Watt, G. D.; White, G. J.; Cronin, N. J. and van Vliet, A. H. F.
Observations of the J = 2 → 1 line of carbon monoxide in the NGC 2024 nebula.
Monthly Notices of the Royal Astronomical Society, 189 pp. 287–291.
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Observations have been made of the J = 2 → 1 transition of 12CO and 13CO in the molecular cloud near NGC 2024. The most intense emission comes from the central regions of the nebula close to areas of high optical extinction. The source contains two hotspots, but no significant velocity gradient or line-broadening was observed. This is consistent with a model in which the emission arises mainly from the high-density interface between the expanding HII region and a neutral cloud. Measurements of the 13CO isotope indicates that τ13 > 1.7, but there is insufficient information to derive the density in the region. Analysis of data for Mon R2, however, using the radially collapsing cloud model, indicates a hydrogen density of ~2 × 104 cm-3 for that source.
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