Interstellar Extinction and Elemental Abundances
  Elements in the interstellar medium (ISM) exist in the form of gas or dust.
The interstellar extinction and elemental abundances provide crucial
constraints on the composition, size and quantity of interstellar dust. Most of
the extinction modeling efforts have assumed the total abundances (both in gas
and in dust) of the dust-forming elements---known as the "interstellar
abundances", the "interstellar reference abundances", or the "cosmic
abundances"---to be solar and the gas-phase abundances to be independent of the
interstellar environments. However, it remains unclear if the solar abundances
are an appropriate representation of the interstellar abundances. Meanwhile,
the gas-phase abundances are known to exhibit appreciable variations with the
local interstellar environments. In this work we explore the viability of the
abundances of B stars, the solar and protosolar abundances, and the protosolar
abundances augmented by Galactic chemical enrichment (GCE) as an appropriate
representation of the interstellar abundances by quantitatively examining the
extinction and elemental abundances of ten interstellar lines of sight for
which both the extinction curves and the gas-phase abundances of all the major
dust-forming elements (i.e., C, O, Mg, Si and Fe) have been observationally
determined. Instead of assuming a specific dust model and then fitting the
observed extinction curves, for each sightline we apply the model-independent
Kramers-Kronig relation, which relates the wavelength-integrated extinction to
the total dust volume, to place a lower limit on the dust depletion. This,
together with the observationally-derived gas-phase abundances, allows us to
rule out the B-star, solar, and protosolar abundances as the interstellar
reference standard. We conclude that the GCE-augmented protosolar abundances
are a viable representation of the interstellar abundances.
