Modelling extragalactic extinction through gamma-ray burst afterglows
  We analyze extragalactic extinction profiles derived through gamma-ray burst
afterglows, using a dust model specifically constructed on the assumption that
dust grains are not immutable but respond time-dependently to the local
physics. Such a model includes core-mantle spherical particles of mixed
chemical composition (silicate core, sp2 and sp3 carbonaceous layers), and an
additional molecular component, in the form of free-flying polycyclic aromatic
hydrocarbons. We fit most of the observed extinction profiles. Failures occur
for lines of sight presenting remarkable rises blueward the bump. We find a
tendency in the carbon chemical structure to become more aliphatic with the
galactic activity, and to some extent with increasing redshifts. Moreover, the
contribution of the moleclar component to the total extinction is more
important in younger objects. The results of the fitting procedure (either
successes and failures) may be naturally interpreted through an evolutionary
prescription based on the carbon cycle in the interstellar medium of galaxies.
