1,747 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
A bimodal dust grain distribution in the IC 434 HII region
Recent studies of dust in the interstellar medium have challenged the
capabilities and validity of current dust models, indicating that the
properties of dust evolve as it transits between different phases of the
interstellar medium. We conduct a multi-wavelength study of the dust emission
from the ionized gas of the IC 434 emission nebula, and combine this with
modeling, from large scales that provide insight into the history of the IC
434/L1630 region, to small scales that allow us to infer quantitative
properties of the dust content inside the H II region. The dust enters the H II
region through momentum transfer with a champagne flow of ionized gas, set up
by a chance encounter between the L1630 molecular cloud and the star cluster of
Ori. We observe two clearly separated dust populations inside the
ionized gas, that show different observational properties, as well as
contrasting optical properties. Population A is colder ( 25 K) than
predicted by widely-used dust models, its temperature is insensitive to an
increase of the impinging radiation field, is momentum-coupled to the gas, and
efficiently absorbs radiation pressure to form a dust wave at 1.0 pc ahead of
Ori AB. Population B is characterized by a constant [20/30] flux ratio
throughout the HII region, heats up to 75 K close to the star, and is
less efficient in absorbing radiation pressure, forming a dust wave at 0.1 pc
from the star. We conclude that the dust inside IC 434 is bimodal. The
characteristics of population A are remarkable and can not be explained by
current dust models. Population B are grains that match the classical
description of spherical, compact dust. Our results confirm recent work that
stress the importance of variations in the dust properties between different
regions of the interstellar medium.Comment: 18 pages, 10 figures, proposed for acceptance in A&
High-Resolution 4.7 Micron Keck/NIRSPEC Spectra of Protostars. II. Detection of the ^(13)CO Isotope in Icy Grain Mantles
The high-resolution (R = 25,000) infrared M-band spectrum of the massive protostar NGC 7538 IRS 9 shows a narrow absorption feature at 4.779 μm (2092.3 cm^(-1)) that we attribute to the vibrational stretching mode of the ^(13)CO isotope in pure CO icy grain mantles. This is the first detection of ^(13)CO in icy grain mantles in the interstellar medium. The ^(13)CO band is a factor of 2.3 narrower than the apolar component of the ^(12)CO band. With this in mind, we discuss the mechanisms that broaden solid-state absorption bands. It is shown that ellipsoidally shaped pure CO grains fit the bands of both isotopes at the same time. Slightly worse but still reasonable fits are also obtained by CO embedded in N_2-rich ices and thermally processed O_2-rich ices. In addition, we report new insights into the nature and evolution of interstellar CO ices by comparing the very high resolution multicomponent solid ^(12)CO spectrum of NGC 7538 IRS 9 with that of the previously studied low-mass source L1489 IRS. The narrow absorption of apolar CO ices is present in both spectra but much stronger in NGC 7538 IRS 9. It is superposed on a smooth broad absorption feature well fitted by a combination of CO_2 and H_2O-rich laboratory CO ices. The abundances of the latter two ices, scaled to the total H_2O ice column, are the same in both sources. We thus suggest that thermal processing manifests itself as evaporation of apolar ices only and not the formation of CO_2 or polar ices. Finally, the decomposition of the ^(12)CO band is used to derive the ^(12)CO/^(13)CO abundance ratio in apolar ices. A ratio of ^(12)CO/^(13)CO = 71 ± 15 (3 σ) is deduced, in good agreement with gas-phase CO studies (~77) and the solid ^(12)CO_2/^(13)CO_2 ratio of 80 ± 11 found in the same line of sight. The implications for the chemical path along which CO_2 is formed are discussed
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
Formation of hydrogen peroxide and water from the reaction of cold hydrogen atoms with solid oxygen at 10K
The reactions of cold H atoms with solid O2 molecules were investigated at 10
K. The formation of H2O2 and H2O has been confirmed by in-situ infrared
spectroscopy. We found that the reaction proceeds very efficiently and obtained
the effective reaction rates. This is the first clear experimental evidence of
the formation of water molecules under conditions mimicking those found in cold
interstellar molecular clouds. Based on the experimental results, we discuss
the reaction mechanism and astrophysical implications.Comment: 12 pages, 3 Postscript figures, use package amsmath, amssymb,
graphic
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
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&
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