2002ApJ...578..885Y

This paper presents Kuiper Airborne Observatory observations of the photodissociation regions (PDRs) in nine reflection nebulae. These observations include the far-infrared atomic fine-structure lines of [O I] 63 and 145 µm, [C II] 158 µm, and [Si II] 35 µm and the adjacent far-infrared continuum to these lines. Our analysis of these far-infrared observations provides estimates of the physical conditions in each reflection nebula. In our sample of reflection nebulae, the stellar effective temperatures are 10,000-30,000 K, the gas densities are 4x10²-2x10⁴ cm^–3, the gas temperatures are 200-690 K, and the incident far-ultraviolet intensities are 300-8100 times the ambient interstellar radiation field strength (1.2x10^–4 ergs.cm^–2.s^–1.sr^–1). Our observations are compared with current theory for low-excitation PDRs. The [C II] 158 µm to [O I] 63 µm line ratio decreases with increasing incident far-ultraviolet intensity. This trend is due in part to a positive correlation of gas density with incident far-ultraviolet intensity. We show that this correlation arises from a balance of pressure between the H II region and the surrounding PDR. The [O I] 145 to 63 µm line ratio is higher (greater than 0.1) than predicted and is insensitive to variations in incident far-ultraviolet intensity and gas density. The stellar temperature has little effect on the heating efficiency that primarily had the value 3x10^–3, within a factor of 2. This result agrees with a model that modifies the photoelectric heating theory to account for color temperature effects and predicts that the heating efficiencies would vary by less than a factor of 3 with the color temperature of the illuminating field. In addition to the single-pointing observations, an [O I] 63 µm scan was done across the molecular ridge of one of our sample reflection nebulae, NGC 1977. The result appears to support previous suggestions that the ionization front of this well-studied PDR is not purely edge-on.