74 research outputs found
Estimation of high-resolution dust column density maps: Empirical model fits
Sub-millimetre dust emission is an important tracer of density N of dense
interstellar clouds. One has to combine surface brightness information at
different spatial resolutions, and specific methods are needed to derive N at a
resolution higher than the lowest resolution of the observations. Some methods
have been discussed in the literature, including a method (in the following,
method B) that constructs the N estimate in stages, where the smallest spatial
scales being derived only use the shortest wavelength maps. We propose simple
model fitting as a flexible way to estimate high-resolution column density
maps. Our goal is to evaluate the accuracy of this procedure and to determine
whether it is a viable alternative for making these maps. The new method
consists of model maps of column density (or intensity at a reference
wavelength) and colour temperature. The model is fitted using Markov chain
Monte Carlo (MCMC) methods, comparing model predictions with observations at
their native resolution. We analyse simulated surface brightness maps and
compare its accuracy with method B and the results that would be obtained using
high-resolution observations without noise. The new method is able to produce
reliable column density estimates at a resolution significantly higher than the
lowest resolution of the input maps. Compared to method B, it is relatively
resilient against the effects of noise. The method is computationally more
demanding, but is feasible even in the analysis of large Herschel maps. The
proposed empirical modelling method E is demonstrated to be a good alternative
for calculating high-resolution column density maps, even with considerable
super-resolution. Both methods E and B include the potential for further
improvements, e.g., in the form of better a priori constraints.Comment: Accepted to A&
Top-down formation of fullerenes in the interstellar medium
[Abridged] Fullerenes have been recently detected in various circumstellar
and interstellar environments, raising the question of their formation pathway.
It has been proposed that they can form by the photo-chemical processing of
large polycyclic aromatic hydrocarbons (PAHs). Following our previous work on
the evolution of PAHs in the NGC 7023 reflection nebula, we evaluate, using
photochemical modeling, the possibility that the PAH CH (i.e.
circumovalene) can lead to the formation of C upon irradiation by
ultraviolet photons. The chemical pathway involves full dehydrogenation,
folding into a floppy closed cage and shrinking of the cage by loss of C
units until it reaches the symmetric C molecule. At 10" from the
illuminating star and with realistic molecular parameters, the model predicts
that 100% of CH is converted into C in 10
years, a timescale comparable to the age of the nebula. Shrinking appears to be
the kinetically limiting step of the whole process. Hence, PAHs larger than
CH are unlikely to contribute significantly to the formation of
C, while PAHs containing between 60 and 66 C atoms should contribute to
the formation of C with shorter timescales, and PAHs containing less
than 60 C atoms will be destroyed. Assuming a classical size distribution for
the PAH precursors, our model predicts absolute abundances of C are up
to several of the elemental carbon, i.e. less than a percent of the
typical interstellar PAH abundance, which is consistent with observational
studies. According to our model, once formed, C can survive much longer
than other fullerenes because of the remarkable stability of the C
molecule at high internal energies.Hence, a natural consequence is that
C is more abundant than other fullerenes in highly irradiated
environments.Comment: Accepted for publication in A&A. Latest version contains the
corrected version of Fig.
Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
Various studies have emphasized variations of the charge state and
composition of the interstellar polycyclic aromatic hydrocarbon (PAH)
population in photodissociation regions (PDRs). We aim to model the spatial
evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on
the specific case of the north-west (NW) PDR of NGC 7023 and also discuss the
case of the diffuse interstellar medium (ISM). The physical conditions in NGC
7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH
chemical evolution model that includes recent experimental data on PAHs and
describes multiphoton events. We consider a family of compact PAHs bearing up
to 96 carbon atoms. The calculated ionization ratio is in good agreement with
observations in NGC 7023 NW. Within the PDR, PAHs evolve into three major
populations: medium-sized PAHs (50<Nc<90) are normally hydrogenated, larger
PAHs (Nc>90) can be superhydrogenated, and smaller species (Nc<50) are fully
dehydrogenated. In the cavity, where the fullerene C60 was recently detected,
all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical
evolution exhibits a complex non-linear behaviour as a function of the UV
radiation field because of multiphoton events. Steady state for hydrogenation
is reached on timescales ranging from less than a year for small PAHs, up to
10000 years for large PAHs at Av=1. We identified critical reactions that need
more studies. Our new model allows us to rationalize the observational
constraints without any fitting parameter. PAHs smaller than 50 carbon atoms
are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is
obtained for the diffuse ISM. Carbon clusters turn out to be end products of
PAH photodissociation, and the evolution of these clusters needs to be
investigated further to evaluate their impact on the chemical and physical
evolution of PDRs.Comment: 16 pages, 10 figures; Accepted for publication in A&
The degeneracy between dust colour temperature and spectral index. Comparison of methods for estimating the beta(T) relation
Sub-millimetre dust emission provides information on the physics of
interstellar clouds and dust. Noise can produce anticorrelation between the
colour temperature T_C and the spectral index beta. This must be separated from
the intrinsic beta(T) relation of dust. We compare methods for the analysis of
the beta(T) relation. We examine sub-millimetre observations simulated as
simple modified black body emission or using 3D radiative transfer modelling.
