We obtained a rather complete view of the infrared emission of the Orion nebula and its interface with the adjacent molecular cloud. The most interesting results are the observation of amorphous, and possibly crystalline, silicates in emission over the entire H II region and in an extended region around the bright O9.5Vpe star Ori A. We have fitted the mid-IR continuum of the H II region and around Ori A with the emission from amorphous silicate and amorphous carbon grains at the equilibrium temperatures predicted for the grains in the given radiation field. This shows that both types of grains can survive in the harsh conditions of the H II region. A number of bands (the 9.6 µm bump seen in Fig. 6; the excess 14 µm emission indicated in Figs. 4 and 10) suggest emission from crystalline silicates (essentially forsterite) in the H II region. Crystalline silicates may also exist around Ori A, but further, longer wavelength observations are required to confirm their presence.
Do the observed crystalline silicates result from processing of amorphous silicates in the H II region or in the environment of Ori A? Silicate annealing into a crystalline form requires temperatures of the order of 1000 K for extended periods (Hallenbeck et al. 1998). The dust temperatures observed in the H II region and around Ori A are considerably lower than this annealing temperature. One might however invoke grain heating following grain-grain collisions in the shock waves that are likely to be present in the H II region. However, grain fragmentation rather than melting is the more likely outcome of such collisions (Jones et al. 1996). It is probable that the crystalline silicates observed here were already present in the parent molecular cloud, and probably originate from oxygen-rich red giants.
Emission by both amorphous and crystalline silicates has been observed with ISO around evolved stars (Waters et al. 1996; Voors et al. 1998). The crystalline silicates there must have been produced locally by annealing of amorphous silicates. Gail & Sedlmayr (1999) have shown that this is possible, and that both amorphous and crystalline forms can be released into the interstellar medium. However, there is no evidence for absorption by crystalline silicates in the general interstellar medium in front of the deeply embedded objects for which amorphous silicate absorption is very strong (Demyk et al. 1999; Dartois et al. 1998). Consequently, crystalline silicates represent only a minor fraction compared to amorphous silicates. It would be difficult to detect the emission from a small crystalline component of dust in the diffuse interstellar medium because the dust is too cool (TK) to emit strongly in the m wavelength region. Observations of H II regions and bright stars provide the opportunity of observing this emission due to the strong heating of dust. Emission from amorphous and crystalline silicates is seen around young stars (Waelkens et al. 1996; Malfait et al. 1998) as well as in comets (Crovisier et al. 1998). There are also silicates in meteorites, but their origin is difficult to determine because of secondary processing in the solar system. Crystalline silicates in comets, and perhaps in interplanetary dust particles believed to come from comets (Bradley et al. 1992), must be interstellar since the material in comets never reached high temperatures. However, the silicates probably experienced changes during their time in the interstellar medium. It is interesting to note that while very small grains of carbonaceous material exist, there seem to be no very small silicate grains in the interstellar medium (Désert et al. 1986).
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000