Interstellar Silicon Depletion and the Ultraviolet Extinction
  Spinning small silicate grains were recently invoked to account for the
Galactic foreground anomalous microwave emission. These grains, if present,
will absorb starlight in the far ultraviolet (UV). There is also renewed
interest in attributing the enigmatic 2175 Angstrom interstellar extinction
bump to small silicates. To probe the role of silicon in the UV extinction, we
explore the relations between the amount of silicon required to be locked up in
silicates [Si/H]_{dust} and the 2175 Angstrom bump or the far-UV extinction
rise, based on an analysis of the extinction curves along 46 Galactic
sightlines for which the gas-phase silicon abundance [Si/H]_{gas} is known. We
derive [Si/H]_{dust} either from {[Si/H]_{ISM} - [Si/H]_{gas}} or from the
Kramers-Kronig relation which relates the wavelength-integrated extinction to
the total dust volume, where [Si/H]_{ISM} is the interstellar silicon reference
abundance and taken to be that of proto-Sun or B stars. We also derive
[Si/H]_{dust} from fitting the observed extinction curves with a mixture of
amorphous silicates and graphitic grains. We find that in all three cases
[Si/H]_{dust} shows no correlation with the 2175 Angstrom bump, while the
carbon depletion [C/H]_{dust} tends to correlate with the 2175 Angstrom bump.
This supports carbon grains instead of silicates as the possible carrier of the
2175 Angstrom bump. We also find that neither [Si/H]_{dust} nor [C/H]_{dust}
alone correlates with the far-UV extinction, suggesting that the far-UV
extinction is a combined effect of small carbon grains and silicates.
