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Astron. Astrophys. 364, 723-731 (2000)

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5. Discussion and conclusion

By comparing visible and infrared images of Sh 152 we have found that:

  • the spatial distribution of the two UIRBs at 3.3 and 6.2 µm are the same, suggesting similar properties for their carriers,

  • the UIRBs emission peak is located at the border of the ionized region,

  • the ERE location coincides with the ionized region and significantly differs from the UIRBs location,

  • the continuum emission observed at 10.5 and 12 µm is in favor of grains as carriers of the ERE.

From spectrophotometric data, we have found that the scattering spectra of Sh 152 could be explained by grains made up of silicates and silicon and that nanoparticles or porous grains of silicon could account for the observed ERE in terms of photoluminescence.

Laboratory studies have shown that luminescence from porous silicon with amorphous (Noguchi et al. 1992) or crystalline (Koshida et al. 1993) nanostrucures might be explained by electron quantum confinement. The shape of the ERE band and its width and peak wavelength would depend on the size distribution of the nanoparticles (see for example Wilson et al. 1993; Ledoux 1999). Moreover, owing to the high luminescence yield of silicon, the observed ERE intensity is compatible with the cosmic abundance of silicon.

Up to now, no other observational evidence exists for the presence of nanoparticles or highly porous grains of silicon in the interstellar medium. However, laboratory results suggest that different kinds of silicon-based nanoparticles might be present in this medium.

  • Nanoparticles of silicon with an oxide shell can be found in micron-sized grains of silicate from non stoechiometric condensation. It has been shown that they can yield luminescence under ultraviolet irradiation (Lu et al. 1995).

  • Silica implanted with Si+ ions can give rise to luminescence phenomena (Kachurin et al. 1997).

  • Hydrogen passivated silicon nanoparticles can also yield luminescence (Estes & Moddel 1996a,b); in this case, the bond-stretching, bending and wagging absorption features due to the Si-H bending (see for example Brodsky et al. 1977) should be observed.

If nanosized silicon particles or highly porous grains of silicon are the carriers of the ERE, it should be noted, however, that this interpretation suffers from some limitations. In a recent paper (Darbon et al. 1999b), we have shown that ERE does not occur in nebulae illuminated by stars whose effective temperature is lower than 10000K. Supposing that nanosized crystals of silicon are present in these nebulae as well as in Sh 152, we found (Fig. 11) that the number of efficient photons needed to induce ERE is much smaller for Sh 152 (region 1) than for VDB 035, a reflection nebula where no ERE is detected. This contradiction could be resolved if the materials and/or the size of the particles in the vicinity of stars colder than A0 are not the same as in the vicinity of hotter stars. Moreover, at least up to now, it seems that nanosized particles of silicon have been found neither in meteorites nor in lunar rocks, even when single crystal X-ray diffraction methods are used (see for example Bradley & Brownlee 1991). Such fine laboratory studies on cosmic materials as well as future space missions directed toward comets are likely to yield fundamental data on these particles.

[FIGURE] Fig. 11. Variation with wavelength of the number of efficient photons needed to induce ERE in region 1 of Sh 152 (5) and in the reflection nebula VDB 035 (3). These functions are derived from curves (1), (4) and (2) which represent the spectral variations of respectively the absorption coefficient [FORMULA] (cm-1) of nanocrystals of silicon (1) (Ledoux 1999), the flux of photons illuminating region 1 in Sh 152 (4) and the flux of photons illuminating the reflection nebula VDB 035 (2).

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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