Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula

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© 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
, , Citation Marc Kassis et al 2006 ApJ 637 823 DOI 10.1086/498404

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Author affiliations

1 Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743

2 Astronomy Department, Boston University, 725 Commonwealth Avenue, Boston, MA 02215

3 Department of Physics and Astronomy, Colby College, 5863 Mayflower Hill, Waterville, ME 04901

4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138

5 Astronomy Department, Boston University, 725 Commonwealth Avenue, Boston, MA 02215

6 University of Hawaii, NASA Infrared Telescope Facility, 640 North A`ohoku Place, Room 209, Hilo, HI 96720

7 Visiting astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under a cooperative agreement with the National Aeronautics and Space Administration

8 Current address: 206 Space Science Building, Cornell University, Ithaca, NY 14853

9 Prior to her death in 2004 April, the principal investigator of the MIRSI, Lynne K. Deutsch, made contributions to results presented in this paper

Dates

  1. Received 2005 June 17
  2. Accepted 2005 September 28
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0004-637X/637/2/823

Abstract

The mid-infrared emission from a photodissociation region (PDR) viewed edge-on in the Orion Nebula is examined through 8.7-20.6 μm images and 8-13 μm spectra. The polycyclic aromatic hydrocarbon (PAH) emission is located between the edges of H II regions and layers of [C I] emission, agreeing with PDR theory. Using a simple model, the spatial variations in the emission from PAHs detected at 8.6, 11.2, and 12.7 μm are demonstrated to be directly proportional to the material column density and the intensity of the UV field. For a homogeneous, neutral cloud illuminated by a bright OB star, PDR theory predicts that the ultraviolet (UV) radiation is attenuated exponentially (e). The predicted UV attenuation is confirmed by observations of broad PAH emission features found at 8.6, 11.2, and 12.7 μm. The PAH emission is found in cool regions having greater optical depths relative to regions where mid-infrared emission from ionized gas is observed. Through modeling we determine a gas density of 9.7 × 104 cm-3. On large and small size scales, the relative strengths of the 8.6, 11.2, and 12.7 μm PAH features at the bar of the Orion Nebula indicate that there is not a simple transition from ionized to neutral PAHs across the PDR.

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10.1086/498404