103 research outputs found
Random mixtures of polycyclic aromatic hydrocarbon spectra match interstellar infrared emission
The mid-infrared (IR; 5-15~m) spectrum of a wide variety of astronomical
objects exhibits a set of broad emission features at 6.2, 7.7, 8.6, 11.3 and
12.7 m. About 30 years ago it was proposed that these signatures are due
to emission from a family of UV heated nanometer-sized carbonaceous molecules
known as polycyclic aromatic hydrocarbons (PAHs), causing them to be referred
to as aromatic IR bands (AIBs). Today, the acceptance of the PAH model is far
from settled, as the identification of a single PAH in space has not yet been
successful and physically relevant theoretical models involving ``true'' PAH
cross sections do not reproduce the AIBs in detail. In this paper, we use the
NASA Ames PAH IR Spectroscopic Database, which contains over 500
quantum-computed spectra, in conjunction with a simple emission model, to show
that the spectrum produced by any random mixture of at least 30 PAHs converges
to the same 'kernel'-spectrum. This kernel-spectrum captures the essence of the
PAH emission spectrum and is highly correlated with observations of AIBs,
strongly supporting PAHs as their source. Also, the fact that a large number of
molecules are required implies that spectroscopic signatures of the individual
PAHs contributing to the AIBs spanning the visible, near-infrared, and far
infrared spectral regions are weak, explaining why they have not yet been
detected. An improved effort, joining laboratory, theoretical, and
observational studies of the PAH emission process, will support the use of PAH
features as a probe of physical and chemical conditions in the nearby and
distant Universe
Contribution of polycyclic aromatic hydrocarbon ionization to neutral gas heating in galaxies: model versus observations
[Abridged] The ionization of polycyclic aromatic hydrocarbons (PAHs), by
ultraviolet (UV) photons from massive stars is expected to account for a large
fraction of the heating of neutral gas in galaxies. Evaluation of this
proposal, however, has been limited by our ability to directly compare
observational diagnostics to the results of a molecular model describing PAH
ionization. The objective of this article is to take advantage of the most
recent values of molecular parameters derived from laboratory experiments and
quantum chemical calculations on PAHs and provide a detailed comparison between
modeled values and observational diagnostics for the PAH charge state and the
heating efficiency for PAHs. Despite the use of a simple analytical model, we
obtain a good agreement between model results and observational diagnostics
over a wide range of radiation fields and physical conditions, in environments
such as star-forming regions, galaxies, and protoplanetary disks. In addition,
we found that the modeled photoelectric heating rates by PAHs are close to the
observed cooling rates given by the gas emission. These results show that PAH
ionization is the main source of neutral gas heating in these environments. The
results of our photoelectric heating model by PAHs can thus be used to assess
the contribution of UV radiative heating in galaxies (vs shocks, for instance).
We provide the empirical formulas fitted to the model results, and the full
python code itself, to calculate the heating rates and heating efficiencies for
PAHs.Comment: Accepted for publication in Astronomy and Astrophysic
Waves on the surface of the Orion molecular cloud
Massive stars influence their parental molecular cloud, and it has long been
suspected that the development of hydrodynamical instabilities can compress or
fragment the cloud. Identifying such instabilities has proved difficult. It has
been suggested that elongated structures (such as the `pillars of creation')
and other shapes arise because of instabilities, but alternative explanations
are available. One key signature of an instability is a wave-like structure in
the gas, which has hitherto not been seen. Here we report the presence of
`waves' at the surface of the Orion molecular cloud near where massive stars
are forming. The waves seem to be a Kelvin-Helmholtz instability that arises
during the expansion of the nebula as gas heated and ionized by massive stars
is blown over pre-existing molecular gas.Comment: Preprint of publication in Natur
Sand bodies at the shelf edge in the Gulf of Lions (Western Mediterranean): Deglacial history and modern processes
What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?
