Abstract
Lines of sight with E(B-V)/N(HI) considerably smaller than the average value for the solar neighbourhood have been selected from the catalogue of Diplas and Savage. In order to develop quantitative considerations, estimates of the molecular hydrogen column density were obtained using the relation of Savage et al. extended to E(B-V)>0.4 with the recent data of Rachford et al. Contrary to the prevaling opinion in the literature for lines of sight with similar behaviour, we found that only 22 per cent of our sample was characterized by both an average gas density larger than 1 cm−3 and a value of RV larger than that in the diffuse interstellar medium. By computing extinction models, we were able to fit the E(B-V)/N(HI) by changing the value of RV only for some lines of sight. For the remaining ones, a ρd/ρH ratio different from the average Galactic value must be invoked. The application of the Kramers-Kronig relation to the observed extinction curves confirmed this possibility. Moreover, attemps to fit such curves with models having grain volumes corresponding to the standard ρd/ρH ratio failed.
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