Testing Models of Low-Excitation Photodissociation Regions with Far-Infrared Observations of Reflection Nebulae

Details Testing Models of Low-Excitation Photodissociation Regions with Far-Infrared Observations of Reflection Nebulae Testing Models of Low-Excitation Photodissociation Regions with Far-Infrared Observations of Reflection Nebulae

Oct 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002apj...578..885y&link_type=abstract

The Astrophysical Journal, Volume 578, Issue 2, pp. 885-896.

Astronomy and Astrophysics

Astronomy

18

Ism: Dust, Extinction, Infrared: Ism, Ism: Lines And Bands, Ism: Reflection Nebulae

Scientific paper

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 4×102-2×104 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 (1.2×10-4 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 3×10-3, 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 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.

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