405 research outputs found
Transformation of amorphous carbon clusters to fullerenes
Transformation of amorphous carbon clusters into fullerenes under high
temperature is studied using molecular dynamics simulations at microsecond
times. Based on the analysis of both structure and energy of the system, it is
found that fullerene formation occurs in two stages. Firstly, fast
transformation of the initial amorphous structure into a hollow sp shell
with a few chains attached occurs with a considerable decrease of the potential
energy and the number of atoms belonging to chains and to the amorphous domain.
Then, insertion of remaining carbon chains into the sp network takes place
at the same time with the fullerene shell formation. Two types of defects
remaining after the formation of the fullerene shell are revealed: 7-membered
rings and single one-coordinated atoms. One of the fullerene structures
obtained contains no defects at all, which demonstrates that defect-free carbon
cages can be occasionally formed from amorphous precursors directly without
defect healing. No structural changes are observed after the fullerene
formation, suggesting that defect healing is a slow process in comparison with
the fullerene shell formation. The schemes of the revealed reactions of chain
atoms insertion into the fullerene shell just before its completion are
presented. The results of the performed simulations are summarized within the
paradigm of fullerene formation due to selforganization of the carbon system.Comment: 35 pages, 9 figure
Infrared Study of Fullerene Planetary Nebulae
We present a study of 16 PNe where fullerenes have been detected in their
Spitzer spectra. This large sample of objects offers an unique opportunity to
test conditions of fullerene formation and survival under different metallicity
environments as we are analyzing five sources in our own Galaxy, four in the
LMC, and seven in the SMC. Among the 16 PNe under study, we present the first
detection of C60 (possibly also C70) fullerenes in the PN M 1-60 as well as of
the unusual 6.6, 9.8, and 20 um features (possible planar C24) in the PN K
3-54. Although selection effects in the original samples of PNe observed with
Spitzer may play a potentially significant role in the statistics, we find that
the detection rate of fullerenes in C-rich PNe increases with decreasing
metallicity (5% in the Galaxy, 20% in the LMC, and 44% in the SMC). CLOUDY
photoionization modeling matches the observed IR fluxes with central stars that
display a rather narrow range in effective temperature (30,000-45,000 K),
suggesting a common evolutionary status of the objects and similar fullerene
formation conditions. The observed C60 intensity ratios in the Galactic sources
confirm our previous finding in the MCs that the fullerene emission is not
excited by the UV radiation from the central star. CLOUDY models also show that
line- and wind-blanketed model atmospheres can explain many of the observed
[NeIII]/[NeII] ratios by photoionization suggesting that possibly the UV
radiation from the central star, and not shocks, are triggering the
decomposition of the circumstellar dust grains. With the data at hand, we
suggest that the most likely explanation for the formation of fullerenes and
graphene precursors in PNe is that these molecular species are built from the
photo-chemical processing of a carbonaceous compound with a mixture of aromatic
and aliphatic structures similar to that of HAC dust.Comment: Accepted for publication in ApJ (43 pages, 11 figures, and 4 tables).
Small changes to fit the proof-corrected article to be published in Ap
On the Excitation and Formation of Circumstellar Fullerenes
We compare and analyze the Spitzer mid-infrared spectrum of three
fullerene-rich planetary nebulae in the Milky Way and the Magellanic Clouds;
Tc1, SMP SMC16, and SMP LMC56. The three planetary nebulae share many
spectroscopic similarities. The strongest circumstellar emission bands
correspond to the infrared active vibrational modes of the fullerene species
C60 and little or no emission is present from Polycyclic Aromatic Hydrocarbons
(PAHs). The strength of the fullerene bands in the three planetary nebulae is
very similar, while the ratio of the [NeIII]15.5um/[NeII]12.8um fine structure
lines, an indicator of the strength of the radiation field, is markedly
different. This raises questions about their excitation mechanism and we
compare the fullerene emission to fluorescent and thermal models. In addition,
the spectra show other interesting and common features, most notably in the
6-9um region, where a broad plateau with substructure dominates the emission.
These features have previously been associated with mixtures of
aromatic/aliphatic hydrocarbon solids. We hypothesize on the origin of this
band, which is likely related to the fullerene formation mechanism, and compare
it with modeled Hydrogenated Amorphous Carbon that present emission in this
region.Comment: 13 pages, 2 tables, 7 figures, Accepted for publication in Ap
The Formation of Fullerenes in Planetary Nebulae
In the last decade, fullerenes have been detected in a variety of astrophysical environments, with the majority being found in planetary nebulae. Laboratory experiments have provided us with insights into the conditions and pathways that can lead to fullerene formation, but it is not clear precisely what led to the formation of astrophysical fullerenes in planetary nebulae. We review some of the available evidence, and propose a mechanism where fullerene formation in planetary nebulae is the result of a two-step process where carbonaceous dust is first formed under unusual conditions; then, the fullerenes form when this dust is being destroyed
Constraints on Exotic Heavily Ionizing Particles from the Geological Abundance of Fullerenes
The C_{60} molecule exhibits a remarkable stability and inertness that leads
to its survival in ancient carbonaceous rocks initially subject to the high
temperatures requisite for its formation. Elementary particles having very high
electronic stopping powers can similarly form C_{60} and higher fullerenes in
their wake. Combined, these two features point at the possibility of using the
C_{60} presence (or absence) in selected bulk geological samples as a new type
of solid-state nuclear track detector, with applications in astro-particle
physics.Comment: Final version (few modifications). Phys. Rev. Lett. (in press). 4
pages LaTeX, 1 eps figure embedde
Formation of fullerenes in H-containing Planetary Nebulae
Hydrogen depleted environments are considered an essential requirement for
the formation of fullerenes. The recent detection of C60 and C70 fullerenes in
what was interpreted as the hydrogen-poor inner region of a post-final helium
shell flash Planetary Nebula (PN) seemed to confirm this picture. Here, we
present evidence that challenges the current paradigm regarding fullerene
formation, showing that it can take place in circumstellar environments
containing hydrogen. We report the simultaneous detection of Polycyclic
Aromatic Hydrocarbons (PAHs) and fullerenes towards C-rich and H-containing PNe
belonging to environments with very different chemical histories such as our
own Galaxy and the Small Magellanic Cloud. We suggest that PAHs and fullerenes
may be formed by the photochemical processing of hydrogenated amorphous carbon.
These observations suggest that modifications may be needed to our current
understanding of the chemistry of large organic molecules as well as the
chemical processing in space.Comment: accepted for publication in The Astrophysical Journal Letters (14
pages, 4 figures and 1 Table). Replaced version with slight changes in the
abstrac
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