In addition to chi^2 fitting, we examine the results of the SIMEX method, basic
Bayesian model, hierarchical models, and one method that explicitly assumes a
functional form for beta(T). All methods exhibit some bias. Bayesian method
shows significantly lower bias than direct chi^2 fits. The same is true for
hierarchical models that also result in a smaller scatter in the temperature
and spectral index values. However, significant bias was observed in cases with
high noise levels. Beta and T estimates of the hierarchical model are biased
towards the relation determined by the data with the highest S/N ratio. This
can alter the recovered beta(T) function. With the method where we explicitly
assume a functional form for the beta(T) relation, the bias is similar to the
Bayesian method. In the case of an actual Herschel field, all methods agree,
showing some degree of anticorrelation between T and beta.
The Bayesian method and the hierarchical models can both reduce the
noise-induced parameter correlations. However, all methods can exhibit
non-negligible bias. This is particularly true for hierarchical models and
observations of varying signal-to-noise ratios and must be taken into account
when interpreting the results.Comment: Submitted to A&A, 18 page
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
30 years of cosmic fullerenes
In 1985, "During experiments aimed at understanding the mechanisms by which
long-chain carbon molecules are formed in interstellar space and circumstellar
shells", Harry Kroto and his collaborators serendipitously discovered a new
form of carbon: fullerenes. The most emblematic fullerene (i.e. C
"buckminsterfullerene"), contains exactly 60 carbon atoms organized in a
cage-like structure similar to a soccer ball. Since their discovery impacted
the field of nanotechnologies, Kroto and colleagues received the Nobel prize in
1996. The cage-like structure, common to all fullerene molecules, gives them
unique properties, in particular an extraordinary stability. For this reason
and since they were discovered in experiments aimed to reproduce conditions in
space, fullerenes were sought after by astronomers for over two decades, and it
is only recently that they have been firmly identified by spectroscopy, in
evolved stars and in the interstellar medium. This identification offers the
opportunity to study the molecular physics of fullerenes in the unique physical
conditions provided by space, and to make the link with other large
carbonaceous molecules thought to be present in space : polycyclic aromatic
hydrocarbons.Comment: To appear in the Proceedings of the annual meeting of the French
society of astronomy and astrophysics (SF2A 2015
A CO survey on a sample of Herschel cold clumps
Context. The physical state of cold cloud clumps has a great impact on the process and efficiency of star formation and the masses of the forming stars inside these objects. The sub-millimetre survey of the Planck space observatory and the far-infrared follow-up mapping of the Herschel space telescope provide an unbiased, large sample of these cold objects. Aims. We have observed (CO)-C-12(1-0) and (CO)-C-13(1-0) emission in 35 high-density clumps in 26 Herschel fields sampling different environments in the Galaxy. Here, we aim to derive the physical properties of the objects and estimate their gravitational stability. Methods. The densities and temperatures of the clumps were calculated from both the dust continuum and the molecular line data. Kinematic distances were derived using (CO)-C-13(1-0) line velocities to verify previous distance estimates and the sizes and masses of the objects were calculated by fitting 2D Gaussian functions to their optical depth distribution maps on 250 mu m. The masses and virial masses were estimated assuming an upper and lower limit on the kinetic temperatures and considering uncertainties due to distance limitations. Results. The derived excitation temperatures are between 8.5-19.5 K, and for most clumps between 10 15 K, while the Herschel-derived dust colour temperatures are more uniform, between 12 16 K. The sizes (0.1-3 pc), (CO)-C-13 column densities (0.5-44 x 10(15) cm(-2)) and masses (from less than 0.1 M-circle dot to more than 1500 M-circle dot) of the objects all span broad ranges. We provide new kinematic distance estimates, identify gravitationally bound or unbound structures and discuss their nature. Conclusions. The sample contains objects on a wide scale of temperatures, densities and sizes. Eleven gravitationally unbound clumps were found, many of them smaller than 0.3 pc, but large, parsec-scale clouds with a few hundred solar masses appear as well. Colder clumps have generally high column densities but warmer objects appear at both low and higher column densities. The clump column densities derived from the line and dust observations correlate well, but are heavily affected by uncertainties of the dust properties, varying molecular abundances and optical depth effects.Peer reviewe
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
Large-scale magnetic field in the Monoceros OB 1 east molecular cloud
