Astron. Astrophys. 358, 708-716 (2000)
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
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
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) 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]](img8.gif) |
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 , 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 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.
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![[FIGURE]](img14.gif) |
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 Ori A. The position of the hottest of the Trapezium stars ( 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.
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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
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 at low
wavelength resolution (R 40). We also
use an ISO Short-Wavelength Spectrometer (SWS) observation which
provides higher-resolution ( )
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.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000
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