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Astron. Astrophys. 358, 708-716 (2000)

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1. Introduction

The Orion nebula is one of the most studied star-forming regions in the Galaxy. The ionizing stars of the Orion nebula (the Trapezium stars, the hottest of which is [FORMULA] Ori C, O6) have eroded a bowl-shaped H II  region into the surface of the Orion molecular cloud. The Orion bar is the limb-brightened edge of this bowl where an ionization front is progressing into the molecular cloud. It is seen as an elongated structure at a position angle of approximately 60o. The Orion nebula extends to the North. The Trapezium stars are located at an angular distance of approximately 2.3 arc minutes from the bar, corresponding to 0.35 pc at a distance of 500 pc. The molecular cloud extends to the other side of the bar, but also to the back of the Orion nebula. The bright star [FORMULA] Ori A (O9.5Vpe) lies near the bar, and is clearly in front of the molecular cloud since its color excess is only E(B-V) [FORMULA] 0.2 mag.

Figs. 1 and 2 illustrate the geometry of the region observed. Fig. 1 shows six representative images of the region of the Orion bar (see the figure caption for details). Fig. 2 shows the contours of the the [Ne III ] 15.5 µm  fine-structure line emission which delineates the H II  region. The emission in one of the mid-IR bands (hereafter called the Aromatic Infrared Bands, AIBs) at 6.2 µm  traces the Orion bar (an edge-on PhotoDissociation Region or PDR). The AIBs are usually strongly emitted by PDRs. The Trapezium region was avoided because of possible detector saturation.

[FIGURE] Fig. 1. Mosaic of six images of the Orion bar area (shown in detector coordinates, a clockwise rotation of 10.4o is needed to display the real sky orientation; see Fig. 2 for the equatorial coordinates). Top row: (left) an image at 5.01 µm , [FORMULA] Ori A is visible in the middle left of the image; (centre) the Orion bar at the AIB wavelength of 6.2 µm ; (right) image at 9.5 µm (the wavelength of one of the silicate features), note that [FORMULA] Ori A is again visible. Bottom row: (left) image at the AIB wavelength of 11.3 µm  and (centre) image at 12.7 µm  (ISOCAM's CVF resolution blends [Ne II ] and the 12.7 µm  AIB feature); image at 15.6 µm (right) , wavelength of the [Ne III ] forbidden line.

[FIGURE] Fig. 2. [Ne III ] map (contours) and 6.2 µm  Aromatic Infrared Band (AIB) map (grey scale). The grey scale corresponds to line intensities from 0.01 to 0.30 erg s-1 cm- 2 sr-1. The contours correspond to band intensities of 0.015 to 0.06 erg s-1 cm- 2 sr-1 by steps of 0.05. The cross is at the position of the O9.5V star [FORMULA] Ori A. The position of the hottest of the Trapezium stars ([FORMULA] Ori C) is also indicated, outside the observed field. The position of the SWS aperture in the direction of the H II  region is shown by a black cross.

Pioneering infrared observations by Stein & Gillet (1969) and Ney et al.  (1973) discovered interstellar silicate emission near 10 µm  in the direction of the Trapezium. This was confirmed by Becklin et al.  (1976) who also noticed extended silicate emission around [FORMULA] Ori A. Since that time, interstellar silicate emission has been found by the Infrared Space Observatory (ISO) in the H II  region N 66 of the Small Magellanic Cloud (Contursi et al.  in preparation) and in a few Galactic compact H II  regions (Cox et al.  in preparation) and Photodissociation Regions (PDRs, Jones et al.  in preparation). The emission consists of two broad bands centered at 9.7 and 18 µm , which show little structure and are clearly dominated by amorphous silicates.

We report in the present article ISO observations of the Orion bar and of a part of the Orion nebula made with the Circular Variable Filter of the ISO camera (CAM-CVF) which allowed imaging spectrophotometry of a field [FORMULA] at low wavelength resolution (R [FORMULA] 40). We also use an ISO Short-Wavelength Spectrometer (SWS) observation which provides higher-resolution ([FORMULA]) spectroscopy at a position within the H II  region (see Fig. 2). This spectrum was taken as part of the MPEWARM guaranteed-time program. We show here that these new ISO data confirm and extend previous observations of the amorphous silicate emission and also give evidence for emission by crystalline silicates.

Sect. 2 of this paper describes the observations and data reduction. In Sect. 3, we discuss the emission of dust and gas. The silicate emission is characterized in Sect. 4 through modelling of the observed continuum IR emission. Finally, conclusions are presented in Sect. 5. Our observations also give information on the fine-structure lines and on the AIBs. This will be presented in Appendices A and B respectively.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000