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Silicon carbide together with amorphous carbon are the main components of dust grains in the atmospheres of C-rich AGB stars. Small gaseous Si-C bearing molecules (such as SiC, SiCSi, and SiC2) are efficiently formed close to the stellar photosphere. They likely condense onto dust seeds owing to their highly refractory nature at the lower temperatures (i.e., below about 2500 K) in the dust growth zone which extends a few stellar radii from the photosphere. Beyond this region, the abundances of Si-C bearing molecules are expected to decrease until they are eventually reformed in the outer shells of the circumstellar envelope, owing to the interaction between the gas and the interstellar UV radiation field. Our goal is to understand the time-dependent chemical evolution of Si-C bond carriers probed by molecular spectral line emission in the circumstellar envelope of IRC+10216 at millimeter wavelengths.
Far-UV photons (FUV, E < 13.6 eV) from hot massive stars regulate, or at least influence, the heating, ionization, and chemistry of most of the neutral interstellar medium (H i and H2 clouds). Investigating the interaction between FUV radiation and interstellar matter (molecules, atoms and grains) thus plays an important role in astrochemistry.
The Orion Bar, an interface region between the Orion A molecular cloud and the H ii region around the Trapezium cluster, is a textbook example of a strongly illuminated dense PDR (photodissociation region). The Bar is illuminated by a FUV field of a few 104 times the mean interstellar radiation field. Because of its proximity and nearly edge-on orientation, it provides a very good template to investigate the chemical content, structure, and dynamics of a strongly irradiated molecular cloud edge. We have used ALMA to mosaic a small field of the Bar where the critical transition from atomic to molecular gas takes place. These observations provide an unprecedented sharp view of this transition layer (≲ 1″ resolution or ≲ 414 AU). The resulting images (so far in the rotational emission of CO, HCO+, H13CO+, SO+, SO, and reactive ions SH+ and HOC+) show the small-scale structure in gas density and temperature, and the steep abundance gradients. The images reveal a pattern of high-density substructures, photo-ablative gas flows and instabilities at the edge of the molecular cloud. These first ALMA images thus show a more complex morphology than the classical clump/interclump static model of a PDR.
In order to quantify the chemical content in strongly FUV-irradiated gas, we have also used the IRAM-30 m telescope to carry out a complete line-survey of the illuminated edge of the Bar in the millimeter domain. Our observations reveal the presence of complex organic molecules (and precursors) that were not expected in such a harsh environment. In particular, we have reported the first detection of the unstable cis conformer of formic acid (HCOOH) in the ISM. The energy barrier to internal rotation (the conversion from trans to cis) is approximately 4827 cm−1 (≈7000 K). Hence, this detection is surprising. The low inferred trans-to-cis abundance ratio of 2.8±1.0 supports a photoswitching mechanism: a given conformer absorbs a FUV stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we have specifically studied with ab initio quantum calculations, was not considered so far in astrochemistry although it can affect the structure of a variety of molecules in PDRs.
In this contribution, we briefly describe how an observed mid-infrared (5.5-14 μm) spectrum can be used to trace key physical conditions along a given line of sight, such as the UV radiation field, the ionization parameter and the dust column density. These parameters are often difficult to determine independently from PDR models. The PAHTAT toolbox offers the opportunity to analyze mid-IR spectra using a limited number of parameters, that are associated with the physical properties of the dust and gas being observed.
Cationic polycyclic aromatic hydrocarbons (PAHs) are attractive candidates for the Diffuse Interstellar Bands, but to date not a single PAH species has been identified on the basis of a spectral agreement. This indicates either that the molecular diversity is very large or that the candidates that have been considered are not the correct ones. In particular, small/medium-sized PAH cations are submitted to photodissociation under UV photons from stars. Therefore it is of interest to characterize the spectroscopic properties of key breakdown products. Furthermore, these studies should be performed under conditions that mimic those found in interstellar space, which leads to additional experimental difficulties. We describe the approach we are developing with the PIRENEA set-up and present results on the 1-Methylpyrene cation and photo-derived species. Experimental measurements are guided by calculations based on density functional theory.
In the early 90's, C60+ was proposed as the carrier of two diffuse interstellar bands (DIBs) at 9577 and 9632 Å, 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 9577 and 9632 Å DIBs to C60+ would represent a significant step forward in the field.
