There are quite a few unsolved or partially solved problems
related to interstellar dust which will be demanding close
attention in the future (see below for a list).
A number of new remote observational facilities
which will be available early in the new millennium
(Atacama Large Millimeter Array [MMA/LSA],
Far Infrared and Submillimeter Telescope [FIRST],
Next Generation Space Telescope [NGST],
Space Infrared Telescope Facility [SIRTF],
Stratospheric Observations for Infrared Astronomy [SOFIA],
Submillimeter Wave Astronomy Satellite [SWAS])
will permit further tests of current dust models and
promise new observational breakthroughs.
- What is the source and nature of the Diffuse Interstellar Bands?
- What is the carrier of the 2175 Å extinction hump?
- What is the carrier of the "Unidentified Infrared Bands"?
if it is PAHs, where are they formed?
are they mainly from carbon star outflows or formed
in situ by ion-molecule reactions
or from the organic refractories derived from
photoprocessing of ice mixtures
(Greenberg et al. 2000)?
- What is the carrier of the Extended Red Emission?
- What are all the sources and sinks (destruction) of
interstellar dust? where are interstellar grains made? are they mainly
made in the cold ISM
or are the silicate cores mainly stardust (serving as
"condensation seeds") while the organic mantles are formed in the ISM
Greenberg & Li
- What are the exact composition and morphology of
interstellar dust? are they separate bare silicate and
graphite grains or silicate core-carbonaceous mantle
grains or composite grains composed of small silicates,
carbon and vacuum? if most of interstellar grain
mass is condensed in the cold ISM, how can pure
silicate and graphite grains form (see
- What are the sizes of large dust grains
(> 0.25 µm)? how much can we learn from
X-ray halos and from spacecraft in situ dust
- Why are crystalline silicates not seen in the ISM
while they are present in stardust and cometary dust?
how do cometary silicates become crystalized?
- How do molecular hydrogen and other simple molecules
form on grain surfaces?
although considerable progress has been made
in recently years in studies of the diffusion rates
of adsorbed hydrogen atoms on the surfaces of variable
dust materials, the recombination reactions,
and the restoration of the new molecules to the gas phase
(Pirronello et al. 1997;
Pirronello et al. 1999;
Manicò et al. 2001),
the formation of molecular hydrogen is still not well understood
- How do interstellar grains accrete and
deplete mantles in dense molecular clouds?
we need high spatial resolution observations of
molecule distributions in the gas and in the solid
as function of depth in the cloud - interiors of
clouds as well as regions of low and high mass star formation.
- How does dust evolve in protosolar regions?
we need higher spatial resolution and sensitivity.
Improvements in the theory of dust/grain chemistry,
particularly in collapsing clouds leading to
star formation as well as in quiescent molecular clouds.
- Will the chemical and morphological analysis of comet nuclei
and dust material reveal the true character of interstellar
dust? will they provide further answers to the question of
- How can we resolve the evolution of interstellar matter
leading to the material measured and analyzed in meteorites,
in interplanetary dust particles?
- What is the true atomic composition of
the interstellar medium?
how variable is it in time and space?
are there global variation over distances of kiloparsecs?
- When did dust first form in a galaxy?
what are the composition and sizes of dust
in extragalactic environments?
A. Li was deeply saddened by the passing away
of Prof. J. Mayo Greenberg on November 29, 2001.
As a pioneer in the fields of cosmic dust, comets,
astrochemistry, astrobiology and light scattering,
Mayo's passing was a great loss for the astro-community.
Mayo had been scientifically active till his very
last days. Just a few weeks before Mayo passed away,
A. Li discussed future collaboration plans
with him on dust in high-z galaxies.
It was a great experience for A. Li to work with Mayo in Leiden.
He will be remembered forever, as a great astrophysicist and as a great
mentor. A. Li is also grateful to Profs. Bruce T. Draine
and Ewine F. van Dishoeck for their continuous advice,
encouragement and support.
A. Li thanks Profs. Lou J. Allamandola, Bruce T. Draine,
Jonathan Lunine, Valerio Pirronello for valuable
discussions, comments and suggestions.
Some of the materials of Section 2 (dust history)
from Greenberg & Shen
and the "Introduction" Chapter of A. Li's PhD thesis
of Section 3.2.1 (dust luminescence) from
Li & Draine (2002a);
and of Section 3.2.2 (dust IR emission) from
Li & Draine (2001b).
This work was supported in part by NASA grant NAG5-7030 and
NSF grant AST-9988126 and by a grant from the Netherlands
Organization for Space Research (SRON).