Comet composition: Models and new observations

Summary We discuss structure and composition of comets as they are inferred on the basis of new theoretical models and very recent observations. Many questions are still open, but the efforts performed in coincidence with P/Halley's apparition in 1986 have led to significant improvements. Today we have a better knowledge of the materials which are stored in comet nuclei and new generations of theoretical models are able to give good representations of the surface and inner structure of these bodies. However, much work remains still to be done, also in preparation for ambitious space missions which shall return to Earth actual samples of cometary nuclei. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Carbon and silicate grains in the laboratory as analogues of cosmic dust

Carbon and silicate grains are the two main components of cosmic dust. There is increasing spectroscopic evidence that their composition varies according to the cosmic environment and the experienced processing. Irradiation from ultraviolet photons and cosmic rays, as well as chemical interactions with the interstellar gas play a crucial role for grain transformation. The study of 'laboratory analogues' represents a powerful tool to better understand the nature and evolution of cosmic materials. In particular, simulations of grain processing are fundamental to outline an evolutionary pathway for interstellar particles. In the present work, we discuss the ultraviolet and infrared spectral changes induced by thermal annealing, ultraviolet irradiation, ion irradiation and hydrogen atom bombardment in carbon and silicate analogue materials. The laboratory results give the opportunity to shed light on the long-standing problems of the attribution of ultraviolet and infrared interstellar spectral features.     

Dehydrogenation study of cosmic-dust analogue grains

Summary New infrared and optical-absorption measurements performed on hydrogenated amorphous-carbon grains are presented. In particular, the spectral modifications produced by dehydrogenation of the samples are analysed. We interpret the observed variations in terms of an increasing dimension of the graphitic clusters disordely arranged to form carbon grains. The results support the hypothesis that the behaviour of the interstellar extinction in the ultraviolet can be attributed to dehydrogenated amorphous-carbon grains. Paper presented a the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Comet P/grigg-Skjellerup: Ground-based observations after the encounter with the Giotto spacecraft

Summary After the encounter with the Giotto spacecraft, the comet P/Grigg-Skjellerup was observed with the 2.2 m ESO telescope. Imaging in theV andR bands and infrared photometry with theJ andH filters were performed. From the radial enhancement of the images we derived that theR frame mainly concerns the dust tail of the comet. Thus we applied the inverse numerical method of dust tail analysis in order to estimate the following cometary-grain parameters: ejection velocity, anisotropy of the emission, size distribution and loss rate. This from the observations back in time to the Giotto encounter. From infrared observation we could calculate theJ andH magnitudes and the colour indexJ−H=0.4 consistent with previous measurements of the comet. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Scattering matrices and reflectance spectra of forsterite particles with different size distributions

We present measurements of the complete scattering matrix as a function of the scattering angle of three different samples of forsterite particles in random orientation at a wavelength of 632.8 nm. The three samples were prepared so that three different size distributions were obtained. The composition and reflection spectra of the three samples have been experimentally determined. The results indicate that the elements of the scattering matrix are affected in a different way by the size differences. Since light scattering by comets is probably caused by particles similar to our forsterite particles, the results potentially contain information on the size of the cometary particles.     

Current in a quantum driven thermostatted system with off-diagonal disorder

We analyze a one dimensional quantum model with off-diagonal disorder, consisting of a sequence of potential energy barriers whose width is a random variable either uniformly or “half-normally” distributed, subjected to an external electric field. We shed light on how the microscopic disorder affects the value of the transmission coefficient and on the structure of the fluctuations around the solutions corresponding to the regular lattice configuration. We also characterize the asymptotic limit obtained by letting the number of barriers diverge. Thus, we explain the novelty of our method with respect to the standard thermodynamic limit discussed in the literature and also evidence the onset of a large deviation principle for the transmission coefficient.