In this Paper we analyze the mid-infrared (mid-IR) emission of very small
dust particles in a sample of 12 protoplanetary disks to see how they are
connected to interstellar dust particles and to investigate the possibility
that their emission can be used as a probe of the physical conditions and
evolution of the disk. We define a basis made of three mid-IR template spectra
PAH, PAH and VSGs that were derived from the analysis of reflection
nebulae, and an additional PAH spectrum that was introduced by Joblin et
al. (2008) for the analysis of the spectra of planetary nebulae. From the
optimization of the fit of 12 star+disk spectra, using a linear combination of
the 4 template spectra, we found that an additional small grain component with
a broad feature at 8.3 m is needed. We find that the fraction of VSG
emission in disks decreases with increasing stellar temperature. VSGs appear to
be destroyed by UV photons at the surface of disks, thus releasing free PAH
molecules, which are eventually ionized as it is observed in photodissociation
regions. On the opposite, we observe that the fraction of PAH increases
with increasing star temperature except in the case of B stars where they are
absent. We argue that this is compatible with the identification of PAH as
large ionized PAHs, most likely emitting in regions of the disk that are close
to the star. Finally, we provide a UV-dependant scheme to explain the evolution
of PAHs and VSGs in protoplanetary disks. We show that A stars modify the size
spectrum of PAHs and VSGs in favor of large PAHs while B stars destroy even the
largest PAHs up to large radii in the disk. These results allow us to put new
constrains on the properties of two sources: IRS 48 and "Gomez's Hamburger"
which are poorly characterized.Comment: Accepted for publication in A&
Spatial distribution of small hydrocarbons in the neighborhood of the Ultra Compact HII region Monoceros R2
We study the chemistry of small hydrocarbons in the photon-dominated regions
(PDRs) associated with the ultra-compact HII region Mon R2. Our goal is to
determine the variations of the abundance of small hydrocarbons in a high-UV
irradiated PDR and investigate their chemistry. We present an observational
study of CH, CCH and c-CH in Mon R2 combining data obtained with the
IRAM 30m telescope and Herschel. We determine the column densities of these
species, and compare their spatial distributions with that of polycyclic
aromatic hydrocarbon (PAH). We compare the observational results with different
chemical models to explore the relative importance of gas-phase, grain-surface
and time-dependent chemistry in these environments. The emission of the small
hydrocarbons show different patterns. The CCH emission is extended while CH and
c-CH are concentrated towards the more illuminated layers of the PDR.
The ratio of the column densities of c-CH and CCH shows spatial
variations up to a factor of a few, increasing from
_3_2 in the envelope to a maximum of
towards the 8m emission peak. Comparing these results
with other galactic PDRs, we find that the abundance of CCH is quite constant
over a wide range of G, whereas the abundance of c-CH is higher in
low-UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account
relatively well for the abundances of CH and CCH in the most exposed layers of
the PDR, but falls short by a factor of 10 to reproduce c-CH.
In the molecular envelope, time-dependent effects and grain surface chemistry
play a dominant role in determining the hydrocarbons abundances. Our study
shows that CCH and c-CH present a complex chemistry in which UV
photons, grain-surface chemistry and time dependent effects contribute to
determine their abundances.Comment: 18 pages, 11 figures, 7 tables. Proposed for acceptance in A&A.
Abstract abridge
Herschel / HIFI observations of CO, H2O and NH3 in Mon R2
Context. Mon R2 is the only ultracompact HII region (UCHII) where the
associated photon-dominated region (PDR) can be resolved with Herschel. Due to
its brightness and proximity, it is the best source to investigate the
chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to
estimate the abundance of H2O and NH3 in this region and investigate their
origin. Methods. We present new observations obtained with HIFI and the
IRAM-30m telescope. Using a large velocity gradient approach, we model the line
intensities and derive an average abundance of H2O and NH3 across the region.
Finally, we model the line profiles with a non-local radiative transfer model
and compare these results with the abundance predicted by the Meudon PDR code.
Results. The variations of the line profiles and intensities indicate complex
geometrical and kinematical patterns. The H2O lines present a strong absorption
at the ambient velocity and emission in high velocity wings towards the HII
region. The spatial distribution of the o-H2^18O line shows that the its
emission arises in the PDR surrounding the HII region. By modeling the o-H2^18O
emission we derive a mean abundance of o-H2O of ~10^-8 relative to H2. The
ortho-H2O abundance is however larger, ~1x10^-7, in the high velocity wings.
