Publication Date:
2013-04-13
Description:
Infrared stellar photometry from the Two Micron All-Sky Survey (2MASS) and spectral line imaging observations of 12 CO and 13 CO J = 1–0 line emission from the Five College Radio Astronomy Observatory (FCRAO) 14-m telescope are analysed to assess the variation of the CO abundance with physical conditions throughout the Orion A and Orion B molecular clouds. Three distinct A v regimes are identified in which the ratio between the 13 CO column density and visual extinction changes corresponding to the photon-dominated envelope, the strongly self-shielded interior, and the cold, dense volumes of the clouds. Within the strongly self-shielded interior of the Orion A cloud, the 13 CO abundance varies by 100 per cent with a peak value located near regions of enhanced star formation activity. The effect of CO depletion on to the ice mantles of dust grains is limited to regions with A v 〉 10 mag and gas temperatures less than ~20 K as predicted by chemical models that consider thermal evaporation to desorb molecules from grain surfaces. Values of the molecular mass of each cloud are independently derived from the distributions of A v and 13 CO column densities with a constant 13 CO-to-H 2 abundance over various extinction ranges. Within the strongly self-shielded interior of the cloud ( A v 〉 3 mag), 13 CO provides a reliable tracer of H 2 mass with the exception of the cold, dense volumes where depletion is important. However, owing to its reduced abundance, 13 CO does not trace the H 2 mass that resides in the extended cloud envelope, which comprises 40–50 per cent of the molecular mass of each cloud. The implied CO luminosity to mass ratios, M / L CO , are 3.2 and 2.9 for Orion A and Orion B, respectively, which are comparable to the value (2.9), derived from -ray observations of the Orion region. Our results emphasize the need to consider local conditions when applying CO observations to derive H 2 column densities.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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