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Session 21 - Massive Star Formation & Ultracompact HII Regions.
Oral session, Monday, January 13
Harbour B,
We have imaged the [O I] 63 \mum line around 5 FIR and radio continuum sources in the southern massive star formation region NGC 6334. We find evidence for [O I] absorption toward the FIR continuum source NGC 6334 V. This is the first instance in which the [O I] 63 \mum line has been observed in absorption against a continuum source. From the strength of the absorption, we calculate the column density of atomic oxygen to be N(O^0)\approx 5.8\pm2.4\times10^18 cm^-2, which corresponds to a hydrogen column density of N(H)\approx 1.2\pm0.5 \times10^22 cm^-2. This is more than an order of magnitude lower than the molecular column density derived from CO excitation models: N(H_2)\approx 1.3\times10^23 cm^-2. We suggest that only a small fraction of the oxygen in the absorbing, foreground material, 5-10%, is in atomic form.
The [O I] 63 \mum line is found in emission toward the other four sources observed: NGC 6334 A, C, D and E. Based on the [O I] 63 \mum data in combination with previously observed [O I] 146 \mum, [C II] 158 \mum, FIR and CO data, the best fits of single-component photodissociation region (PDR) models imply densities of n\sim10^4 cm^-3 and UV field strengths of G_0\sim10^4 for NGC 6334 C, D and E. However, neither single- nor two-component PDR models can explain the anomalously faint [O I] 63 \mum intensity toward NGC 6334 A or NGC 6334 V. We suggest that the low [O I] 63 \mum intensity from NGC 6334 A is due to self-absorption. These results suggest that the 63 \mum [O I] line may commonly be absorbed by cold molecular gas, and its use as a PDR diagnostic is therefore problematic.
