2004MNRAS.353..760B

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 & Savage. In order to develop quantitative considerations, estimates of the molecular hydrogen column density were obtained using the relation of Savage et al. extended at E(B-V) > 0.4 with the recent data of Rachford et al. Contrary to the prevailing opinion in the literature for sightlines with similar behaviour, we found that only 22 per cent of our sample was characterized by both an average gas density larger than 1/cm3 and a value of R_V 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 R_V only for some sightlines. For the remaining ones, a ρ_d/ρ_Hratio 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, attempts to fit such curves with models having grain volumes corresponding to the standard ρ_d/ρ_Hratio failed.

We find a linear relation between ρ_d/ρ_Hand E(B-V)/N(H) for our sightlines. The average Galactic value marks the separation into two groups characterized by lower abundances of C and Si trapped into the grains when E(B-V)/N(H) is smaller than the Galactic value, and by larger abundances when E(B-V)/N(H) is greater.