SIMBAD references

The physical conditions in the OH absorption zones, towards the compact H II regions DR 21 and K 3-50, have been determined by fitting to observational data. The analysis uses approximate {LAMBDA}-iteration, including line overlap, based on techniques adapted from stellar atmosphere codes. Using as parameters the kinetic and dust temperatures, the H₂ number density and the OH/H₂ abundance ratio, the model quantitatively reproduces the absorption line data in the six main line excited state transitions in the J = 5/2, 7/2 and 9/2 levels of the F₁ (=²{PI}_3/2) stack. Physical conditions for DR 21 show that the zone is of thickness of <10^–3pc, and for a kinetic temperature of 175K, the dust temperature is 125±10K, the OH colunm density is 2.2±0.4x10¹⁵cm^–2, the hydrogen number density is 1.8±0.7x10⁷cm^–3 and the OH/H₂ abundance is 1.6±0.9x10^–7, with no velocity gradient. K 3-50 has a column density ∼50% greater than that of DR 21. The dust temperature lies typically 50K below the kinetic temperature in both sources. We compare our results with an earlier model which used the Large Velocity Gradient approximation and find significant differences. Our present results show that OH absorption arises under conditions of much lower OH abundance than is appropriate for maser action and we suggest that it may be possible to trace a sequence of events which leads from a maser epoch into an absorption epoch in massive star formation.