The near-infrared (NIR) spectral range (2–5 μm) contains a number of interesting features for the study of the interstellar medium. In particular, the aromatic and aliphatic components in carbonaceous dust can be investigated most efficiently with the NIR spectroscopy. We analyze NIR spectra of the diffuse Galactic emission taken with the Infrared Camera onboard AKARI and find that the aliphatic to aromatic emission band ratio decreases toward the ionized gas, which suggests processing of the band carriers in the ionized region.
We use the NASA Ames Polycyclic Aromatic Hydrocarbon (PAH) infrared spectroscopic
database to model infared emission of PAHs following absorption of a UV photon. We
calculate emission spectra resulting from the full cooling cascade for each species in the
database. Using a least squares approach, we can find out what PAH mixtures best reproduce
a few typical astronomical observations representing the different classes of UIR spectra.
We find that we can reproduce the observed UIR spectra in the wavelength range 6–14
μm, offering support for the hypothesis that the UIR bands are indeed
due to vibrational modes of PAHs and related molecular species. Spectral decompositions of
our best fit models confirm and reinforce several earlier results: (i)
the 6.2 μm band requires a significant contribution of
nitrogen-substituted PAHs (PANHs); (ii) the reported components and their
variations in the 7.7 μm band are indicative of changes in the size
distribution of the contributing molecules; (iii) there is a significant
contribution of anions to the 7.7 μm band; (iv) the 11.2
μm band is due to large, neutral and pure PAHs; (v)
the 11.0 μm band is due to large PAH cations.
The diffuse interstellar bands (DIBs) have been known of since 1922, but their carrier
molecules remain unidentified to this day. We present a brief history of DIB observations,
followed by a list of constraints any suggested origin must face, and finally a preview of
current research for ultraviolet DIBs using the Cosmic Origins Spectrograph
on the Hubble Space Telescope. We conclude that PAHs are
consistent with all the listed constraints, but that PAHs may not be the only molecular
species responsible for the DIBs.
With advancement of infrared space telescopes during the past decade, infrared wavelength
regime has been a focal point to study various properties of galaxies with respect to
evolution of galaxies. Polycyclic Aromatic Hydrocarbons (PAHs) have emerged as one of the
most important features since these features dominate the mid-infrared spectra of
galaxies. These PAH features provide a great handle to calibrate star formation rates and
diagnose ionized states of grains. However, the PAH 3.3 μm feature has
not been studied as much as other PAH features since it is weaker than others and resides
outside of Spitzer capability, although it will be the only PAH feature accessible by JWST
for high-z galaxies. AKARI mJy Unbiased Survey of Extragalactic Sources in 5MUSES (AMUSES)
intends to take advantage of AKARI capability of spectroscopy in the
2 ~ 5 μm to provide an unbiased library of 44 sample
galaxies selected from a parent sample of 5MUSES, one of Spitzer legacy projects. For
these 3.6 μm flux limited sample galaxies whose redshifts range between
0 < z < 1, AMUSES will calibrate PAH 3.3
μm as a star formation rate (SFR) indicator while measuring ratios
between PAH features. We present preliminary results of AMUSES.
Within the Herschel key project “The Warm And Dense ISM” (WADI) we systematically observe
a number of prominent photon-dominated regions (PDRs) to measure the impact of varying UV
fields on the energy balance, the chemical and dynamical structure of heated molecular
We have investigated the dissociative recombination (DR) of the
ions using the CRYRING heavy-ion storage ring at Stockholm University, Sweden. The
dissociative recombination branching ratios were determined at minimal collision energy,
showing that the DR of both ions was dominated by pathways keeping the carbon atoms
together in one product. The absolute reaction cross sections for the titular ions are
best fitted by σ(Ecm [eV]) = 1.3 ± 0.4 × 10-15
(Ecm [eV])−1.19 ± 0.02 cm2
and σ(Ecm [eV]) = 1.1 ± 0.3 ×
10-15(Ecm [eV])−1.33 ± 0.02 cm2
in the intervals 3-300 meV and 3-200 meV respectively. The thermal rate constants of the
titular reactions are best described by:
k(T) = 1.3 ± 0.4 × 10-6(T/300)−0.69 ± 0.02
cm3s-1 for C6D6+
and k(T) = 2.0 ± 0.6 × 10-6 (T/300)−0.83 ± 0.02
cm3s-1 for C6D7+.
