Testing models of low-excitation photodissociation regions with far-infrared observations of reflection nebulae

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 mum, [C II] 158 mum, and [Si II] 35 mum 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 4 x 10(2)-2 x 10(4) 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 (1.2 x 10(-4) ergs cm(2) s(-1) sr(-1)). Our observations are compared with current theory for low-excitation PDRs. The [C II] 158 mum to [O I] 63 mum 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 mum 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 3 x 10(-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 mum 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.