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might be the formation of amorphous carbon with multiple hybridization
states. This circumstellar carbon dust would be the precursor material for
the solid-state formation of platy carbyne crystals when thermally annealed.
16.12 INTERSTELLAR DUST
In the cold (10-30 K) interstellar clouds the detection of carbon clusters is
still incomplete but the centro-symmetric carbon chain could possibly be
detected by their far-IR bending-rotation transitions. The C
3
molecule
was recently detected in one cold dust cloud [104]. The observations of
hydrogen-containing polar chain molecules towards star
-Ophiuchi and
laboratory studies of the electronic transitions of a number of homologous
series lead to an important conclusion on the role of carbon chains as
potential molecules in the so-called diffuse interstellar bands (DIBs). These
carbon-chain molecules would have to be very long, e.g. HC
n
H, C
2n
,C
n
H
neutral chains, HC
n
H
รพ
cations or (C
n
)
anions (except for odd-numbered
carbon chains [105]. Carbon chain species, as a family, have abundances in
the diffuse ISM on the order of 10
10
(relative to H) that is consistent with
their abundances in dense molecular clouds [106]. Assuming an average
length of 10 carbon atoms per chain, this abundance implies that roughly a
millionth of all cosmically available carbon could occur in the form of these
chains. The discovery that carbon chain molecules are abundant in various
interstellar regions poses a major problem. Their existence and abundan-
ces in the ISM would imply the operation of a facile synthesis process, but
it is unclear how such long carbon chains could be synthesized efficiently
from smaller units by the ion-molecule processes, by gas-phase chemistry
[107,108] or via grain disruption by shock waves that periodically traverse
the ISM [109].
Lacking an experimentally verified explanation for carbon chain forma-
tion at 10-30K suggests that carbyne crystals will probably not form in the
ISM that is generally characterized by dust destruction and little conden-
sation [110]. Yet, polyynes are abundantly and efficiently formed in the
circumstellar environments, at relatively high pressures and temperatures
(see
Section 16.11
). Stars continuously interact with the ISM. In its early
stage a young star will expel much of its enveloping dust and gas. Even when
hydrostatic equilibrium is reached as the star settles onto the main sequence
of the Hertzsprung-Russel diagram, its stellar winds keep supplying some
matter to ISM. The star-ISM interaction becomes dramatic towards the end
of stellar evolution when roughly a half of the stellar mass could be injected
into the ISM. The most massive stars explode as supernovae thereby rapidly
returning their matter, now converted by the nucleosynthesis, to the ISM.
In the asymptotic giant branch (AGB) stage of stellar evolution a star
gradually loses its outer shells until only a small, extremely dense core is
exposed. This particular evolutionary stage is generally believed to be the
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