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 10ʹ*13ʹ( 1.75*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 (~ 1ʹ (0.2 pc)) environment of S 106 IR. A hot ( T>200 K) and dense ( 3...cm -3) gas component (emission peaks of [C II] 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 ʹʹ east) to S 106 IR and one 120 ʹʹ 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 ʹʹ 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 3P2state; 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 -1and 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 10 2 to 10 3.5 G 0 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).