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<HEADER><TITLE>Far-Infrared Spectroscopy of Reflection Nebulae</TITLE>

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<DL><DT><B>Session 34 -- Airbourne Astronomy</B>

<DT><I>Display presentation, Tuesday, 9:30-6:30, Pauley Room</I>

</DL>

<H2>[34.13] Far-Infrared Spectroscopy of Reflection Nebulae</H2>

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

<P><P>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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