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

Abstract

The O star S 106 IR powers a bright, spatially extended 10′ × 3′ (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 μm, [C I] 609 and 370 μm, CO 7→6, and CO 4→3 measurements probe the large scale (1.2 pc) PDR emission, whereas [O I] 63 μm, CN N = 3→2, and CS J = 7→6 observations are focused on the immediate (∼1′ (0.2 pc)) environment of S 106. A hot (T > 200 K) and dense (n > 3 × 105 cm-3) gas component (emission peaks of [C II] 158 μm, 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 seconds east) to S 106 IR and one 120 seconds to the west) seen in the [C I] and low-J (Jup < 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 seconds 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 (Tex, 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 (gt;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 μm line is the most important cooling line, followed by other atomic FIR lines ([O III] 52 μm, [C II] 158 μm) 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 103.5 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 × 105 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 series of papers that high-density (n > 105 cm-3) clumps with a hot PDR surface are embedded in low- to medium density gas (n ≤ 104 cm-3).