Chemistry Reference
In-Depth Information
carbyne chain lengths in the amorphous carbon matrix are unknown. The
thinness of the plates suggests the breakdown of cumulene and polyyne
chains to their smallest units that locally reorganized to bridge graphene
sheets to form carbyne single crystals.
Carbynes are extremely rare in meteorites. The main issue to emerge is
that platey carbyne crystals in these extraterrestrial materials were formed
by solid-state transformations of amorphous sp
2
-hybridized carbon with
embedded carbyne chain molecules during thermal annealing inside a parent
body (asteroid; comet nucleus) with a sustained thermal regime. Pervasive
hydration of CI and CM meteorites and flash-heating of IDPs during
atmospheric entry might have erased most or all of their metastable carbon
minerals. Such amorphous carbons could have formed within the solar
system or around other stars, and after surviving sojourn through the inter-
stellar medium (ISM), might then be incorporated as so-called presolar dust
in meteorites and IDPs. What then is the evidence for carbyne formation
in circumstellar environments?
16.11 CIRCUMSTELLAR DUST
Circumstellar regions range from the winds of very young stars to dust
envelopes around highly evolved stars. Stars of a few solar masses develop
extended atmospheres that, when drifting outwards, will generate warm
(
1000K) and relatively dense circumstellar envelopes wherein interesting
chemistry and dust formation occur. Carbon-rich envelopes show a parti-
cularly rich chemistry. A plethora of carbon-chain molecules take a promi-
nent place in circumstellar envelopes. Vibration-rotation lines of the pure
carbon chain molecule C3, a potential building block for carbon grains,
have been identified in the circumstellar spectrum of the obscured carbon
star IRC
10216 [98]. In the circumstellar shell of the same source C
5
was
also detected [99]. In this well studied, nearby envelope of high mass-
loss rate, the family of carbon chain molecules HC
2n
þ
1
N(n
þ
¼
1-5), C
n
H
(n
1-3) has attracted much attention. It was proposed
that the large cyanopolyynes, HC
2n
þ
1
N, were formed in radical reactions
of C
3
N and C
5
N with acetylene [100]. As abundance calculations using a
photochemical model of the envelope demonstrate that chains will be
efficiently produced in outer circumstellar envelopes [101], within which
dust might form and be ejected in to the ISM. The dust, distributed
throughout the IS with the gas, extinguishes starlight rising almost mono-
tonically with photon energy from the near IR to far UV. The extinction
curve is interpreted as due to grains with a distribution of dust grains sizes
ranging from about 3 nm to 300 nm, being the smallest in the ''molecular''
regime. However, the interpretation of interstellar polarization requires the
presence of macroscopic dust [102]. The chemical pathways towards carbo-
naceous grains, starting from very small radical clusters, such as C
2
,C
3
,C
2
H
¼
2-6) and C
n
S(n
¼
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