A multiwavelength study of the S 106 region

Abstract

The O star S 106 IR powers a bright, spatially extended 10 0 3 0 (1:75 0:5 pc at a distance of 600 pc) photon domi- nated region (PDR) traced by our observations of FIR fine structure lines and submm molecular transitions. The (C II) 158m, (C I) 609 and 370m, CO 7!6, and CO 4!3 measurements probe the large scale (1.2 pc) PDR emission, whereas (O I )6 3m, CN N = 3!2, and CS J = 7!6 observations are focused on the immediate (1 0 (0.2 pc)) environment of S 106 IR. A hot (T> 200 K) and dense (n> 3 10 5 cm 3 ) gas component (emission peaks of (C II) 158m, CO 7!6, and CO 4!3) is found at S 106 IR. Cooler gas associated with the bulk emission of the molecular cloud is characterized by two emission peaks (one close (20 00 east) to S 106 IR and one 120 00 to the west) seen in the (C I )a nd low-J (Jup 500 K), resulting in substantial population of the energetically higher 3 P2 state; the analysis of the mid- and high- J CO excitation con- firms the higher temperature at S 106 IR. At this position, the (O I )6 3m line is the most important cooling line, followed by other atomic FIR lines ((O III )5 2m, (C II) 158m) and high-J CO lines, which are more ecient coolants compared to (C I )2 ! 1a nd 1!0. We compare the observed line ratios to plane-parallel PDR model predictions and obtain consistent re- sults for UV fluxes spanning a range from 10 2 to 10 3:5 G0 and densities around 10 5 cm 3 only at positions away from S 106 IR. Towards S 106 IR, we estimate a density of at least 3 10 5 at temperatures between 200 and 500 K from non-LTE modelling of the CO 16!15/14!13 ratio and the CO 7!6 intensity. Our new observations support the picture drawn in the first part of this serie of papers that high-density ( n> 10 5 cm 3 ) clumps with a hot PDR surface are embedded in low- to medium density gas (n 10 4 cm 3 ).