804 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.
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&
Laboratory studies of polycyclic aromatic hydrocarbons: the search for interstellar candidates
Polycyclic Aromatic Hydrocarbons (PAHs) are considered as a major constituent
of interstellar dust. They have been proposed as the carriers of the Aromatic
Infrared Bands (AIBs) observed in emission in the mid-IR. They likely have a
significant contribution to various features of the extinction curve such as
the 220 nm bump,the far-UV rise and the diffuse interstellar bands. Emission
bands are also expected in the far-IR, which are better fingerprints of
molecular identity than the AIBs. They will be searched for with the Herschel
Space Observatory. Rotational emission is also expected in the mm range for
those molecules which carry significant dipole moments. Despite spectroscopic
studies in the laboratory, no individual PAH species could be identified. This
emphasises the need for an investigation on where interstellar PAHs come from
and how they evolve due to environmental conditions: ionisation and
dissociation upon UV irradiation, interactions with electrons, gas and dust.
There is also evidence for PAH species to contribute to the depletion of heavy
atoms from the gas phase, in particular Si and Fe. This paper illustrates how
laboratory work can be inspired from observations. In particular there is a
need for understanding the chemical properties of PAHs and PAH-related species,
including very small grains, in physical conditions that mimic those found in
interstellar space. This motivates a joint effort between astrophysicists,
physicists and chemists. Such interdisciplinary studies are currently
performed, taking advantage of the PIRENEA set-up, a cold ion trap dedicated to
astrochemistry.Comment: to appear in "Cosmic Dust - Near and Far", Th. Henning, E. Grun, J.
Steinacker (eds.
Formation of buckminsterfullerene (C60) in interstellar space
Buckminsterfullerene (C60) was recently confirmed to be the largest molecule
identified in space. However, it remains unclear how, and where this molecule
is formed. It is generally believed that C60 is formed from the build up of
small carbonaceous compounds, in the hot and dense envelopes of evolved stars.
Analyzing infrared observations, obtained by Spitzer and Herschel, we found
that C60 is efficiently formed in the tenuous and cold environment of an
interstellar cloud illuminated by strong ultraviolet (UV) radiation fields.
This implies that another formation pathway, efficient at low densities, must
exist. Based on recent laboratory and theoretical studies, we argue that
Polycyclic Aromatic Hydrocarbons are converted into graphene, and subsequently
C60, under UV irradiation from massive stars. This shows that alternative -
top-down - routes are key to understanding the organic inventory in space.Comment: 22 pages, 4 figures, 3 sup. figures, 1 sup. vide
Time Resolved Correlation measurements of temporally heterogeneous dynamics
Time Resolved Correlation (TRC) is a recently introduced light scattering
technique that allows to detect and quantify dynamic heterogeneities. The
technique is based on the analysis of the temporal evolution of the speckle
pattern generated by the light scattered by a sample, which is quantified by
, the degree of correlation between speckle images recorded at
time and . Heterogeneous dynamics results in significant
fluctuations of with time . We describe how to optimize TRC
measurements and how to detect and avoid possible artifacts. The statistical
properties of the fluctuations of are analyzed by studying their
variance, probability distribution function, and time autocorrelation function.
We show that these quantities are affected by a noise contribution due to the
finite number of detected speckles. We propose and demonstrate a method to
correct for the noise contribution, based on a extrapolation
scheme. Examples from both homogeneous and heterogeneous dynamics are provided.
Connections with recent numerical and analytical works on heterogeneous glassy
dynamics are briefly discussed.Comment: 19 pages, 15 figures. Submitted to PR
On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica
In this study, 8Â years of high-resolution radiosonde data at nine Antarctic
stations are analysed to provide the first large-scale characterization of
the fine vertical structure of the low troposphere up to 3 km altitude over
the coastal margins of East Antarctica. Radiosonde data show a large spatial
variability of wind, temperature and humidity profiles, with different
features between stations in katabatic regions (e.g., Dumont d'Urville and
Mawson stations), stations over two ice shelves (Neumayer and Halley
stations) and regions with complex orography (e.g., McMurdo). At the Dumont
d'Urville, Mawson and Davis stations, the yearly median wind speed profiles
exhibit a clear low-level katabatic jet. During precipitation events, the
low-level flow generally remains of continental origin and its speed is even
reinforced due to the increase in the continentâocean pressure gradient.
Meanwhile, the relative humidity profiles show a dry low troposphere,
suggesting the occurrence of low-level sublimation of precipitation in
katabatic regions but such a phenomenon does not appreciably occur over the
ice shelves near Halley and Neumayer. Although ERA-Interim and ERA5
reanalyses assimilate radiosoundings at most stations considered here,
substantial â and sometimes large â low-level wind and humidity biases are
revealed but ERA5 shows overall better performance. A free simulation with
the regional polar version of the Weather Research and Forecasting model
(Polar WRF) (at a 35 km resolution) over the entire continent shows
too-strong and too-shallow near-surface jets in katabatic regions especially
in winter. This may be a consequence of an underestimated coastal cold air
bump and associated seaâcontinent pressure gradient force due to the coarse
35 km resolution of the Polar WRF simulation. Beyond documenting the
vertical structure of the low troposphere over coastal East Antarctica, this
study gives insights into the reliability and accuracy of two major
reanalysis products in this region on the Earth. The paper further underlines
the difficulty of modeling the low-level flow over the margins of the ice
sheet with a state-of-the-art atmospheric model.</p
Polycyclic aromatic hydrocarbons with SPICA
Thanks to high sensitivity and angular resolution and broad spectral
coverage, SPICA will offer a unique opportunity to better characterize the
nature of polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs),
to better use them as probes of astrophysical environments. The angular
resolution will enable to probe the chemical frontiers in the evolution process
from VSGs to neutral PAHs, to ionized PAHs and to "Grand-PAHs" in
photodissotiation regions and HII regions, as a function of G/n (UV
radiation field / density). High sensitivity will favor the detection of the
far-IR skeletal emission bands of PAHs, which provide specific fingerprints and
could lead to the identification of individual PAHs. This overall
characterization will allow to use PAH and VSG populations as tracers of
physical conditions in spatially resolved protoplanetary disks and nearby
galaxies (using mid-IR instruments), and in high redshift galaxies (using the
far-IR instrument), thanks to the broad spectral coverage SPICA provides. Based
on our previous experience with ISO and Spitzer we discuss how these goals can
be reached.Comment: To appear in Proc. Workshop "The Space Infrared Telescope for
Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies".
Eds. A.M. Heras, B. Swinyard, K. Isaak, and J.R. Goicoeche
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