A multiwavelength study of the S 106 region. II. Characteristics of the photon dominated region

The O star S 106 IR powers a bright, spatially extended 10'x 3' (1.75x 0.5 pc at a distance of 600 pc) photon dominated region (PDR) traced by our observations of FIR fine structure lines and submm molecular transitions. The [C II] 158 mu m, [C I] 609 and 370 mu m, CO 7->6, and CO 4->3 measurements probe the large scale (1.2 pc) PDR emission, whereas [O I] 63 mu m, CN N=3->2, and CS J=7->6 observations are focused on the immediate ( ~ 1' (0.2 pc)) environment of S 106 IR. A hot (T>200 K) and dense (n>3 *E⁵ cm^-3) gas component (emission peaks of [C I] 158 mu m, 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'' east) to S 106 IR and one 120'' to the west) seen in the [C I] and low-J (J^up<4) CO emission lines. In the immediate environment of the star, the molecular and [C I] lines show high-velocity emission due to the interaction of the cloud with the stellar wind of S 106 IR. The intensities of the FIR lines measured with the KAO are compared to those observed with the ISO LWS towards two positions, S 106 IR and 120'' west. We discuss intensities and line ratios of the observed species along a cut through the molecular cloud/H II region interface centered on S 106 IR. The excitation conditions (T_ex, opacities, column densities) are derived from an LTE analysis. We find that the temperature at the position of S 106 IR obtained from the [C I] excitation is high (>500 K), resulting in substantial population of the energetically higher ³P₂ state; the analysis of the mid- and high-J CO excitation confirms the higher temperature at S 106 IR. At this position, the [O I] 63 mu m line is the most important cooling line, followed by other atomic FIR lines ([O III] 52 mu m, [C II] 158 mu m) and high-J CO lines, which are more efficient coolants compared to [C I] 2->1 and 1->0. We compare the observed line ratios to plane-parallel PDR model predictions and obtain consistent results for UV fluxes spanning a range from 10² to 10^3.5 G₀ and densities around 10⁵ cm^-3 only at positions away from S 106 IR. Towards S 106 IR, we estimate a density of at least 3*E⁵ 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>10⁵ cm^-3) clumps with a hot PDR surface are embedded in low- to medium density gas (nle10 ⁴ cm^-3).

Appendix A is only available in electronic form at http://www.edpsciences.org