423 research outputs found
Top-down formation of fullerenes in the interstellar medium
[Abridged] Fullerenes have been recently detected in various circumstellar
and interstellar environments, raising the question of their formation pathway.
It has been proposed that they can form by the photo-chemical processing of
large polycyclic aromatic hydrocarbons (PAHs). Following our previous work on
the evolution of PAHs in the NGC 7023 reflection nebula, we evaluate, using
photochemical modeling, the possibility that the PAH CH (i.e.
circumovalene) can lead to the formation of C upon irradiation by
ultraviolet photons. The chemical pathway involves full dehydrogenation,
folding into a floppy closed cage and shrinking of the cage by loss of C
units until it reaches the symmetric C molecule. At 10" from the
illuminating star and with realistic molecular parameters, the model predicts
that 100% of CH is converted into C in 10
years, a timescale comparable to the age of the nebula. Shrinking appears to be
the kinetically limiting step of the whole process. Hence, PAHs larger than
CH are unlikely to contribute significantly to the formation of
C, while PAHs containing between 60 and 66 C atoms should contribute to
the formation of C with shorter timescales, and PAHs containing less
than 60 C atoms will be destroyed. Assuming a classical size distribution for
the PAH precursors, our model predicts absolute abundances of C are up
to several of the elemental carbon, i.e. less than a percent of the
typical interstellar PAH abundance, which is consistent with observational
studies. According to our model, once formed, C can survive much longer
than other fullerenes because of the remarkable stability of the C
molecule at high internal energies.Hence, a natural consequence is that
C is more abundant than other fullerenes in highly irradiated
environments.Comment: Accepted for publication in A&A. Latest version contains the
corrected version of Fig.
Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
Various studies have emphasized variations of the charge state and
composition of the interstellar polycyclic aromatic hydrocarbon (PAH)
population in photodissociation regions (PDRs). We aim to model the spatial
evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on
the specific case of the north-west (NW) PDR of NGC 7023 and also discuss the
case of the diffuse interstellar medium (ISM). The physical conditions in NGC
7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH
chemical evolution model that includes recent experimental data on PAHs and
describes multiphoton events. We consider a family of compact PAHs bearing up
to 96 carbon atoms. The calculated ionization ratio is in good agreement with
observations in NGC 7023 NW. Within the PDR, PAHs evolve into three major
populations: medium-sized PAHs (50<Nc<90) are normally hydrogenated, larger
PAHs (Nc>90) can be superhydrogenated, and smaller species (Nc<50) are fully
dehydrogenated. In the cavity, where the fullerene C60 was recently detected,
all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical
evolution exhibits a complex non-linear behaviour as a function of the UV
radiation field because of multiphoton events. Steady state for hydrogenation
is reached on timescales ranging from less than a year for small PAHs, up to
10000 years for large PAHs at Av=1. We identified critical reactions that need
more studies. Our new model allows us to rationalize the observational
constraints without any fitting parameter. PAHs smaller than 50 carbon atoms
are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is
obtained for the diffuse ISM. Carbon clusters turn out to be end products of
PAH photodissociation, and the evolution of these clusters needs to be
investigated further to evaluate their impact on the chemical and physical
evolution of PDRs.Comment: 16 pages, 10 figures; Accepted for publication in A&
Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions
Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic
Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains
(VSGs) by the impinging UV radiation field from a nearby star. Aims. We
investigate quantitatively the transition zone between evaporating Very Small
Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative
contribution of PAHs and eVSGs to the mid-IR emission in a wide range of
excitation conditions. We fit the observed mid-IR emission of PDRs by using a
set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting
tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the
community as an IDL routine. From the results of the fit, we derive the
fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of
the local UV radiation field. Results. We show a clear decrease of f_eVSG with
increasing intensity of the local UV radiation field, which supports the
scenario of photo-destruction of eVSGs. Conversely, this dependence can be used
to quantify the intensity of the UV radiation field for different PDRs,
including non resolved ones. Conclusions. PAHTAT can be used to trace the
intensity of the local UV radiation field in regions where eVSGs evaporate,
which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated
PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.Comment: 13 pages, 11 figures. Accepted for publication in A&A. Typos
correcte
30 years of cosmic fullerenes
In 1985, "During experiments aimed at understanding the mechanisms by which
long-chain carbon molecules are formed in interstellar space and circumstellar
shells", Harry Kroto and his collaborators serendipitously discovered a new
form of carbon: fullerenes. The most emblematic fullerene (i.e. C
"buckminsterfullerene"), contains exactly 60 carbon atoms organized in a
cage-like structure similar to a soccer ball. Since their discovery impacted
the field of nanotechnologies, Kroto and colleagues received the Nobel prize in
1996. The cage-like structure, common to all fullerene molecules, gives them
unique properties, in particular an extraordinary stability. For this reason
and since they were discovered in experiments aimed to reproduce conditions in
space, fullerenes were sought after by astronomers for over two decades, and it
is only recently that they have been firmly identified by spectroscopy, in
evolved stars and in the interstellar medium. This identification offers the
opportunity to study the molecular physics of fullerenes in the unique physical
conditions provided by space, and to make the link with other large
carbonaceous molecules thought to be present in space : polycyclic aromatic
hydrocarbons.Comment: To appear in the Proceedings of the annual meeting of the French
society of astronomy and astrophysics (SF2A 2015
Polycyclic aromatic hydrocarbons and molecular hydrogen in oxygen-rich planetary nebulae: the case of NGC6720
Evolved stars are primary sources for the formation of polycyclic aromatic
hydrocarbons (PAHs) and dust grains. Their circumstellar chemistry is usually
designated as either oxygen-rich or carbon-rich, although dual-dust chemistry
objects, whose infrared spectra reveal both silicate- and carbon-dust features,
are also known. The exact origin and nature of this dual-dust chemistry is not
yet understood. Spitzer-IRS mid-infrared spectroscopic imaging of the nearby,
oxygen-rich planetary nebula NGC6720 reveals the presence of the 11.3 micron
aromatic (PAH) emission band. It is attributed to emission from neutral PAHs,
since no band is observed in the 7 to 8 micron range. The spatial distribution
of PAHs is found to closely follow that of the warm clumpy molecular hydrogen
emission. Emission from both neutral PAHs and warm H2 is likely to arise from
photo-dissociation regions associated with dense knots that are located within
the main ring. The presence of PAHs together with the previously derived high
abundance of free carbon (relative to CO) suggest that the local conditions in
an oxygen-rich environment can also become conducive to in-situ formation of
large carbonaceous molecules, such as PAHs, via a bottom-up chemical pathway.