These expressions correlates well with earlier flowing afterglow studies on the same
Near-infrared (NIR; 2.5–5 μm) low-resolution
(λ/Δλ ~ 100) spectra were obtained for a
number of Galactic and extragalactic objects with the Infrared Camera (IRC) in the AKARI
warm mission. These data provide us with the first opportunity to make a systematic study
of the 3.3–3.5 μm PAH features in a galactic scale as well as within an
object. Whereas the 3.3 μm band is well resolved in most spectra, the
3.5 μm band is not clearly separated from the 3.4 μm
band in the IRC spectrum. The intensity ratio of the summation of the 3.4 and
3.5 μm bands to the 3.3 μm band shows a tendency to
increase towards the Galactic center, although a large variation in the ratio is also seen
in a local scale. A search for deuterated PAH features in the 4 μm region
is carried out in IRC NIR spectra. Emission lines originating from the ionized gas
together with the detector anomaly hamper an accurate search at certain wavelengths, but
little convincing evidence has so far been obtained for the presence of significant
features in 4.2–4.7 μm. A conservative upper limit of a few percents is
obtained for the integrated intensity ratio of the 4.4–4.7 μm possible
features to the 3.3–3.5 μm PAH features in the spectra so far
This symposium has shown that the field of astrophysical PAHs remains extremely active
and lively. Thanks to Spitzer Space Telescope, the number of PAH papers
has spectacularly increased, including now up to the young Universe. Laboratory and
theoretical works have progressed in proportion. Salient features of the six sessions of
the symposium are briefly reviewed. Comprehensive analyses of the rich and complex
infrared spectra of interstellar PAHs are now well established, based on a large database
of observational data. PAHs are fully confirmed as excellent tracers of star formation,
but their emission strongly depends on metallicity. Various observations, especially in
harsh environments, have confirmed the complexity of the lifecycle of PAHs in space, and
the need for multiple formation modes. Electronic properties remain a major issue for
astronomical PAHs, including their possible connection with the diffuse interstellar
bands, and the possible importance of protonated PAHs. Progress in studying complex
carbonaceous compounds, such as those of various soots, and in synthesizing very large
PAHs may give important clues for understanding interstellar PAHs. Significant progress
was also reported in modeling the important role of PAHs in the physics and chemistry of
the interstellar medium.
In the interstellar medium (ISM), PAHs are abundant and also carry most of the dust
surface. They are thus privileged sites for surface reactions such as the formation of
H2. In regions penetrated by UV photons, PAHs loose electrons by the
photoelectric effect and efficiently heat the gas. In more shielded regions, PAH recombine
and may carry an important fraction of the cloud electronic charge which plays an
important role in the gas dynamics and chemistry. We review here processes involving PAHs
which control key aspects of the physics of the ISM. We also discuss the corresponding
observational constraints. Most of these processes involve a detailed knowledge of the
charge of PAHs and we therefore review current models in this area. We argue that more
laboratory measurements of the rate of electronic capture on large PAH cations are
The possible role of neutral PAHs as catalysts for H2 formation in the
interstellar medium is investigated by a combined experimental and density function theory
study of the superhydrogenation of coronene (C24H12). The
calculations suggest efficient hydrogenation of both edge and centre sites, along with
competing abstraction reactions to form H2 in a series of catalytic cycles.
Scanning tunneling microscopy and thermal desorption measurements have been used to
provide direct evidence of the formation of superhydrogenated coronene as a result of
exposure to D atoms. Lower limit estimates for the cross-sections of
1.8 × 10-17, 5.5 × 10-18 and 1.1 × 10-18 cm2
for the formation of singly, doubly and triply hydrogenated coronene are derived. The
results suggest that superhydrogenated PAHs may play an important role in H2
formation in the ISM.
