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R. Young Owl (UCLA), M. Meixner, D. Fong (Univ. of Illinois), M. Haas (NASA Ames), A. Rudolph (Harvey Mudd College), A. G. G. M. Tielens (Kapteyn Astronomical Inst.)
We present the results of a far-infrared (FIR) analysis (cf, Chokshi et al. 1988; Steiman-Cameron et al. 1997) of the low-excitation photodissociation regions (PDRs) associated with nine reflection nebulae illuminated by stars ranging from type B1 -- B9. The major cooling mechanism for PDR gas is line emission in the FIR, the brightest being the fine-structure lines of neutral oxygen ([OI] at 63\mum) and singly ionized carbon ([CII] at 158\mum). Our data were obtained from the Kuiper Airborne Observatory (KAO) and from the ISO archive. Our analysis provides estimates of the gas temperatures (T0) and gas densities (n0) of these low-excitation PDRs, and determines the physical conditions in PDRs as a function of the effective temperature of their illuminating stars. We use these results to test the low-density PDR models calculated by Hollenbach, Takahashi, & Tielens (1991) and Spaans et al. (1994). We also determine the relative importance of photoelectric heating to other possible heating processes (e.g., cosmic rays). In regions of moderate to high far ultraviolet (FUV) flux, the most important heating mechanism for PDR gas is photo-ejection of electrons from the surfaces of dust grains by incident FUV photons, h\nu=6--13.6eV. We examine whether the photoelectric mechanism remains dominant in lower-excitation PDRs.
Young Owl, Meixner and Fong were partially supported by NASA Grants NASA NAG 2-1067 and NASA NAG 5-3350. Additional support was provided by NSF AST 97-33697 (Meixner), the Laboratory for Astronomical Imaging at the University of Illinois and NSF grant AST 96-13999. Rudolph was supported by NSF grant 96-24924 and by a New Faculty Research Grant at Harvey Mudd College. The KAO observations were supported by NASA UPN 352.
The author(s) of this abstract have provided an email address for comments about the abstract: rolaine@astro.ucla.edu