OH ABSORPTION IN DR-21 AND K-3-50 REVISITED

The physical conditions in the OH absorption zones, towards the compac t H II regions DR 21 and K 3-50, have been determined by fitting to ob servational data. The analysis uses approximate LAMBDA-iteration, incl uding line overlap, based on techniques adapted from stellar atmospher e codes. Using as parameters the kinetic and dust temperatures, the H- 2 number density and the OH/H-2 abundance ratio, the model quantitativ ely reproduces the absorption line data in the six main line excited s tate transitions in the J = 5/2, 7/2 and 9/2 levels of the F1 (= 2PI3/ 2) stack. Physical conditions for DR 21 show that the zone is of thick ness of < 10(-3) pc, and for a kinetic temperature of 175 K, the dust temperature is 125 +/- 10 K, the OH colunm density is 2.2 +/- 0.4 x 10 (15) cm-2, the hydrogen number density is 1.8 +/- 0.7 x 10(7) cm-3 and the OH/H-2 abundance is 1.6 +/- 0.9 x 10(-7), with no velocity gradie nt. K 3-50 has a column density approximately 50% greater than that of DR 21. The dust temperature lies typically 50 K below the kinetic tem perature in both sources. We compare our results with an earlier model which used the Large Velocity Gradient approximation and find signifi cant differences. Our present results show that OH absorption arises u nder conditions of much lower OH abundance than is appropriate for mas er action and we suggest that it may be possible to trace a sequence o f events which leads from a maser epoch into an absorption epoch in ma ssive star formation.