1,407 research outputs found
Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 {\mu}m band
In this Letter we report the results of density functional theory
calculations on medium-sized neutral Polycyclic Aromatic Hydrocarbon (PAH)
molecules with armchair edges. These PAH molecules possess strong C-H
stretching and bending modes around 3 {\mu}m and in the fingerprint region
(10-15 {\mu}m), and also strong ring deformation modes around 12.7 {\mu}m.
Perusal of the entries in the NASA Ames PAHs Database shows that ring
deformation modes of PAHs are common - although generally weak. We then propose
that armchair PAHs with NC >65 are responsible for the 12.7 {\mu}m Aromatic
Infrared Band in HII regions and discuss astrophysical implications in the
context of the PAH life-cycle.Comment: Minor editin
Mapping PAH sizes in NGC 7023 with SOFIA
NGC 7023 is a well-studied reflection nebula, which shows strong emission
from polycyclic aromatic hydrocarbon (PAH) molecules in the form of aromatic
infrared bands (AIBs). The spectral variations of the AIBs in this region are
connected to the chemical evolution of the PAH molecules which, in turn,
depends on the local physical conditions. We use the capabilities of SOFIA to
observe a 3.2' x 3.4' region of NGC 7023 at wavelengths that we observe with
high spatial resolution (2.7") at 3.3 and 11.2 um. We compare the SOFIA images
with existing images of the PAH emission at 8.0 um (Spitzer), emission from
evaporating very small grains (eVSG) extracted from Spitzer-IRS spectral cubes,
the ERE (HST and CFHT), and H_2 (2.12 um). We create maps of the 11.2/3.3 um
ratio to probe the morphology of the PAH size distribution and the 8.0/11.2 um
ratio to probe the PAH ionization. We make use of an emission model and of
vibrational spectra from the NASA Ames PAHdb to translate the 11.2/3.3 um ratio
to PAH sizes. The 11.2/3.3 um map shows the smallest PAH concentrate on the PDR
surface (H_2 and extended red emission) in the NW and South PDR. We estimated
that PAHs in the NW PDR bear, on average, a number of carbon atoms (N_c) of ~70
in the PDR cavity and ~50 at the PDR surface. In the entire nebula, the results
reveal a factor of 2 variation in the size of the PAH. We relate these size
variations to several models for the evolution of the PAH families when they
traverse from the molecular cloud to the PDR. The PAH size map enables us to
follow the photochemical evolution of PAHs in NGC 7023. Small PAHs result from
the photo-evaporation of VSGs as they reach the PDR surface. Inside the PDR
cavity, the PAH abundance drops as the smallest PAH are broken down. The
average PAH size increases in the cavity where only the largest species survive
or are converted into C_60 by photochemical processing.Comment: accepted for publication in A&
Nested shells reveal the rejuvenation of the Orion-Eridanus superbubble
The Orion-Eridanus superbubble is the prototypical superbubble due to its
proximity and evolutionary state. Here, we provide a synthesis of recent
observational data from WISE and Planck with archival data, allowing to draw a
new and more complete picture on the history and evolution of the
Orion-Eridanus region. We discuss the general morphological structures and
observational characteristics of the superbubble, and derive quantitative
properties of the gas- and dust inside Barnard's Loop. We reveal that Barnard's
Loop is a complete bubble structure which, together with the lambda Ori region
and other smaller-scale bubbles, expands within the Orion-Eridanus superbubble.
We argue that the Orion-Eridanus superbubble is larger and more complex than
previously thought, and that it can be viewed as a series of nested shells,
superimposed along the line of sight. During the lifetime of the superbubble,
HII region champagne flows and thermal evaporation of embedded clouds
continuously mass-load the superbubble interior, while winds or supernovae from
the Orion OB association rejuvenate the superbubble by sweeping up the material
from the interior cavities in an episodic fashion, possibly triggering the
formation of new stars that form shells of their own. The steady supply of
material into the superbubble cavity implies that dust processing from interior
supernova remnants is more efficient than previously thought. The cycle of
mass-loading, interior cleansing, and star formation repeats until the
molecular reservoir is depleted or the clouds have been disrupted. While the
nested shells come and go, the superbubble remains for tens of millions of
years.Comment: 20 pages, 6 figures, accepted for publication in Ap
Evaluation of the Multiplane Method for Efficient Simulations of Reaction Networks
Reaction networks in the bulk and on surfaces are widespread in physical,
chemical and biological systems. In macroscopic systems, which include large
populations of reactive species, stochastic fluctuations are negligible and the
reaction rates can be evaluated using rate equations. However, many physical
systems are partitioned into microscopic domains, where the number of molecules
in each domain is small and fluctuations are strong. Under these conditions,
the simulation of reaction networks requires stochastic methods such as direct
integration of the master equation. However, direct integration of the master
equation is infeasible for complex networks, because the number of equations
proliferates as the number of reactive species increases. Recently, the
multiplane method, which provides a dramatic reduction in the number of
equations, was introduced [A. Lipshtat and O. Biham, Phys. Rev. Lett. 93,
170601 (2004)]. The reduction is achieved by breaking the network into a set of
maximal fully connected sub-networks (maximal cliques). Lower-dimensional
master equations are constructed for the marginal probability distributions
associated with the cliques, with suitable couplings between them. In this
paper we test the multiplane method and examine its applicability. We show that
the method is accurate in the limit of small domains, where fluctuations are
strong. It thus provides an efficient framework for the stochastic simulation
