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

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3. Gas and dust emission

The spectra towards the H II  region of the Orion nebula are shown in Figs. 3 and 4. The CAM-CVF spectrum of Fig. 3 is representative of the whole field because CVF spectra obtained at different positions in the H II  region and around [FORMULA] Ori A look qualitatively similar (compare Figs. 3 and 10 which show the CVF spectra of different pixels; note particularly the rising long wavelength portion of the spectra).

[FIGURE] Fig. 4. SWS spectrum in the Orion nebula at the position shown in Fig. 2. A fit to the spectrum (see Sect. 3 for details) is shown which uses amorphous astronomical silicate (130 K: bold dashed-dotted, and 80 K: light dashed-dotted), amorphous carbon (155 K: bold dashed, and 85 K: light dashed), and amorphous carbon VSGs (300 K: dotted). The total calculated spectrum is given by the thin solid line. The identification of the strongest spectral features is indicated.

In the SWS spectrum, a large number of unresolved lines from atoms, ions and molecules are visible. We note the Pf[FORMULA] recombination line of hydrogen (emitted by the warm, ionized gas of the H II   region) and the molecular hydrogen pure rotation lines S(2) and very faintly S(3) and S(5) (stemming from the cooler, molecular PDR gas). The simultaneous presence of these lines reflects the variety of physical conditions present along the line of sight. Clearly, we are looking at emission from the H II  region mixed with some emission from the background PDR. These unresolved lines are briefly discussed in Appendix A.

The other striking fact of the SWS spectrum is the strong continuum peaking at about 25 µm . It is emitted by warm dust in the H II   region, but dust from the background PDR probably also contributes. The broad emission bands of amorphous silicates centered at 10 and 18 µm  are visible. The classical AIBs at 6.2, 7.7, 8.6, 11.3 and 12.7 µm  dominate the mid-IR part of the spectrum. As discussed by Boulanger et al.  1998, the mid-IR spectrum can be decomposed into Lorentz profiles (the AIBs) and an underlying polynomial continuum. Maps of the various AIBs constructed in this way all show the same morphology originating mainly from the PDR gas in the Orion bar (see Appendix B). We will hereafter use the 6.2 µm -band as representative of the behaviour of the AIBs.

In Fig. 5 we compare the behaviour of the mid-IR continuum emission and of the AIBs. Clearly, the AIB emission is concentrated in the Orion bar whereas the 15.5 µm -continuum emission extends throughout the whole CAM field and shows a local peak around [FORMULA] Ori A (note that the mid-IR emission around this star is foreground because the star lies in front of the nebula). The continuum emission, however, appears to peak towards [FORMULA] Ori C, outside the region observed with ISOCAM.

[FIGURE] Fig. 5. Continuum emission at 15.5 µm  (contours) superimposed on the AIB 6.2 µm  map (grey scale). The continuum flux was taken to be the average of the flux on each side of the [Ne III ]15.5 µm  line. The 6.2 µm  feature strength was estimated as explained in the Appendix C. The contours are from 10 to 80 Jy/pixel (1 pixel = [FORMULA]), by steps of 5 Jy/pixel; the grey scale map spans 0.01 to 0.3 erg s- 1 cm-2 sr-1. The position of [FORMULA] Ori A is indicated by a cross.

The contrast in the emission morphology between the bands and continuum can be interpreted in terms of the photodestruction of the AIB carriers in the hard UV-radiation field of the H II  region. The AIB carriers must be efficiently destroyed while the larger grains are much more resistant (e.g.  Allain et al.  1996). We detail the modelling of the dust thermal emission in the next section.

3.1. Modelling the dust emission

To account for the observed SWS spectra, we have calculated the thermal equilibrium temperature of dust in the Orion H II  region as a function of distance of the Orion bar from the Trapezium stars, assuming that [FORMULA] Ori C (an O6 star) dominates the local radiation field. We use the optical constants of the amorphous astronomical silicate of Draine (1985) and of the amorphous carbon AC1 of Rouleau & Martin (1991). Assuming typical interstellar grain sizes (e.g. Draine & Lee 1984), we find a temperature range of 85-145 K for amorphous silicates and a range of 110-200 K for amorphous carbon, corresponding to grains of radius 1500 and 100 Å respectively, at a distance of [FORMULA] pc from [FORMULA] Ori C (the distance of the Orion Bar to the Trapezium stars).

Using, for simplicity, discrete dust temperatures consistent with those calculated above ([FORMULA] = 80 K and 130 K, [FORMULA] = 85 K and 155 K) we are able to satisfactorily model the continuum emission spectrum from the dust in the Orion H II  region at the position of the ISO-SWS spectrum. In Fig. 4 we show the calculated emission spectrum from our model where we adopt the carbon/silicate dust mass ratios of Draine & Lee (1984). In the calculated spectrum we have included the emission from carbon grains at 300 K, containing [FORMULA] 1 percent. of the total carbon dust mass, in order to fit the short wavelength continuum emission. The hot carbon grain emission mimics that of the stochastically-heated Very Small Grains (VSGs, Désert et al.  1990). The 300 K temperature represents a mean of the temperature fluctuations for these small particles in the radiation field of [FORMULA] Ori C, and therefore indicates a lower mass limit of [FORMULA] 1 percent for the mass of the available carbon in VSGs.

The results of our model show that the emission feature in the 10 µm   region is dominated by amorphous silicates at temperatures of the order of 130 K, but that there may also be a small contribution from amorphous carbon grains in the 12 µm  region (Fig. 4). We also note broad "features" in the SWS spectrum, above the modelled continuum in Fig. 4, at [FORMULA] µm , [FORMULA] µm  and longward of 32 µm , that are not explained by our model. These features bear a resemblance to the major bands at 19.5, 23.7 and 33.6 µm  seen in the crystalline forsterite spectra of Koike et al.  (1993) and of Jaeger et al.  (1998). Bands in these same wavelength regions were noted by Jones et al.  (1998) in the SWS spectra of the M 17 H II  region and were linked with the possible existence of crystalline Mg-rich olivines in this object. Thus, similar broad emission bands are now observed in the 15-40 µm  wavelength region of the SWS spectra of two H II  regions (Orion and M 17). These bands resemble those of the crystalline Mg-rich silicate forsterite. Another band at 9.6 µm  is probably due to some sort of crystalline silicate, and will be discussed in more details in the next section.

This dust model is simple-minded but emphasizes dust spectral signatures in the mid-IR continuum which was the main aim here. More detailed modelling treating temperature fluctuations and taking into account the grain size distribution is underway (Jones et al.  in preparation).

The broad continua that lie above the model fit (i.e. [FORMULA] µm  and [FORMULA] 32 µm , Fig. 4) can be associated with crystalline silicate emission bands. This seems to be a robust conclusion of this study. The features are too narrow to be explained by single-temperature blackbody emission and are therefore likely to be due to blended emission features from different materials. Unfortunately, having only one full SWS spectrum and CVF spectra that do not extend beyong 18 µm , we are unable to say anything about the spatial variation of these broad bands in the Orion region.

Interestingly, broad plateaux in the [FORMULA] µm  region have been associated with large aromatic hydrocarbon species containing of the order of a thousand carbon atoms (van Kerckhoven et al.  2000). However, in this study the integrated intensity of the [FORMULA] µm  plateaux do vary by a factor of up to 10 relative to the aromatic carbon features shortward of 13 µm . Thus, the origin of these broad emission features does remain something of an open question at this time.

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Online publication: June 8, 2000