Silicon and iron dust in gamma-ray burst host galaxy absorbers
  Depletion studies provide a way to understand the chemical composition of
interstellar dust grains. We here examine 23 gamma-ray bursts (GRB) optical
afterglow spectra (spanning 0.6<z<5.0) and compare their silicon and iron
dust-phase column densities with different extinction curve parameters to study
the composition of the interstellar dust grains in these high-redshift GRB host
galaxies. The majority of our sample (87%) show featureless extinction curves
and only vary in shape. We observe strong correlations (with >96% significance)
between the total-to-selective extinction, R_V, and the dust-phase column
densities of Si and Fe. Since a large fraction of interstellar iron is locked
in silicate grains, this indicates that high Si and Fe depletion leads to an
increase in the fraction of large silicate grains and vice versa. This suggests
that silicates play a vital role to induce the entire extinction at any
wavelength. On the other hand, the far-UV extinction is usually attributed to
the presence of small silicates. However, we find no trend between the far-UV
parameter of the extinction curve, c_4, and the abundance of Si and Fe in the
dust phase. We, therefore, propose that the far-UV extinction could be a
combined effect of small (probably nanoparticles) dust grains from various
species.
