From: Eugene.Leitl@lrz.uni-muenchen.de
Date: Sun Dec 03 2000 - 10:52:18 MST
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4. ASTROPHYSICS: ON STARDUST
In astrophysics, the term "dust" refers to various entities: a)
interplanetary and cometary dust are found in the Solar System;
b) circumstellar dust is found around stars; c) interstellar dust
is found between stars. Individual dust particles are usually
called "dust grains" and range in size from approximately 10
nanometers up to the micron range (with an average size about the
size of particles in cigarette smoke). Interstellar dust
extinguishes and reddens starlight, can be detected by its
absorption and emission of infrared radiation, and can be
detected by its polarizing effect on starlight. The exact
composition of interstellar dust is uncertain, but infrared
absorption measurements indicate that a significant fraction of
the material is organic. In general, interstellar dust is
believed to be carbon, iron, and silicates mixed with or coated
with frozen water.
... ... J. Mayo Greenberg (University of Leiden, NL) presents a
review of recent research on interstellar dust and cometary dust,
the author making the following points:
1) The extinction curve for interstellar dust, which
indicates the reduction of light intensity at each wavelength,
indicates there must be 3 types of dust grains: a) The particles
that block light in the visible spectrum are elongated grains
nearly 0.2 microns in diameter and approximately 0.4 microns in
length. These particles account for approximately 80 percent of
interstellar dust, with each grain containing a rocky core
surrounded by a mantle of organic materials and ice. b) A hump in
the ultraviolet part of the extinction curve suggests the
presence of smaller particles of approximately 5 nanometers
diameter, which comprise approximately 10 percent of the total
dust mass. These grains are most likely amorphous carbonaceous
solids that probably contain some hydrogen but little or no
nitrogen or oxygen. c) Finally, an even smaller type of particle,
approximately 2 nanometers in diameter, is apparently responsible
for blocking light in the far ultraviolet region. These smallest
particles, which constitute the remaining 10 percent of the dust
mass, are believed to be large molecules similar to the
polycyclic aromatic hydrocarbons emitted in automobile exhaust.
2) The author postulates a 100-million-year "dust cycle",
which dust grains undergo approximately 50 times before their
destruction:
... ... a) In diffuse dust clouds, where gas is sparse, the dust
is a mixture of core-mantle grains, carbonaceous particles, and
polycyclic aromatic hydrocarbon-like (PAH-like) molecules.
... ... b) When the dust enters a dense gas cloud, atoms and
molecules of gas adhere to the core-mantle grains and form an
outer mantle of ice. The carbonaceous particles and PAH-like
molecules also accrete on the core-mantle grains.
... ... c) Ultraviolet radiation affects the material in the ice
mantle, creating a layer of complex organic compounds of
yellowish color.
... ... d) As the cloud of dust and gas contracts to form a star,
some of the core-mantle dust grains clump together and become
comet nuclei. But the vast majority of the dust is dispersed.
... ... e) Returning to a diffuse cloud, the core-mantle grain is
exposed to harsher radiation that evaporates the ice mantle and
further processes the organic material. The complex of organic
compounds turns from yellowish to brown.
... ... f) Supernova shock fronts accelerate the dust grains,
causing violent collisions that shatter the organic mantles. The
debris becomes the carbonaceous particles and PAH-like molecules.
3) The author points out that as astronomers make new
discoveries about the chemical composition of both comets and
interstellar dust, they are becoming convinced that comets
originally formed as clumps of dust grains. In addition, comet
dust may have played a role in seeding life on Earth. Each loose
cluster of comet dust not only contains organic materials, but
also has a structure that is ideal for chemical evolution once it
is immersed in water. Experiments have indicated that small
molecules could easily penetrate such clumps from the outside,
while large molecules would remain sequestered in the interior.
The author states: "Such a structure could stimulate the
production of ever larger and more complex molecules, possibly
serving as a tiny incubator for the first primitive life forms. A
single comet could have deposited up to 10^(25) of these 'seeds'
on the young Earth."
