360 research outputs found
Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions
Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic
Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains
(VSGs) by the impinging UV radiation field from a nearby star. Aims. We
investigate quantitatively the transition zone between evaporating Very Small
Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative
contribution of PAHs and eVSGs to the mid-IR emission in a wide range of
excitation conditions. We fit the observed mid-IR emission of PDRs by using a
set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting
tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the
community as an IDL routine. From the results of the fit, we derive the
fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of
the local UV radiation field. Results. We show a clear decrease of f_eVSG with
increasing intensity of the local UV radiation field, which supports the
scenario of photo-destruction of eVSGs. Conversely, this dependence can be used
to quantify the intensity of the UV radiation field for different PDRs,
including non resolved ones. Conclusions. PAHTAT can be used to trace the
intensity of the local UV radiation field in regions where eVSGs evaporate,
which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated
PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.Comment: 13 pages, 11 figures. Accepted for publication in A&A. Typos
correcte
Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 m ratio
In photon-dominated regions (PDRs), UV photons from nearby stars lead to the
evaporation of very small grains (VSGs) and the production of gas-phase
polycyclic aromatic hydrocarbons (PAHs). Our goal is to achieve better insight
into the composition and evolution of evaporating very small grains (eVSGs) and
PAHs through analyzing the infrared (IR) aliphatic and aromatic emission bands.
We combined spectro-imagery in the near- and mid-IR to study the spatial
evolution of the emission bands in the prototypical PDR NGC 7023. We used
near-IR spectra obtained with AKARI to trace the evolution of the 3.3m and
3.4m bands, which are associated with aromatic and aliphatic C-H bonds on
PAHs. The spectral fitting involves an additional broad feature centred at
3.45m. Mid-IR observations obtained with Spitzer are used to discriminate
the signatures of eVSGs, neutral and cationic PAHs. We correlated the spatial
evolution of all these bands with the intensity of the UV field to explore the
processing of their carriers. The intensity of the 3.45m plateau shows an
excellent correlation with that of the 3.3m aromatic band (correlation
coefficient R = 0.95), indicating that the plateau is dominated by the emission
from aromatic bonds. The ratio of the 3.4m and 3.3m band intensity
() decreases by a factor of 4 at the PDR interface from the
more UV-shielded to the more exposed layers. The transition region between the
aliphatic and aromatic material is found to correspond spatially with the
transition zone between neutral PAHs and eVSGs. We conclude that the
photo-processing of eVSGs leads to the production of PAHs with attached
aliphatic sidegroups that are revealed by the 3.4m emission band. Our
analysis provides evidence for the presence of very small grains of mixed
aromatic and aliphatic composition in PDRs.Comment: Accepted for publication in A&A. Abstract abridged, language editing
applied in v
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
Detection of the buckminsterfullerene cation (C60+) in space
In the early 90s, C60+ was proposed as the carrier of two diffuse
interstellar bands (DIBs) at 957.7 and 963.2 nm, but a firm identification
still awaits gas-phase spectroscopic data. Neutral C60, on the other hand, was
recently detected through its infrared emission bands in the interstellar
medium and evolved stars. In this contribution, we present the detection of
C60+ through its infrared vibrational bands in the NGC 7023 nebula, based on
spectroscopic observations with the Spitzer space telescope, quantum chemistry
calculation, and laboratory data from the literature. This detection supports
the idea that C60+ could be a DIB carrier, and provides robust evidence that
fullerenes exist in the gas-phase in the interstellar medium. Modeling efforts
to design specific observations, combined with new gas-phase data, will be
essential to confirm this proposal. A definitive attribution of the 957.7 and
963.2 nm DIBs to C60+ would represent a significant step forward in the field.Comment: To appear in "Proceedings of IAU 297 symposium on the Diffuse
Interstellar Bands", eds. J. Cami and N. Cox (5 pages
Variations in solar wind fractionation as seen by ACE/SWICS over a solar cycle and the implications for Genesis Mission results
We use ACE/SWICS elemental composition data to compare the variations in
solar wind fractionation as measured by SWICS during the last solar maximum
(1999-2001), the solar minimum (2006-2009) and the period in which the Genesis
spacecraft was collecting solar wind (late 2001 - early 2004). We differentiate
our analysis in terms of solar wind regimes (i.e. originating from interstream
or coronal hole flows, or coronal mass ejecta). Abundances are normalized to
the low-FIP ion magnesium to uncover correlations that are not apparent when
normalizing to high-FIP ions. We find that relative to magnesium, the other
low-FIP elements are measurably fractionated, but the degree of fractionation
does not vary significantly over the solar cycle. For the high-FIP ions,
variation in fractionation over the solar cycle is significant: greatest for
Ne/Mg and C/Mg, less so for O/Mg, and the least for He/Mg. When abundance
ratios are examined as a function of solar wind speed, we find a strong
correlation, with the remarkable observation that the degree of fractionation
follows a mass-dependent trend. We discuss the implications for correcting the
Genesis sample return results to photospheric abundances.Comment: Accepted for publication in Ap
Kinematics of the ionized-to-neutral interfaces in Monoceros R2
Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only
ultra-compact H ii region (UC H ii) where its associated photon-dominated
region (PDR) can be resolved with the Herschel Space Observatory. Aims. Our aim
is to investigate observationally the kinematical patterns in the interface
regions (i.e., the transition from atomic to molecular gas) associated with Mon
R2. Methods. We used the HIFI instrument onboard Herschel to observe the line
profiles of the reactive ions CH+, OH+ and H2O+ toward different positions in
Mon R2. We derive the column density of these molecules and compare them with
gas-phase chemistry models. Results. The reactive ion CH+ is detected both in
emission (at central and red-shifted velocities) and in absorption (at
blue-shifted velocities). OH+ is detected in absorption at both blue- and
red-shifted velocities, with similar column densities. H2O+ is not detected at
any of the positions, down to a rms of 40 mK toward the molecular peak. At this
position, we find that the OH+ absorption originates in a mainly atomic medium,
and therefore is associated with the most exposed layers of the PDR. These
results are consistent with the predictions from photo-chemical models. The
line profiles are consistent with the atomic gas being entrained in the ionized
gas flow along the walls of the cavity of the H ii region. Based on this
evidence, we are able to propose a new geometrical model for this region.
