939 research outputs found
Absolute evaporation rates of non-rotating neutral PAH clusters
Clusters of polycyclic aromatic hydrocarbons (PAHs) have been proposed as
candidates for evaporating very small grains, which are thought to be
precursors of free-flying PAHs. Evaporation rates have been calculated so far
only for species containing up to a few 100 C atoms, whereas interstellar PAH
clusters could contain up to ~1000 C atoms. We present a method that
generalises the calculation of the statistical evaporation rate of large PAH
clusters and provides rates for species containing up to ~1000 C-atoms. The
evaporation of non-rotating neutral homo-molecular PAH clusters containing up
to 12 molecules from a family of highly symmetric compact PAHs is studied.
Statistical calculations were performed and completed with molecular dynamics
simulations at high internal energies to provide absolute values for the
evaporation rate and distributions of kinetic energy released. The calculations
used explicit atom-atom Lennard-Jones potentials in the rigid molecule
approximation. A new method is proposed to take both inter- and intra-molecular
vibrations into account. Without any parameter adjustment, the calculated
evaporation rates agree well with available experimental data. We find that the
non-rotation assumption has a limited impact on the evaporation rates. The
photostability of PAH clusters increases dramatically with the size of
molecules in the clusters, and to a lesser extent with the number of molecules
in the clusters. For values of the UV radiation field that are typical of the
regions where evaporating very small grains are observed, the smallest clusters
in this study (~50 C-atoms) are found to be quickly photo-evaporated, whereas
the largest clusters (~1000 C-atoms) are photostable. Our results support the
idea that large PAH clusters are good candidates for evaporating very small
grains.Comment: 13 pages, 10 figure
Detection of the buckminsterfullerene cation (C60+) in space
In the early 90s, C60+ was proposed as the carrier of two diffuse
interstellar bands (DIBs) at 957.7 and 963.2 nm, but a firm identification
still awaits gas-phase spectroscopic data. Neutral C60, on the other hand, was
recently detected through its infrared emission bands in the interstellar
medium and evolved stars. In this contribution, we present the detection of
C60+ through its infrared vibrational bands in the NGC 7023 nebula, based on
spectroscopic observations with the Spitzer space telescope, quantum chemistry
calculation, and laboratory data from the literature. This detection supports
the idea that C60+ could be a DIB carrier, and provides robust evidence that
fullerenes exist in the gas-phase in the interstellar medium. Modeling efforts
to design specific observations, combined with new gas-phase data, will be
essential to confirm this proposal. A definitive attribution of the 957.7 and
963.2 nm DIBs to C60+ would represent a significant step forward in the field.Comment: To appear in "Proceedings of IAU 297 symposium on the Diffuse
Interstellar Bands", eds. J. Cami and N. Cox (5 pages
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&
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.
Blind Signal Separation Methods for the Identification of Interstellar Carbonaceous Nanoparticles
The use of Blind Signal Separation methods (ICA and other approaches) for the
analysis of astrophysical data remains quite unexplored. In this paper, we
present a new approach for analyzing the infrared emission spectra of
interstellar dust, obtained with NASA's Spitzer Space Telescope, using FastICA
and Non-negative Matrix Factorization (NMF). Using these two methods, we were
able to unveil the source spectra of three different types of carbonaceous
nanoparticles present in interstellar space. These spectra can then constitute
a basis for the interpretation of the mid-infrared emission spectra of
interstellar dust in the Milky Way and nearby galaxies. We also show how to use
these extracted spectra to derive the spatial distribution of these
nanoparticles
The infrared signatures of very small grains in the Universe seen by JWST
The near- and mid-IR spectrum of many astronomical objects is dominated by
emission bands due to UV-excited polycyclic aromatic hydrocarbons (PAH) and
evaporating very small grains (eVSG). Previous studies with the ISO, Spitzer
and AKARI space telescopes have shown that the spectral variations of these
features are directly related to the local physical conditions that induce a
photo-chemical evolution of the band carriers. Because of the limited
sensitivity and spatial resolution, these studies have focused mainly on
galactic star-forming regions. We discuss how the advent of JWST will allow to
extend these studies to previously unresolved sources such as near-by galaxies,
and how the analysis of the infrared signatures of PAHs and eVSGs can be used
to determine their physical conditions and chemical composition.Comment: To appear in the Proceedings of the annual meeting of the French
society of astronomy and astrophysics (SF2A 2015
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
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Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 m ratio
In photon-dominated regions (PDRs), UV photons from nearby stars lead to the
evaporation of very small grains (VSGs) and the production of gas-phase
polycyclic aromatic hydrocarbons (PAHs). Our goal is to achieve better insight
into the composition and evolution of evaporating very small grains (eVSGs) and
PAHs through analyzing the infrared (IR) aliphatic and aromatic emission bands.
We combined spectro-imagery in the near- and mid-IR to study the spatial
evolution of the emission bands in the prototypical PDR NGC 7023. We used
near-IR spectra obtained with AKARI to trace the evolution of the 3.3m and
3.4m bands, which are associated with aromatic and aliphatic C-H bonds on
PAHs. The spectral fitting involves an additional broad feature centred at
3.45m. Mid-IR observations obtained with Spitzer are used to discriminate
the signatures of eVSGs, neutral and cationic PAHs. We correlated the spatial
evolution of all these bands with the intensity of the UV field to explore the
processing of their carriers. The intensity of the 3.45m plateau shows an
excellent correlation with that of the 3.3m aromatic band (correlation
coefficient R = 0.95), indicating that the plateau is dominated by the emission
from aromatic bonds. The ratio of the 3.4m and 3.3m band intensity
() decreases by a factor of 4 at the PDR interface from the
more UV-shielded to the more exposed layers. The transition region between the
aliphatic and aromatic material is found to correspond spatially with the
transition zone between neutral PAHs and eVSGs. We conclude that the
photo-processing of eVSGs leads to the production of PAHs with attached
aliphatic sidegroups that are revealed by the 3.4m emission band. Our
analysis provides evidence for the presence of very small grains of mixed
aromatic and aliphatic composition in PDRs.Comment: Accepted for publication in A&A. Abstract abridged, language editing
applied in v
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