2003A&A...406..915S

The O star S106 IR powers a bright, spatially extended 10'x3' (1.75x0.5pc at a distance of 600pc) photon dominated region (PDR) traced by our observations of FIR fine structure lines and submm molecular transitions. The [CII] 158µm, [CI] 609 and 370µm, CO 7-6, and CO 4-3 measurements probe the large scale (1.2 pc) PDR emission, whereas [OI] 63µm, CN N=3-2, and CS J=7-6 observations are focused on the immediate (∼1' (0.2pc)) environment of S106 IR. A hot (T>200K) and dense (n>3x10⁵cm^–3) gas component (emission peaks of [CII] 158µm, CO 7-6, and CO 4-3) is found at S106 IR. Cooler gas associated with the bulk emission of the molecular cloud is characterized by two emission peaks (one close (20'' east) to S106 IR and one 120'' to the west) seen in the [CI] and low-J (J_up<4) CO emission lines. In the immediate environment of the star, the molecular and [CI] lines show high-velocity emission due to the interaction of the cloud with the stellar wind of S106 IR. The intensities of the FIR lines measured with the KAO are compared to those observed with the ISO LWS towards two positions, S106 IR and 120'' west. We discuss intensities and line ratios of the observed species along a cut through the molecular cloud/HII region interface centered on S106 IR. The excitation conditions (T_ex, opacities, column densities) are derived from an LTE analysis. We find that the temperature at the position of S106 IR obtained from the [CI] excitation is high (>500K), 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 S106 IR. At this position, the [OI] 63µm line is the most important cooling line, followed by other atomic FIR lines ([OIII] 52µm, [CII] 158µm) and high-J CO lines, which are more efficient coolants compared to [CI] 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 10⁵cm^–3 only at positions away from S106 IR. Towards S106 IR, we estimate a density of at least 3x10⁵ at temperatures between 200 and 500K 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>10⁵cm^–3) clumps with a hot PDR surface are embedded in low- to medium density gas (n≤10⁴cm^–3).