 2 goes to 1) mapping of the giant molecular clouds in Orion A large area CO (J = 2 goes to 1) mapping of the giant molecular clouds in Orion

Apr 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...425..641s&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 425, no. 2, p. 641-652

Astronomy and Astrophysics

Astronomy

73

Interstellar Matter, Mapping, Molecular Clouds, Orion Nebula, Sky Surveys (Astronomy), Emission Spectra, Radiative Transfer, Radio Observation, Space Observations (From Earth), Stellar Luminosity, Transfer Functions

Scientific paper

We made a large area mapping of the CO (J = 2 goes to 1) emission toward the entire extent of the Orion A and B giant molecular clouds using the Tokyo-NRO 60 cm survey telescope, covering approximately 35 square degrees with an angular resolution of 9 min. The data were compared with the CO (J = 1 goes to 0) data taken by Maddalena et al. on the same observing grids with the same angular resolution. Although the J = 2 goes to 1 emission exhibits basically similar spatial distribution to the J = 1 goes to 0 emission, a closer comparison shows that the CO (J = 2 goes to 1)/CO (J = 1 goes to 0) intensity ratio systematically varies over the entire extent of the Orion A and B clouds. The ratio is approximately unity on the main ridges of the clouds and declines down to 0.5 in the peripheral regions. This variation of the intensity ratio is understood mainly in terms of the density variation of unresolved clumps( or filaments) that comprise the clouds. With the aid of large velocity gradient radiative transfer calculations, we conclude that the observed variation of the intensity ratio is explained if the gas density of CO emitting regions is more than approximately 3 x 103/cu cm on the main ridges and it declines down to approximately 2 x 102/cu cm in the peripheral regions. Surface and volume filling factors of the clumps are derived to be greater than or approximately 0.7 and approximately 0.1, respectively, in the peripheral regions. This suggests the existence of small-scale (is less than or approximately 2 pc) substructures even in the cloud peripheries, allowing penertration of external UV radiation. The CO (J = 2 goes to 1)/CO (J = 1 goes to 0) luminosity ratios derived by integrating intensities over the mapping area are 0.77 and 0.66 for the Orion A and B clouds, respectively. Correcting for the unobserved extended low-level emission with small intensity ratios, we estimate the luminosity ratios of the entire GMCs to be 0.75 and 0.62 for the respective clouds. These values are consistent with those often observed in disks of external galaxies, but are significantly less than those observed toward active star-forming regions in galaxies.

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