AAS Meeting #193 - Austin, Texas, January 1999
Session 65. Interstellar Dust and Gas
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[65.03] ISO Mid-Infrared Spectra of Reflection Nebulae

K.I. Uchida, K. Sellgren (OSU), M. W. Werner (JPL), M.L. Houdashelt (Johns Hopkins Univ.)

We present the results of imaging spectroscopy of the reflection nebulae vdB\,17 (NGC 1333), vdB\,59 (NGC 2068), vdB\,101, vdB\,111, vdB\,133 and vdB\,135, obtained with the infrared camera (ISOCAM) and circular variable filter wheel (CVF) on the Infrared Space Observatory (ISO). The sample nebulae were chosen for their wide range of illuminating stellar temperatures, from {T\rm eff} = 3,600 K to 19,000 K, corresponding to ultraviolet (UV; \lambda\ < 400 nm) to total flux ratios, of between 0.01 and 0.87. We have previously reported the detection of interstellar infrared emission features (IEFs) at 6.2, 7.6, 8.6, 11.2, and 12.7 \micron, broad emission features at 6 - 9 \mum and 11 - 13 \mum, and associated 6 - 15 \mum continuum emission, from vdB\,133, a nebula illuminated by stars with a very low UV to total flux ratio of 0.21 (Uchida, Sellgren, & Werner 1998). Here we report new detections of these same IEFs, broad emission features, and continuum features toward the UV-rich nebulae vdB\,17 and vdB\,59.

We have quantified the relative strengths of the Class A interstellar IEFs at 6.2, 7.7, 8.6, 11.3, and 12.7 \mum, of the broad bumps at 6 - 9 \mum and 11 - 13 \mum, and of the associated continuum amongst reflection nebulae with a wide range in UV illumination. We find the relative strengths are quite sensitive to the adopted method of determining the continuum level. However, within a given method the relative strengths show no dependence on UV illumination.

We evaluate the dependence of the spectrum on the absolute intensity of the illuminating UV radiation field; our results combined with analysis of published spectra of NGC\,7023, \rho Ophiuchus and the Orion Bar suggest the interstellar mid-infrared emission spectrum is remarkably similar over UV fields of G0 = 10 to 105. This presents a challenge to identifying the IEFs with a mixture of neutral and ionized PAHs, because the mean PAH ionization state, and thus the spectrum of the PAH mixture, depends on the degree of UV illumination. We conclude the carrier(s) of the IEFs, broad emission features, and mid-infrared continuum must absorb at both UV and visible wavelengths, and are a ubiquitious and hardy component of the ISM.

The author(s) of this abstract have provided an email address for comments about the abstract: kuchida@astronomy.ohio-state.edu

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