of complex reaction networks with strong fluctuations, for which rate equations
fail and direct integration of the master equation is infeasible. The method
also applies in the case of large domains, where it converges to the rate
equation results
The Propagation and Survival of Interstellar Grains
In this paper we discuss the propagation of dust through the interstellar
medium (ISM), and describe the destructive effects of stellar winds, jets, and
supernova shock waves on interstellar dust. We review the probability that
grains formed in stellar outflows or supernovae survive processing in and
propagation through the ISM, and incorporate themselves relatively unprocessed
into meteoritic bodies in the solar system. We show that very large (radii >= 5
micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the
pre-solar SiC and diamond grains extracted from meteorites, can survive the
passage through 100\kms shock waves relatively unscathed. High velocity (>= 250
km/s) shocks destroy dust efficiently. However, a small (~10%) fraction of the
stardust never encountered such fast shocks before incorporation into the solar
system. All grains should therefore retain traces of their passage through
interstellar shocks during their propagation through the ISM. The grain
surfaces should show evidence of processing due to sputtering and pitting due
to small grain cratering collisions on the micron-sized grains. This conclusion
seems to be in conflict with the evidence from the large grains recovered from
meteorites which seem to show little interstellar processing.Comment: 19 pages, 5 figures (.eps), LaTeX, to appear in "Astrophysical
Implications of the Laboratory Study of Presolar Materials" AIP Conference
Proceedings, 1997 T.J. Bernatowicz and E. Zinner (eds.
Pore evolution in interstellar ice analogues: simulating the effects of temperature increase
Context. The level of porosity of interstellar ices - largely comprised of
amorphous solid water (ASW) - contains clues on the trapping capacity of other
volatile species and determines the surface accessibility that is needed for
solid state reactions to take place. Aims. Our goal is to simulate the growth
of amorphous water ice at low temperature (10 K) and to characterize the
evolution of the porosity (and the specific surface area) as a function of
temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are
used to mimic the formation and the thermal evolution of pores in amorphous
water ice. We follow the accretion of gas-phase water molecules as well as
their migration on surfaces with different grid sizes, both at the top growing
layer and within the bulk. Results. We show that the porosity characteristics
change substantially in water ice as the temperature increases. The total
surface of the pores decreases strongly while the total volume decreases only
slightly for higher temperatures. This will decrease the overall reaction
efficiency, but in parallel, small pores connect and merge, allowing trapped
molecules to meet and react within the pores network, providing a pathway to
increase the reaction efficiency. We introduce pore coalescence as a new solid
state process that may boost the solid state formation of new molecules in
space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&
Crystallinity versus mass-loss rate in Asymptotic Giant Branch stars
Infrared Space Observatory (ISO) observations have shown that O-rich
Asymptotic Giant Branch (AGB) stars exhibit crystalline silicate features in
their spectra only if their mass-loss rate is higher than a certain threshold
value. Usually, this is interpreted as evidence that crystalline silicates are
not present in the dust shells of low mass-loss rate objects. In this study,
radiative transfer calculations have been performed to search for an
alternative explanation to the lack of crystalline silicate features in the
spectrum of low mass-loss rate AGB stars. It is shown that due to a temperature
difference between amorphous and crystalline silicates it is possible to
include up to 40% of crystalline silicate material in the circumstellar dust
shell, without the spectra showing the characteristic spectral features. Since
this implies that low mass-loss rate AGB stars might also form crystalline
silicates and deposit them into the Interstellar Medium (ISM), the described
observational selection effect may put the process of dust formation around AGB
stars and the composition of the predominantly amorphous dust in the
Interstellar Medium in a different light. Our model calculations result in a
diagnostic tool to determine the crystallinity of an AGB star with a known
mass-loss rate.Comment: accepted by A&A, 10 pages, 11 figure
Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
We present the results of a systematic study of mid-IR spectra of Galactic
regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral,
starburst), observed by the satellites ISO and Spitzer. We study the relative
variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially
resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well
as among 90 integrated spectra of 50 objects. Our main results are that the
6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios
between these bands and the 11.3 micron band varies by one order of magnitude.
This implies that the properties of the PAHs are remarkably universal
throughout our sample, and that the relative variations of the band ratios are
mainly controled by the fraction of ionized PAHs. In particular, we show that
we can rule out both the modification of the PAH size distribution, and the
mid-infrared extinction, as an explanation of these variations. Using a few
well-studied Galactic regions (including the spectral image of the Orion Bar),
we give an empirical relation between the I(6.2)/I(11.3) ratio and the
ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful
quantitative diagnostic tool of the physical conditions in the regions where
the PAH emission originates. Finally, we discuss the physical interpretation of
the I(6.2)/I(11.3) ratio, on galactic size scales.Comment: Accepted by the ApJ, 67 pages, 70 figure
- …