Dust properties along anomalous extinction sightlines. II. Studying extinction curves with dust models

摘要

The large majority of extinction sight lines in our Galaxy obey a simplerelation depending on one parameter, the total-to-selective extinctioncoefficient, Rv. Different values of Rv are able to match the whole extinctioncurve through different environments so characterizing normal extinctioncurves. In this paper more than sixty curves with large ultraviolet deviationsfrom their best-fit one parameter curve are analyzed. These curves are fittedwith dust models to shed light into the properties of the grains, the processesaffecting them, and their relations with the environmental characteristics. Theextinction curve models are reckoned by following recent prescriptions on grainsize distributions able to describe one parameter curves for Rv values from 3.1to 5.5. Such models, here extended down to Rv=2.0, allow us to compare theresulting properties of our deviating curves with the same as normal curves ina self-consistent framework, and thus to recover the relative trends overcomingthe modeling uncertainties. Such curves represent the larger and homogeneoussample of anomalous curves studied so far with dust models. Results show thatthe ultraviolet deviations are driven by a larger amount of small grains thanpredicted for lines of sight where extinction depends on one parameter only.Moreover, the dust-to-gas ratios of anomalous curves are lower than the samevalues for no deviating lines of sight. Shocks and grain-grain collisionsshould both destroy dust grains, so reducing the amount of the dust trappedinto the grains, and modify the size distribution of the dust, so increasingthe small-to-large grain size ratio. Therefore, the extinction propertiesderived should arise along sight lines where shocks and high velocity flowsperturb the physical state of the interstellar medium living their signature onthe dust properties. (Abridged version)