Possible explanations for this larger abundance include an expanding hot PDR
and/or an outflow. Ammonia seems to be present only in the envelope with an
average abundance of ~2x10^-9 relative to H2. Conclusions. The Meudon PDR code
can account for the measured water abundance in the high velocity gas as long
as we assume that it originates from a <1 mag hot expanding layer of the PDR,
i.e. that the outflow has only a minor contribution to this emission. To
explain the abundances in the rest of the cloud the molecular freeze out and
grain surface chemistry would need to be included.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication in A&A.
Abstract shortened. Updated references, language editing applied in v
Multiple shells around G79.29+0.46 revealed from near-IR to millimeter data
Aiming to perform a study of the warm dust and gas in the luminous blue
variable star G79.29+0.46 and its associated nebula, we present infrared
Spitzer imaging and spectroscopy, and new CO J=2-->1 and 4-->3 maps obtained
with the IRAM 30m radio telescope and with the Submillimeter Telescope,
respectively. We have analyzed the nebula detecting multiple shells of dust and
gas connected to the star. Using Infrared Spectrograph-Spitzer spectra, we have
compared the properties of the central object, the nebula, and their
surroundings. These spectra show a rich variety of solid-state features
(amorphous silicates, polycyclic aromatic hydrocarbons, and CO2 ices) and
narrow emission lines, superimposed on a thermal continuum. We have also
analyzed the physical conditions of the nebula, which point to the existence of
a photo-dissociation region.Comment: Received by ApJ 2009 November 20, accepted for publication 2010
February 25, Published 2010 March 2
Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023
We investigate the physics and chemistry of the gas and dust in dense
photon-dominated regions (PDRs), along with their dependence on the
illuminating UV field. Using Herschel-HIFI observations, we study the gas
energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023
is the prototype of a PDR illuminated by a B2V star and is one of the key
targets of Herschel. Our approach consists in determining the energetics of the
region by combining the information carried by the mid-IR spectrum (extinction
by classical grains, emission from very small dust particles) with that of the
main gas coolant lines. In this letter, we discuss more specifically the
intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by
HIFI and provide information on the emitting gas. We show that both the [CII]
emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs)
arise from the regions located in the transition zone between atomic and
molecular gas. Using the Meudon PDR code and a simple transfer model, we find
good agreement between the calculated and observed [CII] intensities. HIFI
observations of NGC 7023 provide the opportunity to constrain the energetics at
the surface of PDRs. Future work will include analysis of the main coolant line
[OI] and use of a new PDR model that includes PAH-related species.Comment: Accepted for publication in Astronomy and Astrophysics Letters
(Herschel HIFI special issue), 5 pages, 5 figure
Coupled Blind Signal Separation and Spectroscopic Database Fitting of the Mid Infrared PAH Features
The aromatic infrared bands (AIBs) observed in the mid infrared spectrum are
attributed to Polycyclic Aromatic Hydrocarbons (PAHs). We observe the NGC
7023-North West (NW) PDR in the mid-infrared (10 - 19.5 micron) using the
Infrared Spectrometer (IRS), on board Spitzer. Clear variations are observed in
the spectra, most notably the ratio of the 11.0 to 11.2 micron bands, the peak
position of the 11.2 and 12.0 micron bands, and the degree of asymmetry of the
11.2 micron band. The observed variations appear to change as a function of
position within the PDR. We aim to explain these variations by a change in the
abundances of the emitting components of the PDR. A Blind Signal Separation
(BSS) method, i.e. a Non-Negative Matrix Factorization algorithm is applied to
separate the observed spectrum into components. Using the NASA Ames PAH IR
Spectroscopic Database, these extracted signals are fit. The observed signals
alone were also fit using the database and these components are compared to the
BSS components. Three component signals were extracted from the observation
using BSS. We attribute the three signals to ionized PAHs, neutral PAHs, and
Very Small Grains (VSGs). The fit of the BSS extracted spectra with the PAH
database further confirms the attribution to ionized and neutral PAHs and
provides confidence in both methods for producing reliable results. The 11.0
micron feature is attributed to PAH cations while the 11.2 micron band is
attributed to neutral PAHs. The VSG signal shows a characteristically
asymmetric broad feature at 11.3 micron with an extended red wing. By combining
the NASA Ames PAH IR Spectroscopic Database fit with the BSS method, the
independent results of each method can be confirmed and some limitations of
each method are overcome
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