In this scenario, the same stellar source can enrich the interstellar medium
with both oxygen-rich dust and large carbonaceous molecules.Comment: Accepted by MNRAS. 5 page
Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023
Extended Red Emission (ERE) was recently attributed to the photo-luminescence
of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH), or
charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust
emission in the North-West and South photo-dissociation regions of the
reflection nebula NGC 7023.Using a blind signal separation method, we extracted
the map of ERE from images obtained with the Hubble Space Telescope, and at the
Canada France Hawaii Telescope. We compared the extracted ERE image to the
distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral
and ionized PAHs (PAH and PAH) obtained with the Spitzer Space
Telescope and the Infrared Space Observatory. ERE is dominant in transition
regions where VSGs are being photo-evaporated to form free PAH molecules, and
is not observed in regions dominated by PAH. Its carrier makes a minor
contribution to the mid-IR emission spectrum. These results suggest that the
ERE carrier is a transition species formed during the destruction of VSGs.
Singly ionized PAH dimers appear as good candidates but PAH molecules
seem to be excluded.Comment: Accepted for publication in A&
The infrared spectra of very large, compact, highly symmetric, polycyclic aromatic hydrocarbons (PAHs)
The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are
determined computationally using Density Functional Theory. Trends in the band
positions and intensities as a function of PAH size, charge and geometry are
discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions
hold as with small PAHs.
This does not hold for the other features. The larger PAH cations and anions
produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron
becomes prominent and shifts slightly to the red. In addition, the average
anion peak falls slightly to the red of the average cation peak. The similarity
in behavior of the 7.8 and 8.6 micron bands with the astronomical observations
suggests that they arise from large, cationic and anionic PAHs, with the
specific peak position and profile reflecting the PAH cation to anion
concentration ratio and relative intensities of PAH size. Hence, the broad
astronomical 7.7 micron band is produced by a mixture of small and large PAH
cations and anions, with small and large PAHs contributing more to the 7.6 and
7.8 micron component respectively.
For the CH out-of-plane vibrations, the duo hydrogens couple with the solo
vibrations and produce bands that fall at wavelengths slightly different than
their counterparts in smaller PAHs. As a consequence, previously deduced PAH
structures are altered in favor of more compact and symmetric forms. In
addition, the overlap between the duo and trio bands may reproduce the
blue-shaded 12.8 micron profile.Comment: ApJ, 36 pages, 9 fig
Stacked clusters of polycyclic aromatic hydrocarbon molecules
Clusters of polycyclic aromatic hydrocarbon (PAH) molecules are modelled
using explicit all-atom potentials using a rigid body approximation. The PAH's
considered range from pyrene (C10H8) to circumcoronene (C54H18), and clusters
containing between 2 and 32 molecules are investigated. In addition to the
usual repulsion-dispersion interactions, electrostatic point-charge
interactions are incorporated, as obtained from density functional theory
calculations. The general electrostatic distribution in neutral or singly
charged PAH's is reproduced well using a fluctuating charges analysis, which
provides an adequate description of the multipolar distribution. Global
optimization is performed using a variety of methods, including basin-hopping
and parallel tempering Monte Carlo. We find evidence that stacking the PAH
molecules generally yields the most stable motif. A structural transition
between one-dimensional stacks and three-dimensional shapes built from mutiple
stacks is observed at larger sizes, and the threshold for this transition
increases with the size of the monomer. Larger aggregates seem to evolve toward
the packing observed for benzene in bulk.Difficulties met in optimizing these
clusters are analysed in terms of the strong anisotropy of the molecules. We
also discuss segregation in heterogeneous clusters and vibrational properties
in the context of astrophysical observations.Comment: 12 pages, 7 figure
Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs
Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and
molecular hydrogen emission are a potentially powerful tool to derive physical
properties of dense environments irradiated by intense UV fields. We present
new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high
UV-field and dense photodissocation region (PDR) around Monoceros R2, the
closest ultracompact \hII region, revealing the spatial structure of ionized
gas, PAHs and H emissions. Using a PDR model and PAH emission feature
fitting algorithm, we build a comprehensive picture of the physical conditions
prevailing in the region. We show that the combination of the measurement of
PAH ionization fraction and of the ratio between the H 0-0 S(3) and S(2)
line intensities, respectively at 9.7 and 12.3 m, allows to derive the
fundamental parameters driving the PDR: temperature, density and UV radiation
field when they fall in the ranges K, cm,
respectively. These mid-IR spectral tracers thus provide a tool
to probe the similar but unresolved UV-illuminated surface of protoplanetary
disks or the nuclei of starburst galaxies.Comment: Accepted for publication in ApJ Letter
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