1,149 research outputs found
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&
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
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
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
Physical conditions in photodissociation regions: Application to galactic nuclei
Infrared and sub-millimeter observations are used in a simple procedure to determine average physical properties of the neutral interstellar medium in Galactic photodissociation regions as well as in ensembles of clouds which exist in the nuclei of luminous infrared galaxies. The relevant observations include the Infrared Astronomy Satellite (IRAS) infrared continuum measurements, infrared spectroscopy of the fine-structure lines of SiII 35 microns, OI 63 microns, and CII 158 microns, and the 2.6 mm CO (J=1-0) rotational transition. The diagnostic capabilities of the OI 145 microns line is also addressed. Researchers attribute these emission lines as well as the continuum to the atomic/molecular photodissociation region on the surfaces of molecular clouds which are illuminated by strong ultraviolet fields. They use the theoretical photodissociation region models of Tielens and Hollenbach (1985, Ap. J., 291, 722) to construct simple diagrams which utilize line ratios and line to continuum ratios to determine the average gas density n, the average incident far-ultraviolet flux G sub o, and the temperature of the atomic gas T
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
- …