\def\M{M\kern.06em} \def\IC{IC\kern.33em} \def\NGC{NGC\kern.33em} \def\I.{\kern.2em{\sc i}} \def\II.{\kern.2em{\sc ii}} \def\mm.{$\mu$m} \def\simless{\mathbin{\lower 3pt\hbox {$\rlap{\raise 5pt\hbox{$\char'074$}} \mathchar"7218$}}}
Far-Infrared Spectroscopy of Reflection Nebulae

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Session 34 -- Airbourne Astronomy
Display presentation, Tuesday, 9:30-6:30, Pauley Room

[34.13] Far-Infrared Spectroscopy of Reflection Nebulae

M.R.Haas, A.G.G M.Tielens, A.L.Rudolph, E.F.Erickson (NASA/Ames)

We have measured the far-infrared, fine-structure lines of [O\I.]~63\mm. and [C\II.]~158\mm. from the reflection nebulae \NGC 2245, \NGC 2247, \IC 446, CED~201, and P18, using the Cooled Grating Spectrometer aboard the Kuiper Airborne Observatory. Far-infrared continuum maps (Casey 1991, ApJ, 731, 183) show that these nebulae are isolated and relatively compact ($\simless 1'$). We interpret the observed line emission as arising from warm, atomic gas which has been photodissociated by the ultraviolet (UV) radiation field of the exciting stars. Our measured line ratios ([C\II.]/[O\I.] $\sim 3$), combined with the continuum data of Casey and the photodissociation region (PDR) models of Tielens and Hollenbach (1985, ApJ, 291, 722), are used to infer gas densities $\sim$ few $\times1000$~cm$^{-3}$, maximum gas temperatures $\sim200$~K, and incident UV fields for the individual reflection nebulae. These nebulae have central stars with spectral types of B2--B9. Their PDRs have densities and UV fields which are an order of magnitude lower than those associated with the hot stars in regions like Orion and \M 17SW.

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