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	Figure 1: The final band 1 spectra of  Orionis: either based on uncorrected data (black) or on data corrected for presaturation (grey). The spurious spectral feature around m has disappeared.
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In the text

	Figure 2: Up scan (green), down scan (red) and final spectrum (black) in band 2a and 2b. Memory effects are strong at the blue side and at 5.4 m. The final flux may not be estimated correctly from 4.08 to 4.3 m and between 4.8 and 5.5 m. We keep this in mind when comparing to the models.
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In the text

Figure 3: Main contributions to the total photospheric absorption in the atmosphere model with the stellar parameters from Lambert et al. (1984) at 3600 K. CO and SiO are clearly dominant, while there are only minor traces of absorption by water. The relevant line lists used are those of Goldman et al. (1998) for OH, Goorvitch (1994) for CO, Langhoff & Bauschlicher (1993) for SiO and those of Ames (Partridge & Schwenke 1997) for H2O.

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In the text

	Figure 4: The structure of the models presented here: a photospheric model which includes limb darkening and the relevant molecular opacity, surrounded by layers of possibly mixed composition, each with its temperature, density, inner radius and outer radius. For the silicate feature modelling, the resulting spectrum is fed into the dust radiative transfer code MODUST (Bouwman et al. 2000).
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In the text

	Figure 5: An example of an intensity profile at 2.9 m for our MARCS photosphere, surrounded by a single layer with , , a temperature of 2300 K and a H2O column density of  cm-2. The dip in the IP at   is an artefact due to the assumption that the central star is optically thick also in the limb.
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In the text

	Figure 6: A comparison between the ISO-SWS-spectrum (grey) and our synthetic MARCS spectrum (black). A blackbody at T=3600 K is shown by the dotted line for comparison. Clearly, our model can reproduce the global spectral shape very well. Nevertheless, there are some interesting discrepancies.
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In the text

Figure 7: The ISO-SWS spectrum (black) is compared to the MARCS photosphere model (green), and the MARCS photosphere model embedded in a molecular layer (red) at 1.45 , with a temperature of 1750 K and H2O, CO and OH column densities of  cm-2,  cm-2, and  cm-2 respectively. The improvement is significant. The problems at 4.2 and 5 m can be attributed to memory effects in the ISO-SWS observation (see Fig. 2).

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In the text

	Figure 8: Observed K and L band visibilities are compared to our best single layer model. The match is excellent for a photospheric LD diameter of 45.6 mas. The very discrepant points at low spatial frequency are TISIS L band observations, which might be corrupted by a poor subtraction of the thermal background.
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In the text

Figure 9: Average MIDI N-band spectrum compared to the MARCS prediction for the photospheric emission. Upper panel: the calibrated MIDI spectrum and the MARCS photospheric spectrum. There is no trace of olivine emission in the MIDI slit, which is 0.54 arcsec wide, confirming that the olivine dust is indeed more than 10  out. Lower panel: the MIDI spectrum divided by the MARCS spectrum. There is excess emission within the PSF which increases with wavelength up to about 11-12 m and then levels out.

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In the text

Figure 10: The ISO-SWS spectrum of  Orionis (black) in the mid-IR is dominated by the amorphous olivine resonances at 9.7 and 18 m respectively. If the dust features are modeled with an olivine outflow (parameters listed in Table 4), a large discrepancy from 8-17 m suggests another source of emission/opacity. Indeed, when we subtract from the ISO-SWS spectrum the excess emission found in the MIDI N-band spectrum, the agreement is far better (solid line).

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In the text

Figure 11: Upper panel: comparison of different models with the ISI observations (diamonds) at 11.15 m made by Weiner et al. (2003). To match these data with an H2O layer, we need a high H2O column density of about  cm-2, which is neither compatible with the near-IR spectrum, nor with the MIDI N-band spectrum. Lower panel: opacity in the ISI bandpass (shaded region) of  cm-2 of H2O (solid line) and  cm-2 of SiO (dotted line), both at a temperature of 1750 K. The opacity profile is redshifted by 21 km s-1, which is the radial velocity of  Orionis (Wilson 1953).

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In the text

	Figure 12: Both the visibilities and the excess emission at 11 m can be explained as due to a halo covered with hot spots. Allowed layer diameters range from about 1.3 to 1.8 . The derived hot spot temperatures remain well below the expected chromospheric temperature (10 000 K, Harper et al. 2001).
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In the text

Figure 13: By including  g cm-2 of amorphous Al2O3 at 1900 K in the layer at 1.45 , we find a good fit to the ISI data (diamonds in the upper panel), and to the N-band excess ( lower panel). The minor but systematic deviation at small spatial frequency suggests that the layer is actually located a little closer to the photosphere, i.e. 1.35 . The grey box indicates the flux excess allowed by the flux ratio between the detached dust shell and the central object, i.e. the scaling factor used on the interferometric observations in the upper panel. The excess of an optically thin isothermal chromosphere as in Skinner & Whitmore (1987) is also shown (dotted line).

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In the text

	Figure 14: Light curve of  Orionis at the time of the observations presented in this paper. Constructed from observations by the American Association of Variable Star Observers (AAVSO) (Waagen 2004, private communication).
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In the text

	Figure 15: ISO-SWS spectrum (black), MARCS photosphere model (green) and a photosphere+layer model (red) using the layer parameters by Ohnaka (2004b).
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In the text