AAS 199th meeting, Washington, DC, January 2002
Session 97. Interstellar Medium - III
Display, Wednesday, January 9, 2002, 9:20am-6:30pm, Exhibit Hall

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[97.13] Do the Infrared Emission Features Need UV Excitation? The PAH Model in UV-poor Reflection Nebulae

A. Li, B.T. Draine (Princeton University Observatory)

One of the major challenges of identifying the 3.3, 6.2, 7.7, 8.6, and 11.3\mum interstellar infrared emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the recent detection of these bands in regions with little ultraviolet (UV) illumination since small, neutral PAH molecules have little or no absorption at visible wavelengths and thus are excited primarily by UV photons.

The ``astronomical'' PAH model (Li & Draine 2001), incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases (Allamandola, Hudgins, & Sandford 1999), is shown to be able to closely reproduce the observed infrared emission bands of vdB 133, a UV-poor reflection nebula (Uchida, Sellgren, & Werner 1998) as well as the 6.2, 7.7, and 11.3\mum band ratios of the UV-deficient ring in the Andromeda galaxy M31 (Pagani et al.\ 1999).

It is also shown that ``astronomical'' PAHs can be heated sufficiently by a T\rm eff=3000\,{\rm K} black-body to emit at 6.2, 7.7, 8.6, and 11.3\mum. Illustrative mid-IR emission spectra are calculated for reflection nebulae illuminated by cool stars with T\rm eff=3600, 4500, 5000\,{\rm K}. These will allow comparison with future Space Infrared Telescope Facility (SIRTF) observations of vdB 135 (T\rm eff=3600\,{\rm K}), vdB 47 (T\rm eff=4500\,{\rm K}), and vdB 101 (T\rm eff=5000\,{\rm K}) (Houck 2001).

This research was supported in part by NASA grant NAG5-7030 and NSF grant AST-9619429.

{\noindent References:}\\ Allamandola, L.J., Hudgins, D.M., & Sandford, S.A. 1999, ApJ, 511, L115\\ Houck, J.R. 2001, SIRTF Observations of the Mid IR Features in Reflection Nebulae, {\sf http://sirtf.caltech.edu/ROC/pid19}\\ Li, A., & Draine, B.T. 2001, ApJ, 554, 778\\ Pagani, L., et al. 1999, A&A, 351, 447\\ Uchida, K.I., Sellgren, K., & Werner, M.W. 1998, ApJ, 493, L109

The author(s) of this abstract have provided an email address for comments about the abstract: agli@astro.Princeton.EDU

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