Minchin, Nigel R.; White, Glenn J.; Stutzki, Juergen and Krause, Dirk
CI emission from the outflow and PDR in S140.
Astronomy & Astrophysics, 291 pp. 250–260.
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We present high resolution maps (10-14 arcsec FWHM) of the atomic carbon (CI) 3P1 → 3P0(492.1607 GHz) fine structure line and the C17O J = 3 → 2 (337.0611 GHz) rotational line from the outflow and photon-dominated region (PDR) in S 140. These observations reveal remarkable, previously unresolved structure.
There is a marked variation in CI line profiles across the mapped region. Towards the outflow the lines are broad (FWHM ~5-6 km s-1) with peak values of Tmb ~10-12K, yet towards the PDR the lines are distinctly narrower (~3-4 km s-1) with higher values of peak Tmb up to 18K.
Both the CI and C17O intergrated intensity maps show a similar morphology. The emision peak lies towards the molecular outflow source, and there is an arc of emission extending from the peak towards the South. The CI and C17O arcs are offset, with the CI arc offset to the northeast. This CI arc feature is observed at blue and redshifted velocities, while the C17 arc feature is only observed in blueshifted emission. This implies the CI emission lies on the inner edge of the blueshifted molecular outflow wall (traced by the C17O emission). Towards the molecular outflow source the abundance ratio N(CI)/N(CO) = 0.14, with a similar ratio found for the blue wing and core velocity intervals (0.12). The red wing has a particularly high abundance, N(CI)/N(CO) = 0.67. The most plausible mechanism for producing the CI emission is the effect of shocks on the chemical and physical processes at the interface between the stellar wind and the blueshifted outflow cavity wall.
The PDR is a clumpy, narrow (approximately 0.1-0.15pc) elongated ridge-like feature, adjacent to the south-western edge of the molecular cloud. Emission from the PDR is observed across a relatively narrow velocity range, -10 to -6 km s-1. There is a localized intergrated emission peak (at offset position -28, -42 arcsec), which we have designated PDRc1 (PDR clump 1). The abundance of CI is particularly high, N(CI)/N(CO) = 0.29 at the position of PDRc1. The observed column density (9.7±0.8 x 10¹⁷cm-2) and emergent intensity (8.7±0.7 x 10⁻⁶ erg cm-2 s-1 sr-1) towards PDRc1 are in close agreement with recent modelling of low-density PDRs.
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