-----------
J. Mayo Greenberg: The secrets of stardust.
(Scientific American December 2000)
QY: J. Mayo Greenberg, University of Leiden, NL
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Summary by SCIENCE-WEEK http://scienceweek.com 1Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON CARBON IN THE UNIVERSE
Carbon is a major factor in the evolutionary scheme of the
Universe because of its abundance and its ability to form complex
chemical entities. It is apparently also a key element in the
evolution of prebiotic molecules. The different forms of cosmic
carbon range from carbon atoms and carbon-bearing molecules to
complex solid-state carbonaceous structures, and evidence
gathered during the past decade has considerably enhanced our
understanding of the physical and chemical properties of carbon
materials in space. ... ... Th. Henning and F. Salama (2
installations, DE US) present a detailed review of the subject,
the authors making the following points: 1) More than 75 percent
of the 118 *interstellar and circumstellar molecules identified
to date are carbon-bearing molecules, and one component of
interstellar dust is evidently carbonaceous. The cosmic evolution
of carbon from the interstellar medium into *protoplanetary disks
and *planetesimals, and finally into habitable bodies, is
intrinsic to the study of the origin of life. 2) Carbon plays an
important role in the physical evolution of the interstellar
medium because it is the main supplier of free electrons in
diffuse interstellar clouds, thus contributing to the heating of
interstellar gas. 3) The observation of unidentified ubiquitous
molecular and solid-state features in astronomical spectra, and
the realization that these features are linked to carbonaceous
materials, have resulted in major scientific progress in the past
decade. Laboratory and theoretical studies stimulated by these
astronomical observations have led to a better understanding of
the various forms of cosmic carbon such as polycyclic aromatic
hydrocarbons, carbon-chain molecules, carbon clusters, and
carbonaceous solids. These investigations have also led to the
detection of novel forms of carbon and laid the foundations for
the chemistry of *fullerenes. 4) The authors present the
following categorization of carbon in space:
... a) Carbon-rich circumstellar envelopes around *red giant and
*asymptotic giant branch (AGB) stars: CO, C(sub2)H(sub2), complex
hydrocarbons, gas-phase polycyclic aromatic hydrocarbons.
... b) Diffuse interstellar medium: C+, simple diatomic
molecules, gas-phase polycyclic aromatic hydrocarbons and carbon
chains.
... c) Dense interstellar medium: CO, complex hydrocarbons.
... d) Interstellar material in primitive meteorites: polycyclic
aromatic hydrocarbons.
5) The authors suggest that the widespread distribution of
complex organics in the interstellar medium has profound
implications for our understanding of a) the chemical complexity
of the interstellar medium, b) the evolution of prebiotic
molecules, c) the impact of this evolution on the origin and
evolution of life on early Earth through the exogenous delivery
(by cometary encounters and meteoritic bombardments) of prebiotic
organics.
-----------
Th. Henning and F. Salama: Carbon in the Universe.
(Science 18 Dec 98 282:2204)
QY: Th. Henning, Astrophysikalisches Institut und Universitats-
Sternwarte, Schillergabchen 2-3, D-07745, Jena DE.
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Text Notes:
... ... *interstellar and circumstellar molecules: In this
context, an interstellar molecule is any molecule that occurs
naturally in clouds of gas and dust in space. In general, a
circumstellar molecule is any molecule that occurs in gas and
dust surrounding a star.
... ... *protoplanetary disks: These are dust disks surrounding
young stars; it is from these disks that planets presumably form.
... ... *planetesimals: Planetesimals are bodies with dimensions
of 10^(-3) to 10^(3) meters that are believed to form planets by
a process of accretion. The term "accretion" refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk. A stellar accretion disk is a swarm of dust grains
that evolve into planetesimals and then planets.
... ... *fullerenes: Fullerenes are large molecules composed
entirely of carbon, with the chemical formula C(sub n), where n
is any even number from 32 to over 100. They apparently have the
structure of a hollow spheroidal cage with a surface network of
carbon atoms connected in hexagonal and pentagonal rings.
... ... *red giant: A red giant star is a star in a late
stage of evolution. Having exhausted the hydrogen fuel in its
core, the star is burning elements heavier than hydrogen. It has
a surface temperature of less than 4700 degrees Kelvin and a
diameter 10 to 100 times that of the Sun.
... ... *asymptotic giant branch (AGB) stars: These are stars
that occupy a strip in the *Hertzsprung-Russell diagram that is
almost parallel to and just above what is called the "giant
branch" off the *Main Sequence. Stars evolve from the horizontal
H-R branch to the asymptotic giant branch when they have
exhausted the helium in their cores and are instead burning
helium in a shell.
... ... *Hertzsprung-Russell diagram: The Hertzsprung-Russell
diagram is a plot of stellar absolute magnitude against spectral
type, and is perhaps the most useful diagrammatic aid in
astrophysics. It allows the portrayal of the evolution of a star
as occurring along various paths in the diagram.
... ... *Main Sequence: The Main Sequence is a region on the
Hertzsprung-Russell diagram where most stars lie, including our
own Sun. The evolution of a star can be diagrammed as a movement
along the Main Sequence and an eventual branching off the Main
Sequence to regions associated with various types of old stars.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 26Feb99
For more information: http://scienceweek.com/swfr.htm
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