135 research outputs found
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
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
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&
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
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
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
PDRs4All: A JWST Early Release Science Program on Radiative Feedback from Massive Stars
Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the universe, from the era of vigorous star formation at redshifts of 1-3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter-and circumstellar media including diffuse clouds, proto-planetary disks, and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template data sets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template data sets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations.</p
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&
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
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&
- âŠ