Astron. Astrophys. 319, 331-339 (1997)

5. Astrophysical relevance

5.1. The fate of perylene in the interstellar medium

The fragmentation process we observed in perylene may in fact exist in the ISM.

As we have seen, the jet produces cold, gas-phase molecules as in the ISM. Nevertheless, the photon excitation events occuring in our experiment bear, at first sight, no relationship to what happens in space. We show below that, on the contrary, the final X-product of our experiment may indeed be the same than the one produced if perylene had been placed in the ISM.

PAHs of sizes comparable to that of perylene can be shown to absorb average photons of about 10 eV in the diffuse ISM (Léger et al. 1989a). The interstellar radiation field is nevertheless dramatically weaker than that of our excimer laser: perylene in conditions of the diffuse ISM would absorb a photon every year or so (Léger et al. 1989a). In the ISM, the sequence is:

C₂₀ H₁₂ + I I or C₂₀ H₁₂ + R R
I + X X or R + X X

where relaxations () are radiative and are completed after a few 0.1 s. In our experiment, it is as if perylene absorbed sequentially two 10 eV-photons separated by 80 ps (see Sect. 4.3). The X -radical produced by these absorptions has not enough internal energy (a few eV, see Sect. 4.3), however, to fragment (or ionize) and relaxes via collisions towards the same species X as in the ISM. The X-fragment may not be an abundant species of the ISM because it easily dissociates to yield C₂. Photofragmentation of small PAHs may thus be an efficient formation mechanism of C₂ in the ISM. This could occur in irradiated regions, where small PAHs are produced by photodissociation or shock processing (Giard et al. 1992, Schmidt and Witt 1991). Some interesting environments are also objects like the RCB star V854, where at minimum brightness C₂ Swan bands have been observed with emission bands associated to DIBs (Rao and Lambert, 1993a and b).

Our experimental results illustrate the fragility of small PAHs in the ISM as had been pointed out earlier from statistical arguments (Léger et al. 1989b, Jochims et al. 1994). But it is not unlikely that some PAH photofragments may be more stable than their parent.

5.2. Implications for the DIBs

The X-radical observed in this experimental work cannot be a DIB carrier because it would be easily dissociated by visible photons in the ISM. Some conclusions relevant for DIBs can however be drawn from our results.

First, the absorption spectrum of a PAH radical at 0.1 cm^-1 resolution has shown a high electronic multiplicity, which may be a general characteristics of large PAH radicals. The width of each set of bands is 50 cm^-1, which is similar to the widths of the broadest DIBs. We can reasonably expect other derivatives of PAH molecules to possess similar spectral features. Moreover, the pattern of this fine structure depends highly upon the specific electronic properties of the molecular species, and therefore it is natural that many different patterns exist.

It seems of great importance now to search for the physical meaning of DIB profiles. Indeed, if we can understand which phenomenon broadens the bands (rotational, vibrational, electronic, intramolecular couplings...), we can draw conclusions on the carrier of these unidentified features. That is why many recent observational studies are conducted, with high-resolution spectroscopy (Sarre et al., 1995, Ehrenfreund and Foing, 1996, and Jenniskens et al., 1996). These observations are made usually on strong and narrow DIBs, such as 5797, 6613, 5850 Å... They reveal that profiles are rotational for these narrow DIBs, with 2 or 3 components: P, R and sometimes Q branches. For broad DIBs, no high-resolution spectrum exists and nothing is known about their broadening mechanism.

We wish to propose here a mechanism that may explain some patterns and profiles. Each individual line in the excitation spectrum of X (Fig. 6) is a rotational contour of a large molecular radical with a very low rotational temperature (a few K typically). In the ISM, it is very likely that such radicals have a much higher , at least equal to the gas temperature (30-100 K) (Rouan et al. 1992). Therefore, the narrow lines are broadened and may overlap. We have studied this broadening in the case of perylene, by using the same code as presented in section 4.1. Then we simulated the rotational contour by a gaussian profile, because it seems the best approximation to the spectra found in Cossart-Magos and Leach (1990) for coronene, showing weak wings. We evaluate the relationship between and the global rotational width FWHM for low temperatures, and extrapolate to higher temperatures where the computation is no more possible. The dependence is the following: , in good agreement with the results of Cossart-Magos and Leach (1990). Fig. 8 shows some tentative broadenings for both types of bands ( and ), and the comparison of the resulting profile with some DIBs which have been selected for their shape regardless of their wavelengths. The spectrum of the X-species in the region of the origin band has been convolved with a = 3 cm^-1 gaussian and the result is displayed in panel 8a along with the DIB survey spectrum near 5780 Å (Jenniskens and Désert 1994). Although the frequency scales differ, a striking similarity appears when the sequence of small peaks on both sides of the main DIB are considered. Indeed these peaks were found to be decorrelated in intensity from the main 5780 Å feature and its pedestal (dotted line in the figure), but correlated together (Krelowski and Sneden 1995). The fine structure spacings being molecule-dependent, the appropriate value of about 15 cm^-1 could occur in the proper radical carrier. On the other hand, similar sequences of peaks, with different spacings, have been recognized throughout the DIB spectrum, in particular 11 cm^-1 intervals around 6320 Å and 35 cm^-1 around 6800 Å (Herbig and Leka, 1991).
In panel 8b the spectrum of the vibronic sequence at 94 cm^-1 from the origin is convolved with a = 12 cm^-1 gaussian. The unstructured resulting profile can be interestingly compared with that of the 4430 Å famous DIB, although the latter is 2.5 times broader. Other DIBs exhibiting 25 cm^-1 width and regular shape can be found in the survey (like the 7930 Å for instance).

Fig. 8. a Comparison of a portion of the laboratory excitation spectrum showing a -band convoluted with a 3 cm-1 FWHM gaussian, and a representative interstellar spectrum in the region of the 5780 Å DIB. Note that the scales differ by a factor of 2 in extension. b Similar comparison showing a -band convoluted with a 13 cm-1 FWHM gaussian and a recording of the 4430 Å DIB towards HD 210839 (Moutou et al., not published). Note that the scales differ by a factor of 2.5.

The rotational temperature of the DIB carrier required in such an interpretation, is = 40 K for case (a) and = 680 K for case (b), if this carrier would be of the size of perylene. The first value is perfectly reasonable for the diffuse ISM, and the second would invoke a heating mechanism like the "rocket" effect (Rouan et a., 1992) to be at work. These values would have to be corrected according to the size of the molecular carrier.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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