Conclusions. The kinematical patterns of the OH+ and CH+ absorption indicate
the existence of a layer of mainly atomic gas for which we have derived, for
the first time, some physical parameters and its dynamics.Comment: 6 pages, 5 figures. Accepted for publication in A&
The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
We study the spatial distribution and chemistry of small hydrocarbons in the
Orion Bar PDR. We used the IRAM-30m telescope to carry out a millimetre line
survey towards the Orion Bar edge, complemented with ~2'x2' maps of the C2H and
c-C3H2 emission. We analyse the excitation of the detected hydrocarbons and
constrain the physical conditions of the emitting regions with non-LTE
radiative transfer models. We compare the inferred column densities with
updated gas-phase photochemical models including 13CCH and C13CH isotopomer
fractionation. ~40% of the lines in the survey arise from hydrocarbons (C2H,
C4H, c-C3H2, c-C3H, C13CH, 13CCH, l-C3H and l-H2C3). We detect new lines from
l-C3H+ and improve its rotational spectroscopic constants. Anions or deuterated
hydrocarbons are not detected: [C2D]/[C2H]<0.2%, [C2H-]/[C2H]<0.007% and
[C4H-]/[C4H]<0.05%. Our gas-phase models can reasonably match the observed
column densities of most hydrocarbons (within factors <3). Since the observed
spatial distribution of the C2H and c-C3H2 emission is similar but does not
follow the PAH emission, we conclude that, in high UV-flux PDRs,
photodestruction of PAHs is not a necessary requirement to explain the observed
abundances of the smallest hydrocarbons. Instead, gas-phase endothermic
reactions (or with barriers) between C+, radicals and H2 enhance the formation
of simple hydrocarbons. Observations and models suggest that the [C2H]/[c-C3H2]
ratio (~32 at the PDR edge) decreases with the UV field attenuation. The
observed low cyclic-to-linear C3H column density ratio (<3) is consistent with
a high electron abundance (Xe) PDR environment. In fact, the poorly constrained
Xe gradient influences much of the hydrocarbon chemistry in the more
UV-shielded gas. We propose that reactions of C2H isotopologues with 13C+ and H
atoms can explain the observed [C13CH]/[13CCH]=1.4(0.1) fractionation level.Comment: 30 pages, 23 figures, 15 tables. Accepted for publication in A&A
(English edited, abstract abridged
Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs
Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and
molecular hydrogen emission are a potentially powerful tool to derive physical
properties of dense environments irradiated by intense UV fields. We present
new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high
UV-field and dense photodissocation region (PDR) around Monoceros R2, the
closest ultracompact \hII region, revealing the spatial structure of ionized
gas, PAHs and H emissions. Using a PDR model and PAH emission feature
fitting algorithm, we build a comprehensive picture of the physical conditions
prevailing in the region. We show that the combination of the measurement of
PAH ionization fraction and of the ratio between the H 0-0 S(3) and S(2)
line intensities, respectively at 9.7 and 12.3 m, allows to derive the
fundamental parameters driving the PDR: temperature, density and UV radiation
field when they fall in the ranges K, cm,
respectively. These mid-IR spectral tracers thus provide a tool
to probe the similar but unresolved UV-illuminated surface of protoplanetary
disks or the nuclei of starburst galaxies.Comment: Accepted for publication in ApJ Letter
Interstellar C60+
Buckminsterfullerene (C60) was recently detected through its infrared
emission bands in the interstellar medium (ISM), including in the proximity of
massive stars, where physical conditions could favor the formation of the
cationic form, C60+. In addition, C60+ was proposed as the carrier of two
diffuse interstellar bands in the near-IR, although a firm identification still
awaits for gas-phase spectroscopic data. We examined in details the Spitzer IRS
spectra of the NGC 7023 reflection nebula, at a position close (7.5") to the
illuminating B star HD 200775, and found four previously unreported bands at
6.4, 7.1, 8.2 and 10.5 \mu m in addition to the classical bands attributed to
Polycylic Aromatic Hydrocarbons (PAHs) and neutral C60. These 4 bands are
observed only in this region of the nebula, while C60 emission is still present
slightly further away from the star, and PAH emission even further away. Based
on this observation, on theoretical calculations we perform, and on laboratory
studies, we attribute these bands to C60+. The detection of C60+ confirms the
idea that large carbon molecules exist in the gas-phase in these environments.
In addition, the relative variation of the C60, and C60+, band intensities
constitutes a potentially powerful probe of the physical conditions in highly
UV-irradiated regions.Comment: Accepted for publication in A&A, v2: minor corrections of typos and
language and additional reference include
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
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