Dust properties along anomalous sightlines  

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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.

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 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).

Mazzei & Barbaro (2011)  analysed more than sixty curves with large ultraviolet deviations from their best-fit one-parameter curve.The extinction curves are fitted with dust models to shed light on the properties of the grains along selected lines of sight, the processes affecting them, and their relations with the environmental characteristics.
The extinction curve models are developed using the latest prescriptions for grain-size distributions capable of describing
one-parameter curves for RV values from 3.1 to 5.5. These models, extended here down to RV = 2.0,  allow us to compare the resulting  properties of our deviating curves with those of normal curves in a self-consistent framework, and thus to recover the relative trends overcoming the modeling uncertainties.
These represent the largest, homogeneous sample of extinction curves exhibiting large UV deviations from simple extiction laws
 analyzed so far with dust models.
We find that the UV deviations are driven by a larger amount of small grains than predicted for lines of sight where extinction depends on only one parameter. Moreover, the dust-to-gas ratios of anomalous curves are lower than the same values for no deviating lines of sight. Our results show that the mechanisms working in the environments of anomalous curves differ from those in the environments of normal curves.  Both shocks and grain-grain collisions destroy dust grains, hence reduce the amount of the dust trapped in the grains, modify the size distribution of the dust, and increase the small-to-large grain size ratio. Therefore, the anomalous extinction properties should correspond to sight lines where shocks and high velocity flows perturb the physical state of the interstellar medium leaving their signature on the dust properties.

People: P. Mazzei

Collaboration:G. Barbaro, A. Geminale (Padova Univ.)

Publications: Mazzei & Barbaro (2008), MNRAS, 390,706 ; Mazzei & Barbaro (2011), A&A, 527,34

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