Aromatic carbon, in some form, has been an essential ingredient by and large in all
models of the extinction curve, since the original proposal to attribute the bump at 217.5
nm to “astronomical graphite”. This aromatic carbon is most naturally identified, in up to
date models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/or
clustered. In all models, this PAH population accounts for the far–UV nonlinear rise in
the extinction curve, contributes to the bump and possibly part of the large set of
unidentified, discrete absorption features in the visible (the Diffuse Interstellar
Bands). We review the current state of our understanding of the contribution of PAHs to
interstellar extinction, and what constraints can be imposed on the PAH population by
fitting extinction models to observations.
Polycyclic aromatic hydrocarbons are a class of molecules of broad interest that has long
been explored by various spectroscopic techniques. The electronic spectroscopy of these
species is of particular interest since it provides a framework for the understanding of
the electronic structure of large polyatomic molecules. Such studies also allow the
systematic investigation of electronic relaxation mechanisms in large molecules. In this
review, we focus on the gas-phase experimental work on such systems and present the latest
progress. We also underline the challenges that remain to be tackled. A focus on the
understanding of the electronic relaxation pathways at work in gas-phase PAHs will also be
presented, as well as their possible manifestation in space.
Near infrared observations of reflection nebulae have set the historical ground for the
discovery of interstellar PAHs, but since, space observations have focused on their mid-IR
features, and data shortward of 5 μm have remained scarce. The
Spitzer/IRAC images in the 3.6 and 4.5 μm channels do show that the
near-IR emission from small dust particles is ubiquitous across the Galaxy, but provide no
spectroscopic information. To investigate the nature of this near-IR dust emission, we
have obtained AKARI spectroscopic observations, over the 2.5−5 μm
spectral range, for a set of archetype PDRs mapped with the Spitzer spectrometer at mid-IR
wavelengths. These AKARI data supplement earlier observations with the SWS ISO
spectrometer, in providing the gain in sensitivity needed to observe low excitation
sources, and the spatial information required to spatially correlate near-IR spectroscopic
signatures with physical conditions and observed changes in mid-IR spectra. This paper
presents the first results of the data analysis, in relation to two open questions on
interstellar PAHs. (1) Is there an evolutionary link from aliphatic carbon dust to PAHs?
(2) What is the origin of the near-IR dust continuum? The AKARI spectra display features
longward of the main 3.29 μm PAH feature, and continuum emission. The
intensity ratio between the features ascribed to aliphatic CH bonds and the
3.29 μm aromatic band, varies spatially in a way that may be
interpreted as evidence for aromatization of the smallest dust particles by
photo-processing. The continuum displays a striking step-increase across the
3.29 μm feature. We also present a spectrum of a photodissociation
region with a feature at 4.65 μm, which has been speculated to be
related to the CD stretch in aliphatic hydrocarbon side-groups on PAHs.
This review describes five categories of the reactions of polycyclic aromatic
hydrocarbons, including photochemistry, electron attachment/detachment, recombination
processes, radical reactions, and ion-neutral chemistry. For each class of reaction, an
overview of the studies and their general results are presented, as well as references to
the literature. The thermochemistry of PAHs and relevant species is described, including
bond dissociation energies, ionization energies, electron affinities, basicities,
acidities, and the interrelationships of these quantities. Modeling of the chemistry of
PAHs and their ions is discussed for both diffuse and dark clouds. The role of PAH cations
in the catalytic formation of molecular hydrogen is considered. Finally, this review
concludes with a discussion of current challenges in the chemical characterization of
PAHs, and a perspective for future studies.
The mechanisms of nucleation and growth of carbon dust particles in circumstellar
envelopes of carbon-rich stars in the red giant and AGB phases of their evolution are
poorly understood. It has been proposed that the transition of gas phase species to solid
particles, is achieved by the formation of a critical nucleus composed of two PAHs held
together by van der Waals forces. Some insights into the validity of the nucleation of PAH
molecules in the envelope can be gained through the investigation of the thermodynamics of
dimers, representing the first stage towards condensation. We have performed experiments
to identify the temperature range over which small PAH clusters form in saturated uniform
supersonic flows. The kinetics of the formation has also been investigated. The
experimental data have been combined with theoretical calculations. We unambiguously
demonstrate that the association of small PAHs such as pyrene
(C16H10) is slower than the destruction of the dimer in warm and
hot environments such as IRC +10216. Our findings challenge a formation model based on the
physical stacking of small PAH units in circumstellar shells of carbon rich stars.