Context. The role of large-scale magnetic fields in the evolution of star-forming regions remains elusive. Its investigation requires the observational characterization of well-constrained molecular clouds. The Monoceros OB 1 molecular cloud is a large complex containing several structures that have been shown to be engaged in an active interaction and to have a rich star formation history. However, the magnetic fields in this region have only been studied on small scales. Aims. We study the large-scale magnetic field structure and its interplay with the gas dynamics in the Monoceros OB 1 east molecular cloud. Methods. We combined observations of dust polarized emission from the Planck telescope and CO molecular line emission observations from the Taeduk Radio Astronomy Observatory 14-metre telescope. We calculated the strength of the plane-of-sky magnetic field using a modified Chandrasekhar-Fermi method and estimated the mass-over-flux ratios in different regions of the cloud. We used the comparison of the velocity and intensity gradients of the molecular line observations with the polarimetric observations to trace dynamically active regions. Results. The molecular complex shows an ordered large-scale plane-of-sky magnetic field structure. In the northern part, it is mostly orientated along the filamentary structures, while the southern part shows at least two regions with distinct magnetic field orientations. Our analysis reveals a shock region in the northern part right between two filamentary clouds that, in previous studies, were suggested to be involved in a collision. The magnetic properties of the north-main and north-eastern filaments suggest that these filaments once formed a single one, and that the magnetic field evolved together with the material and did not undergo major changes during the evolution of the cloud. In the southern part, we find that either the magnetic field guides the accretion of interstellar matter towards the cloud or it is dragged by the matter falling towards the main cloud. Conclusions. The large-scale magnetic field in the Monoceros OB 1 east molecular cloud is tightly connected to the global structure of the complex. In the northern part, it seems to serve a dynamically important role by possibly providing support against gravity in the direction perpendicular to the field and to the filament. In the southern part, it is probably the most influential factor governing the morphological structure by guiding possible gas inflow. A study of the whole Monoceros OB 1 molecular complex at large scales is necessary to form a global picture of the formation and evolution of the Monoceros OB 1 east cloud and the role of the magnetic field in this process.Peer reviewe
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A self-consistent dynamical model of the Milky Way disc adjusted to Gaia data
Context. Accurate astrometry achieved by Gaia for many stars in the Milky Way provides an opportunity to reanalyse the Galactic stellar populations from a large and homogeneous sample and to revisit the Galaxy gravitational potential. Aims. This paper shows how a self-consistent dynamical model can be obtained by fitting the gravitational potential of the Milky Way to the stellar kinematics and densities from Gaia data. Methods. We derived a gravitational potential using the Besancon Galaxy Model, and computed the disc stellar distribution functions based on three integrals of motion (E, Lz, I3) to model stationary stellar discs. The gravitational potential and the stellar distribution functions are built self-consistently, and are then adjusted to be in agreement with the kinematics and the density distributions obtained from Gaia observations. A Markov chain Monte Carlo (MCMC) is used to fit the free parameters of the dynamical model to Gaia parallax and proper motion distributions. The fit is done on several sets of Gaia data, mainly a subsample of the GCNS (Gaia catalogue of nearby stars to 100 pc) with G< 17, together with 26 deep fields selected from eDR3, widely spread in longitudes and latitudes. Results. We are able to determine the velocity dispersion ellipsoid and its tilt for subcomponents of different ages, both varying with R and z. The density laws and their radial scale lengths for the thin and thick disc populations are also obtained self-consistently. This new model has some interesting characteristics that come naturally from the process, such as a flaring thin disc. The thick disc is found to present very distinctive characteristics from the old thin disc, both in density and kinematics. This lends significant support to the idea that thin and thick discs were formed in distinct scenarios, as the density and kinematics transition between them is found to be abrupt. The dark matter halo is shown to be nearly spherical. We also derive the solar motion with regards to the Local Standard of Rest (LSR), finding U· = 10.79 ± 0.56 km s-1, V· = 11.06 ± 0.94 km s-1, and W· = 7.66 ± 0.43 km s-1, in close agreement with recent studies. Conclusions. The resulting fully self-consistent gravitational potential, still axisymmetric, is a good approximation of a smooth mass distribution in the Milky Way and can be used for further studies, including finding streams, substructures, and to compute orbits for real stars in our Galaxy
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