Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003a%26a...406..915s&link_type=abstract
Astronomy and Astrophysics, v.406, p.915-935 (2003)
Astronomy and Astrophysics
Astrophysics
29
Ism: Atoms, Ism: Clouds, Ism: Individual Objects: S 106, Ism: Structure, Radio Lines: Ism
Scientific paper
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 (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'' 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 (>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 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*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
Kraemer Karl
Kramer Carsten
Mookerjea Bhaswati
Schneider Nicholas
Simon Robert
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