Far-infrared and submillimeter emission from Galactic and extragalactic photodissociation regions

Photodissociation region (PDR) models are computed over a wide range of phy sical conditions, from those appropriate to giant molecular clouds illumina ted by the interstellar radiation field to the conditions experienced by ci rcumstellar disks very close to hot massive stars. These models use the mos t up-to-date values of atomic and molecular data, the most current chemical rate coefficients, and the newest grain photoelectric heating rates, which include treatments of small grains and large molecules. In addition, we ex amine the effects of metallicity and cloud extinction on the predicted line intensities. Results are presented for PDR models with densities over the range n = 10(1)-10(7) cm(-3) and for incident far-ultraviolet radiation fie lds over the range G(0) = 10(-0.5)-10(6.5) (where G(0) is the far-ultraviol iet [FUV] flux in units of the local interstellar value), for metallicities Z = 1 and 0.1 times the local Galactic value, and for a range of PDR cloud sizes. We present line strength and/or line ratio plots for a variety of u seful PDR diagnostics: [C II] 158 mu m, [O I] 63 mu m and 145 mu m, [C I] 3 70 mu m and 609 mu m, CO J = 1-0, J = 2-1, J = 3-2, J = 6-5, and J = 15-14, as well as the strength of the far-infrared continuum. These plots will be useful for the interpretation of Galactic and extragalactic far-infrared a nd submillimeter spectra observable with the Infrared Space Observatory (IS O), the Stratospheric Observatory for Infrared Astronomy, the Submillimeter Wave Astronomy Satellite, the Far Infrared and Submillimeter Telescope, an d other orbital and suborbital platforms. As examples, we apply our results to ISO and ground-based observations of M82, NGC 278, and the Large Magell anic Cloud. Our comparison of the conditions in M82 and NGC 278 show that b oth the gas density and FUV flux are enhanced in the starburst nucleus of M 82 compared with those in the normal spiral NGC 278. We model the high [C I I]/CO ratio observed in the 30 Doradus region of the LMC and find that it c an be explained either by lowering the average extinction through molecular clouds or by enhancing the density contrast between the atomic layers of P DRs and the CO-emitting cloud cores. The ratio L[CO]/M[H-2] implied by the low extinction model gives cloud masses too high for gravitational stabilit y. We therefore rule out low-extinction clouds as an explanation for the hi gh [C II]/CO ratio and instead appeal to density contrast in A(V) = 10 clou ds.