  What might be the most appropriate set of interstellar reference abundances
of chemical elements (both in gas and in dust) relative to hydrogen has been a
subject of much discussion in the past decade. While historically the Sun has
been taken as the reference standard, it has recently been suggested that the
interstellar abundances might be better represented by those of B stars
(because of their young ages) which are just ~60--70% of the widely adopted
solar values (``subsolar''). On the other hand, the most recent estimates of
the solar carbon and oxygen abundances are also close to those of B stars. If
the interstellar abundances are indeed ``subsolar'' like B stars or the
newly-determined solar C,O values, there might be a lack of raw material to
form the dust to account for the interstellar extinction. In literature it has
been argued that this problem could be solved if interstellar grains have a
fluffy, porous structure since fluffy grains are more effective in absorbing
and scattering optical and ultraviolet starlight than compact grains (on a per
unit mass basis). However, we show in this work that, using the Kramers-Kronig
relation, fluffy dust is not able to overcome the abundance shortage problem. A
likely solution is that the abundances of refractory elements in stellar
photospheres may under-represent the composition of the interstellar material
from which stars are formed, resulting either from the possible underestimation
of the degree of heavy-element settling in stellar atmospheres, or from the
incomplete incorporation of heavy elements in stars during the star formation
process.

Astrophysics