The large body of near infrared observations presented in Testi et al. (1997; are analysed with the aim of characterizing the young stellar clusters surrounding Herbig Ae/Be stars. The results confirm the tendency of early Be stars to be surrounded by dense clusters of lower mass "companions", while Ae stars are never found to be associated with conspicuous groups. The transition between the different environments appears to occur smoothly from Ae to Be stars without a sharp threshold.No correlation of the richness of the stellar groups detected is found with the galactic position or the age of the central Herbig Ae/Be star.The stellar volume densities estimated for the groups surrounding pre-main-sequence stars of intermediate mass show the transition from the low density aggregates of T Tauri stars and the dense clusters around massive stars.Only the most massive stars (10-20 M ⊙ ) are found to be associated with dense (∼ 10 3 pc −3 ) stellar clusters. This is exactly the mass regime at which the conventional accretion scenario for isolated star formation faces theoretical problems. Thus our findings strongly supports the idea that the formation of high-mass stars is influenced by dynamical interaction in a young cluster environment.

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The study of a gas turbine plant fed by solid fuel is discussed in this paper. The plant presented is a small one, 3 MWel, externally fired by the combustion of solid biomass. The aim of the technical discussion is to find both the energetic optimisation and the actual feasibility of the plant through available industrial components (gas turbine, heat exchanger, biomass combustor). The final optimal configuration found in the present study allows for a mix of internal (gas) and external (solid fuel) combustion. In this way higher maximum cycle temperature than in standard biomasss combustors are reached through a small addition of gaseous or liquid fuel. The technical study is based on an optimum size and configuration of the power plant, previously defined, with respect to the performance and the complexity of the plant, and in comparison with other energy conversion processes of biomass such as pyrolysis or gasification. A sensitivity analysis permits the determination an optimal gas turbine in terms of pressure ratio and TIT for the current application and indicates the most important parameters that affect the power plant performance, i.e. the components on which the performance of the plant may depend. Economic data show that the direct external combustion of solid fuel has a more favourable trade-off than the configuration of the plant with gasifier.

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