1996A&A...307..915K

We present extended maps of ¹²CO and ¹³CO (J=2->1 and J=3->2) of the southern part of the Orion B region covering a ∼40'x70' area (in [21), and encompassing the NGC 2024 HII region, the interface region between NGC 2024 and the optical nebula IC434, the reflection nebula NGC 2023, and the Horsehead Nebula B33. The physical conditions in these different regions vary significantly due to the influence of the OB association lying to the west and due to the embedded stars. The CO emission originates from many clumps, most apparent in individual velocity channel maps. The observed low-J CO and isotopic CO line ratios are inconsistent with the standard interpretation in terms of a single temperature gas component along the line of sight in LTE. They require an outward temperature gradient on the cloud surface, as is naturally expected to arise from external UV heating. A consistent interpretation for both absolute line brightnesses and the intensity ratios can be reached in terms of PDR models. These indicate densities within the bulk of Orion B of at least close to 10⁵ cm^–3. In addition we analyse the structure by subdividing the [21 emission of Orion B into clumps using an automated procedure. The shape, orientation, LTE mass, virial mass, and stability of these clumps is examined. The mass spectrum of the clumps follows a power law, dN/dM∝M^–α, with α=1.65±0.24. We find an average volume filling factor of the clumps of 0.3 and a contrast between local and clump densities of 60 or higher.