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OATS-DAUT SEMINAR
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Speaker:
Franco CATALDO (Lupi Chemical Institute, Rome, I)
Title:
Astrochemical aspects of elemental carbon: chains, clusters and grains
Date: Wednesday, December 3rd, 2008
Time: 12:00
Venue: Villa Bazzoni
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Abstract:
Elemental carbon is ejected in the interstellar medium by late-type
carbon rich stars. The rich organic chemistry occurring in the
circumstellar envelope of post-AGB (asymptotic giant branch)
carbon-rich giant stars is essentially due to the peculiar properties
of the carbon vapour. In fact carbon vapour in the circumstellar
environment forms quite easily carbon chains of acetylenic nature
known as polyynes. The polyynes have been detected by radioastronomy
and one of the largest organic molecule till known to exist in such
environment is the monocyanopolyyne:
H-C≡C-C≡C-C≡C-C≡C-C≡C-C≡N
Monocyanopolyynes and dicyanopolyynes can be synthesized quite easily
in laboratory with a process developed by the undersigned. A process
mimicking the process occurring in the circumstellar envelope. In
fact, monocyanopolyynes having the general formula can be synthesized
together with ordinary polyynes series H-(C≡C)n-H by
arcing graphite electrodes in acetonitrile or in presence of
ammonia. Dicyanopolyynes N≡C-(C≡C)n-C≡N
are produced almost pure by arcing graphite electrodes directly into
liquid nitrogen. As previously mentioned, these molecules are present
in the envelope of post-AGB carbon-rich giant stars and also in dark
molecular clouds. Our work underlines the parallelism existing
between the polyynes formed by laser ablation experiments of graphite
targets (made by H. W. Kroto group) and those produced from the
submerged carbon arc. In both cases the same products are
obtained. The products consist of hydrogen-terminated polyynes if
water is present, monocyanopolyynes (mixed with hydrogen-terminated
polyynes) if the carbon arc is sparked in acetonitrile or ammonia and
dicyanopolyynes if the arc is struck in liquid nitrogen. The mechanism
of formation of polyynes in the submerged carbon arc involves
essentially neutral species; similar species and pathway may occur
also in the circumstellar environment where polyynes have been
detected by radioastronomy. It is shown that the relative abundances
of the polyynes formed in the submerged carbon arc or in a carbon arc
in vacuum decrease by a factor between 3 and 5 as the chain length
increases by a C2 unit. Exactly the same trend has been observed by
radioastronomy both for polyynes and cyanopolyynes in the
circumstellar environment around red giants and AGB (asymptotic gian
branch) stars. This fact may be a simple coincidence or may suggests
that the mechanism of formation of the polyynes in the carbon arc is
the same occurring in the surroundings of the carbon-rich stars.
The
discovery of the easy formation of polyynes molecules in laboratory
conditions may explain why these molecules are so ubiquitous in the
space. Polyynes are also just the precursors of more complex molecules
of astrochemical interest. For instance, in a hydrogen-rich atmosphere
the polyynes are converted into a mixture of polycyclic aromatic
hydrocarbon (PAHs) and this has been verified experimentally. In their
turn, the PAHs are just the building blocks of graphene sheets and
graphene sheets tends to clump together to form carbon dust. The
interstellar carbon dust may account of 1-2% of the mass of the
interstellar matter. In our laboratory experiments with carbon arc,
the polyynes, the PAHs and the carbon dust are formed together. By
changing the conditions it is possible to exalt or suppress the
formation of one or more chemical species. Additionally, also
fullerenes can be produced together with carbon dust from a carbon
arc. They are commonly considered as carbon clusters and their
formation require special conditions like a rich helium atmosphere
which ensure the optimum cooling. In any case, it has been
demonstrated that polyynes are the precursors also of fullerene
formation.
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contact: Giovanni Vladilo (OATS)