Dust properties along anomalous sightlines
Context
Our understanding of the dust composition and its extinction properties plays a key role into our understanding of star formation and evolution but also of galaxy formation and evolution. 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 in our own galaxy: diffuse gas and dark clouds as well as star formation regions. The meaning of such a finding in terms of the responsible grains and of their physical properties has been extensively studied in various fundamental papers. However this relation does not apply along the majority of sightlines in the Magellanic Clouds and M31, and in several starburst galaxies and quasi-stellar objects (QSOs). Moreover, anomalous sightlines still exist in our own Galaxy as pointed out by several authors. They differ from the "normal" curves since cannot be matched by any value of R(V). We aim at shedding light on the properties of the grains along these lines of sight, the processes affecting them, and their environmental characteristics. For the majority of the anomalous sightlines discussed in literature, there is convincing evidence that their environments have been processed by shock waves.
Results
By defining anomalous curves in term of their deviation from the "normal" best-fit curve at UV wavelengths, as in a previous work, (Barbaro et al. 2004) found that anomalous behaviour arises in environments with dust-to-gas ratio different from the average value in the Galaxy.
Type A anomalous curves, with bump lower than the normal one (see figure above), have dust-to-gas ratios lower than the average whereas B anomalous curves, with bump higher than the normal one (see figure below), dust-to-gas ratios higher. We built up extinction models to interpret the behaviour of such anomalous curves.
B type sightlines require both a lower small-to-large silicate grain ratios and larger ratios of carbonaceous ones, compared with the normal curves. This can be achieved with a relatively high velocity shock which drives a sputtering process which sensibly destroys small silicate grains while it produces only a partial destruction of carbonaceous ones, so increasing the number of the smaller particles. On the contrary, type A curves require a lower velocity of the shock in order to make effective the grain-grain collisions which increase the number of small silicate grains. When applied to the sightlines in the Chamaeleon (Cham) dark cloud, it would suggest that the shock velocity in Cham I is lower than the shock in Cham III (Mazzei & Barbaro, 2008).
Future work
We aim at investigating further anomalous curves by analysing the consequence of a third type of anomaly, i.e., C type, as defined in Mazzei & Barbaro (2008), with new extinction models.
People: P. Mazzei
Collaboration:G. Barbaro, A. Geminale (Padova Univ.)
Publications: Mazzei & Barbaro (2008), MNRAS, 390,706