Astronomy & Astrophysics

Free access article

Issue		A&A Volume 406, Number 3, August II 2003
Page(s)		915 - 935
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20030726

A&A 406, 915-935 (2003)
DOI: 10.1051/0004-6361:20030726

A multiwavelength study of the S 106 region

II. Characteristics of the photon dominated region

N. Schneider^{1, 2, 3}, R. Simon^{1, 4}, C. Kramer¹, K. Kraemer⁴, J. Stutzki¹ and B. Mookerjea¹

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
² IRAM and Observatoire de Grenoble, BP 53, 38406 Saint Martin d'Hères, France
³ Observatoire de Bordeaux, BP 89, 33270 Floirac, France
⁴ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA

(Received 11 October 2002 / Accepted 6 May 2003 )

Abstract
The O star S 106 IR powers a bright, spatially extended $10'\times 3'$ ( $1.75\times 0.5$ 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 $\to$ 6, and CO 4 $\to$ 3 measurements probe the large scale (1.2 pc) PDR emission, whereas [O I] 63 $\mu$ m, CN $N=3{\to}2$ , and CS $J=7{\to}6$ observations are focused on the immediate (~ 1' (0.2 pc)) environment of S 106 IR. A hot ( T>200 K) and dense ( $n>3 \times10^5$ cm ^-3) gas component (emission peaks of [C II] 158 $\mu$ m, CO 7 $\to$ 6, and CO 4 $\to$ 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 ( $J_{\rm up}<4$ ) 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 ( $T_{\rm ex}$ , 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 $\rm ^{3}P_2$ 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 $\to$ 1 and 1 $\to$ 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 $3\times10^5$ at temperatures between 200 and 500 K from non-LTE modelling of the CO 16 $\to$ 15/14 $\to$ 13 ratio and the CO 7 $\to$ 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 ( $n\le10^4$ cm ^-3).

Key words: ISM: atoms -- ISM: clouds -- ISM: individual objects: S 106 -- ISM: structure -- radio lines: ISM

Offprint request: N. Schneider, schneider@obs.u-bordeaux1.fr

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