The laboratory results reported in the previous section delineate a reference frame that can be applied to interpret the spectral features observed in various space conditions in terms of detailed chemical and structural composition of the carriers. Moreover, differences observed for various cosmic environments may find a justification in the processing of materials.

Important, long standing questions concerning the attribution of typical features observed in space seem to have recently found reasonable explanation thanks to data coming from laboratory experiments. This is the case, for example, of the so-called interstellar extinction "bump", falling at 217.5 nm. This is a broad band due to absorption of carbon-based materials dispersed in the ISM. Thanks to the use of laboratory data on carbon grains subject to various degrees of UV processing it has been possible to demonstrate that all the peculiarities observed for the bump (peak stability within 1 % and bandwidth variations within 25 %) are accounted for. Blends of amorphous carbon grains, which have suffered different degrees of UV treatment, provide a very satisfactory spectral fit to observations [20]. The solution to the puzzle is based on a material processing perfectly compatible with actual conditions in the ISM.

Similarly, the 3.4 (im aliphatic band observed in the ISM and in PPN's (see section 1) may find a clear attribution not only based on the good spectral match (see Figure 3) but also in a plausible justification of its genesis in terms of re-hydrogenation of amorphous carbon grains with the activation of C-H aliphatic bonds [17], as demonstrated by experiments (see section 2).

The complete infrared spectrum of comet Hale-Bopp obtained thanks to ISO [7] has been reproduced satisfactorily by a combination of silicates and carbon, where crystalline olivine plays a relevant role [21]. Once again, the good spectral match (Figure 4) is not enough to explain the observations, as questions arise about the occurrence of crystalline silicates in comets. As a matter of fact, comets should contain a record of the primordial material from which our Solar System originated about 4.5 x 109 years ago. Since observations on the ISM give no evidence of crystalline silicates, some specific mechanism must be invoked to account for the formation of this component during the early stages of the proto-planetary nebula evolution. It is unlike that grains participating to the comet formation in the external regions of the nebula have been thermally treated for a sufficient time at temperatures higher that 1000 K (see section 2.2). It must be, then, assumed that a significant mixing between internal and external zones of the pre-solar nebula has occurred during the solar system formation, allowing the involvement of inner processed materials also in the externally formed comets. A similar problem is faced to interpret ISO spectra of oxygen-rich stars [5,6] displaying

Figure 3. Comparison of the absorption spectrum towards GC IRS-6 E [1] (points) and laboratory data on re-hydrogenated carbon [17] (curve).
Figure 4. Comparison of the Hale-Bopp emission spectrum [7] with a simple model based on laboratory data for amorphous and crystalline silicates and amorphous carbon [21].

crystalline silicate features. In this case, a possible non-thermal crystallisation mechanism has been invoked.

Was this article helpful?

0 0

Post a comment