4. Spectrophotometric results
The spectra in Fig. 3 exhibit a continuum which, as a mean, dramatically increases from 4000 to 9500 Å. One possible explanation could be that the optical depth of the dust surrounding the star Sh 152.1 is larger in the direction toward the four studied regions than toward the observer (Witt 1985). However, the 12 µm continuous emission maps in Fig. 8 & Fig. 9 suggest that the dust density is smaller between the star and region 4 than between the star and region 1. The two regions are at the same distance from the star and the mean gradient in region 4 is much more larger than the gradient in region 1 (Fig. 3). As a result, the above explanation does not seem to be valid and the physical properties of the grains should be invoked to explain why the nebula appears to be much redder than the star.
Consequently, we have tried to fit the observed continuous spectra of Sh152 (Fig. 3) by scattering calculations using several grain size distributions, several scattering angle intervals and several materials of astrophysical interest. We have used only simple scattering since the nebula optical depth is small outside the star cocoon. Also, we retained only the model scattering spectra that (i) fit the observed spectra at least in their bluest (Å) and reddest (Å) parts and (ii) undergo only very small changes when scattering parameters are slightly modified (stationary solution).
Scattering spectra of classical HII regions, which exhibit negative or barely positive gradients, are explained by stellar light scattered by HAC grains (Sivan & Perrin 1993). The same explanation could be applied to Sh 152, assuming a larger proportion of HAC grains with a size parameter greater than 1. Such large grains can exist in the vicinity of an O9 (or later-type) star according to Lamy & Perrin (1997): the radiative pressure acting on the smallest grains being greater than the gravitational attraction, these grains are expelled far from the star, thus leading to an increase in the relative abundance of micron-sized grains.
The scattering spectra calculated with HAC grains failed to fit the observations. Same negative results were obtained for various other carbonaceous materials: tholins, icetholin, kerogen, poly-HCN, glassy carbon, amorphous carbons, graphite. We tried to model the scattering component using a number of other materials: silicates (olivine, andesite, augite,...), metals (iron, nickel), iron sulphides (pyrrhotite, troilite), silicon carbides and meteoritic materials. Homogeneous grains, coated grains and solid particles with inclusions were tested. We also considered several grain size distributions in the line of sight. Only grains composed of silicon-based elements were found to fit the observations satisfactorily . This result is in agreement with the recent detection of interstellar grains in the solar system (Landgraf et al. 1999).
The best fit to the scattering component was given for region 1 by a model using grains formed by a core of silicon with a coating of silicon carbide (Fig. 10a) and for region 3 by a model using grains with a silicon core and an obsidian coating (Fig. 10b). For regions 2 and 4, the best fit (Figs. 10e & f) was obtained by using grains composed of olivine with inclusions of silicon (respectively 10% and 5% of the total mass). Note that for region 4 we used two size distributions to fit the scattering satisfactorily in particular below 5000 Å; we cannot exclude, however, that the rising of the spectrum in this spectral domain is due to radioluminescence (Blair & Edgington 1968; Nash 1966; Nash et al. 1975). The parameters of the fit are given in Table 3. The values of the exponent of the size distribution p used in these models, which are lower than those found in the diffuse interstellar medium (), are in agreement with the relative abundance of micron-sized grains, as found by Landgraf et al. (1999).
As is apparent from Figs. 10g & h, regions 2 and 4 do not exhibit ERE. On the contrary, for regions 1 and 3, an ERE band is unambiguously detected (Figs. 10c & d). This is in agreement with the ERE map (Fig. 6) derived from our visible continuum images. The ERE band in Sh 152 looks similar to those found in galactic HII regions (Perrin & Sivan 1992; Sivan & Perrin 1993), with a peak wavelength around 7200 Å and a width around 1000 Å as defined by Witt & Boroson (1990). The ratio of integrated ERE intensity in the 6000-9000 Å range to the scattered light intensity is for region 1 and for region 3. These values fall within the average of those found in reflection nebulae (Witt & Boroson 1990).
In recent studies (Perrin et al. 1995; Darbon et al. 1998), we showed that the same grain material, namely carbonaceous material, could account simultaneously for the scattering and the ERE in the observed nebulae. For Sh 152, the question is whether silicates and silicon based materials, found to explain the scattering in the nebula, can also explain the ERE in terms of luminescence. The answer is a priori negative since the bulk materials we have used to model the scattering component of the spectra are known to be poor light emitters. However, porous silicon (see for example Canham 1990) as well as nanosized silicon particles (Ledoux et al. 1998; Witt et al. 1998) can give rise to photoluminescence phenomena several orders of magnitude brighter than those yielded by the bulk material.
Can such porous and/or small particles be included inside grains (or in the size distribution) without modifying the results of the scattering calculations? If we consider the complex index of refraction, it appears that, for nanosized particles, the mean free path of electrons being disturbed by the boundaries, in the visible (i.e. at frequencies much lower than the plasma frequency), the real part of the refractive index decreases with respect to that of the bulk material (Bohren & Huffman 1983). Also, as the size of the particle decreases, the space between lattice planes increases (Hofmeister et al. 1999) and the complex index of refraction decreases. Another indication comes from studies of optical properties of porous silicon: although there is no reliable way to link roughness to scattering by a surface, a model of the reflectance of a rough surface of silicon is obtained using a decrease in the imaginary part of the index (Theiss 1997).
These results are confirmed by measurements of reflectivity on porous silicon whose upper layer consists of a great number of isolated or connected monocrystalites. The complex index of refraction is obtained from the Kramers-Kronig analysis of the reflectance spectrum (Koshida et al. 1993; von Behren et al. 1995). In the visible the real and imaginary parts of the index are found to be much more lower than the value obtained for the bulk crystaline silicon. These results should be valid even if the upper layer is completely amorphous (Noguchi et al. 1992). Then, it appears that the scattering properties of nanosized or porous grains of silicon could be given by "effective indexes" which look, in the visible, like the refractive indexes of silicate (Zubko et al. 1999), thus inducing only small changes in the scattering models.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001