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

Appendix A: the mid-IR line emission from the Orion nebula

We presented in Fig. 2 a map of the studied region in the [Ne III ] line at 15.5 µm. Fig. A.1 displays the map of the [Ar II ] line at 7.0 µm superimposed on the map of the [Ar III ] 9.0µm line. These maps illustrate the ionization structure of the Orion nebula. The spectral resolution of the CVF does not allow a separation of the the [Ar II ] line at 6.99 µm from the S(5) pure rotation line of H₂ at 6.91 µm. However, the bulk of the H₂ emission come from deeper in the molecular cloud than that of [Ar II ], ie.  more to the south-west (see Fig. 9) and the contamination by the S(5) line is probably minor. The SWS spectrum shown here and that taken towards the bar (Verstraete et al.  1999, in preparation) in which the [Ar II ] and the H₂ S(5) line are well separated from each other, show that the H₂ line is a factor 4 or 5 weaker and hence cannot seriously contaminate the [Ar II ] map.

Fig. A.1. Map in the line of [Ar II ] at 7.0 µm (contours) superimposed on the map of the [Ar III ] 9.0 µm line (grey scale spanning 0.01 to 0.05 erg s-1 cm- 2 sr-1). The contours for [ArII] are from 10-3 to 10-2 erg s- 1 cm-2 sr-1 by steps of 10-3. The position of Ori A is indicated by a cross. The peak at 7 µm at this position is probably due to the S(5) line of H2 rather than to [Ar II ] (see text).

The emission by the singly-charged ion [Ar II ] is concentrated near the ionization front on the inner side of the bar. This is very similar to what is seen in the visible lines of [N II ] 6578 and [S II ] 6731 (Pogge et al.  1992 Fig. 1c and 1d). The detailed correspondence between the maps in these three ions is excellent: note that the optical maps are not much affected by extinction. The ionization potentials for the formation of these ions are 15.8, 14.5 and 10.4 eV for ArII, NII, S II  respectively, and are thus not too different from each other.

The emission from the doubly-charged ions [Ne III ] and [Ar III ] shows a very different spatial distribution, with little concentration near the bar but increasing towards the Trapezium. The [Ne III ] map (Fig. 2) is very similar to the [O III ]5007 line map (Pogge et al.  1992 Fig. 1e), as expected from the similarity of the ionization potentials of [Ne II ] and [O II ], respectively 41.1 and 35.1 eV. However, the distribution of the [Ar III ] line (Fig. A.1) is somewhat different, with a trough where the [Ne III ] and the [O III ] lines exhibit maxima. Ar III  is ionized to Ar IV  at 40.9 eV, almost the same ionization potential as that of Ne II , so that Ar IV  (not observable) should co-exist with Ne III  and Ar III  with Ne II . A map (not displayed) in the 12.7 µm feature, which is a blend of the 12.7 µm AIB and of the [Ne II ] line at 12.8 µm, is indeed qualitatively similar to the [Ar III ] line map in the H II  region. It differs in this region from the maps in the other AIBs, showing that it is dominated by the [Ne II ] line.

As expected, the dereddened distribution of the H line (Pogge et al.  1992 Fig. 3b), an indicator of density, is intermediate between that of the singly-ionized and doubly-ionized lines.

Appendix B: the AIB emission

Maps of the emission of the 6.2 and 11.3µm AIBs are shown in Fig. B.1. We do not display the distribution of the other AIBs because they are very similar. All the spectra of Figs. 3, 4 and 6 show the classical UIBs at 6.2, 7.7, 8.6, 11.3 and 12.7 µm (in the CAM-CVF data the latter is blended with the [Ne II ] line at 12.8 µm). There are fainter bands at 5.2, 5.6, 11.0,, 13.5 and 14.2 µm visible in the SWS spectrum of Fig. 3: they may be AIBs as well. All the main bands visible in the CVF spectra are strongly concentrated near the bar. Emission is observed everywhere, because of the extension of the PDR behind the Orion nebula and the presence of fainter interfaces to the South-East of the bar. We confirm the general similarity between the distributions of the different AIBs through the Orion bar observed by Bregman et al.  (1989).

Fig. B.1. Map of the 11.3 µm UIB (contours) superimposed on the 6.2 µm UIB map (grey scale from 0.05 to 0.3 erg s-1 cm- 2 sr-1). The contours correspond to integrated band intensities from 0.016 to 0.16 erg s- 1 cm-2 sr-1 by steps of 0.016. The distributions of the two UIBs are extremely similar. The position of Ori A is indicated by a cross.

We thus conclude that, although the excitation conditions vary greatly from the Trapezium region towards the South-West of the bar, the mixing of fore- and background material along the line of sight does not allow us to observe spectroscopical changes in the AIB emission features (due e.g. to ionization or dehydrogenation as in M17-SW, Verstraete et al.  1996).

Appendix C: estimates of emission strengths

Spectral emission maps have been obtained using one or another of three different methods. The emission from well defined and rather narrow spectral features, viz. AIBs and ions, can be estimated either by numerical integration of the energy within the line and an ad-hoc baseline (method 1 ), or by simultaneous fit of Lorentz (Boulanger et al. , 1998) and/or gauss profiles, including a baseline, determined by a least square fitting algorithm (method 2 ). The strength of features not amenable to an analytical expression, like the suspected amorphous silicate emission (see Fig. 6) has been estimated using the following method (method 3 ). We have constructed an emission template consisting of all the observed emission features, each one arbitrarily normalised to unit peak intensity, see Fig. C.1. A least square computer code was then used to obtain, for each of the lines of sight, a set of multiplying coefficients for each feature present in the template plus a global parabolic baseline so as to minimize the distance between the model and the data points. The number of free parameters is then eleven "line intensities" and three polynomial coefficients, for a total of 14 free parameters to be determined from 130 observed spectral points per line of sight. The main drawback of this method is that it does not allow for varying line widths or line centres; however, given the low resolution of ISOCAM's CVF this is not a serious drawback. We have found that integrated line emission estimated from methods 2 and 3 give results that agree to within 20 percent; numerical integration of Lorentzian line strengths, on the other hand, badly underestimates the energy carried in the extended line widths and hence this method has not been used.

Fig. C.1. The eleven line and band templates, normalized to unit peak intensity, used by the least square fitting code (first eleven panels; the 12th panel, labeled Sum, shows the combined template). The low three panels give examples of the fit goodness for three lines of sight (from left to right): towards Ori A; towards a "hot spot" in the H II  region; and towards a "hot spot" on the AIB emission. For all lines of sight the fit was stopped at 15 µm  since a simple parabola could not account for the steep rise at longer wavelengths.

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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