2008MNRAS.390..706M

The large majority of extinction curves in our own Galaxy obey to a simple relation depending on one parameter, the total-to-selective extinction coefficient, R_V. Different values of R_V are able to match the whole extinction curve through different environments. However, anomalous curves still exist in our own Galaxy. In this paper, we aim at analysing the behaviour of some anomalous extinction curves in order to shed light into both the properties of the dust and the physical properties of their environments. From the catalogue of Savage et al., by adding the infrared data from Two-Micron All-Sky Survey catalogue, 84 lines of sight (i.e. ≃11 per cent) have been selected which deviate at the 2σ level from the normal extinction law corresponding to their best-fitting R_V value. We single out a subsample of 20 lines of sight to analyse their ultraviolet (UV) deviations, in particular, at λ = 0.22 and 0.15µm, which correspond to the bump and the far-UV rise. The lines of sight with bump lower and far-UV rise higher than their normal extinction law are defined as type A anomalous lines, and those with bump higher and far-UV rise lower as type B anomalous lines. The results here discussed are derived from extinction curve models. A anomalous types require small-to-large grain-size ratios larger than the normal curves both for silicates and carbonaceous grains; B curves also require larger ratios for carbonaceous ones but lower ratios for silicates grains. In order to interpret these results, the environments of type A curves require low-velocity shocks in order to make effective the grain-grain collisions, which increase the number of both small silicate and carbonaceous grains. However, type B lines of sight would be characterized by relatively high-velocity shocks, implying a sputtering process which sensibly destroys small silicate grains while it produces only a partial destruction of the carbonaceous ones with the consequence of increasing the number of these smaller particles.