Astron. Astrophys. 322, 633-645 (1997)

6. Physical mechanisms of relevance

In this section we discuss some of the laboratory studies examining the UV properties of some carbonaceous materials of astrophysical relevance and assess their usefulness in interpreting the results of the preceding sections.

Fink et al. (1984) have derived of HAC thin films for various annealing temperatures, . As was increased, the - peak (associated with sp² bonding) shifted to smaller wavenumbers and its strength increased. Equating a larger with less hydrogenation, this means that we expect to shift to smaller values and to increase as the hydrogen content decreases. Figs. 4 and 5 show qualitatively that trend. On the other hand, as was increased, the - peak (observed for virtually all carbonaceous materials, including diamond) initially at , shifted to larger wavenumbers ( at C), but its strength stayed roughly the same. Thus from this, one expects qualitatively less hydrogenated material (larger ) to exhibit larger (and thus, smaller curvature ). This appears to be contradicted by the results of Fig. 8, though the comparison may not be relevant since the widths observed in Fink et al. (1984) are much broader than those considered here.

A dehydrogenation study of small HAC grains in extinction has been carried out by Mennella et al. (1995a). The HAC grains "as produced" show no UV bump, just a UV rise which is the small x tail of the - electronic transition of carbon (observed at about ). As the annealing temperature is increased from C to C, a weak UV bump develops at around and gradually shifts to smaller x, gaining in strength. This is compatible with the above trend observed in . When all hydrogen is lost (at around C), the very broad UV peak centered around resembles that of arc-evaporated soot produced in an inert gas atmosphere (and UV features associated with hydrogen-poor circumstellar environments; Blanco et al. 1995). These features are much weaker and much broader than the interstellar UV bump.

Simple dehydrogenation, by itself, cannot be the only process responsible for the interstellar UV feature since it violates the observed stability of the bump peak position. This stability endures despite wide variations in temperature, density, and UV flux arising from various interstellar environments.

Very recently, Mennella et al. (1996) have reported a UV feature in small carbonaceous grains falling close (at ) to the peak position of the interstellar UV bump. This feature was produced by subjecting small HAC grains to UV radiation (corresponding to doses about 7 times less than typical in the diffuse interstellar medium). More importantly, this feature was shown to be stable in peak position when subjected to various doses of UV radiation. However, the precise mechanism causing this observed stability is still sketchy at the moment. After the UV processing, the grains still contained a considerable amount of hydrogen (about 0.3 relative to carbon by number, about half the value found in the starting material). Unfortunately, the UV features produced are considerably broader () than the interstellar UV feature. This could be due to the extreme clustering observed for the grains and possibly size effects (since the individual grains have a radius of about ). This laboratory cosmic dust analogue nevertheless looks extremely promising in providing a better understanding of the optical properties of the interstellar UV feature carrier.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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