1,720,994 research outputs found
H2 excitation in turbulent interstellar clouds
No full textWe discuss the observational differences between lines of sight that intercept a group of turbulent dissipative structures and lines of sight that cross less-active regions. Using time-dependent calculations we show that the energy level distribution of the hydrogen molecule evolves in time in response to the local thermal phase. We find that relatively simple models can explain the observed properties of molecular hydrogen in diffuse interstellar clouds in terms of time evolution induced by collisional excitation in a low-density, high-temperature gas
Radiative transfer modelling in protoplanetary disks with the P-N approximation and Monte Carlo techniques
No full textA protoplanetary disk is a quasi-toroidal gas-dust mixture rotating around a protostar that creates a complex intensity field in the internal disk medium in which planets are believed to form. In this work we start to model an idealized disk describing the radiation intensity propagating along a vertical disk section using the P-N Approximation. This unidimensional approach can be extended to a flared surface case but the problem rapidly raises complexity in solving a simple cylindrical mid-plane region. The full 3D case is then handled with a Monte Carlo (MC) code reusing the idealized model to test the software reliability. The Monte Carlo lack in precision is fully repaid by the flexibility in describing the light-to-dust interaction moving the model towards a more realistic micro-physics based approach. Copyright © 2010 John Wiley & Sons, Ltd
The diffuse clouds towards Cyg OB2 No. 5 and No. 12
No full textWe have investigated, in the 13CO(1–0) line, the region between the stars Cyg OB2 No. 5 and No. 12. The integrated intensity contour map reveals the existence of a condensation at 12 km s−1 (core C), north-west of Cyg OB2 No. 5, with a core–halo structure. The morphology and the derived physical parameters make it quite similar to core A, previously discovered towards Cyg OB2 No. 12. A second condensation at 7 km s−1, near Cyg OB2 No. 12, has been mapped in the same line (1–0) of 13CO. A summary of different observations, in all spectral regions, towards Cyg OB2 No. 5 and No. 12 is presented to provide the material for a further comprehensive discussion of this intricate and still intriguing region
Modeling Galactic Extinction with Dust and "Real" Polycyclic Aromatic Hydrocarbons
No full textWe investigate the remarkable apparent variety of galactic extinction curves by modeling extinction profiles with core-mantle grains and a collection of single polycyclic aromatic hydrocarbons. Our aim is to translate a synthetic description of dust into physically well-grounded building blocks through the analysis of a statistically relevant sample of different extinction curves. All different flavors of observed extinction curves, ranging from the average galactic extinction curve to virtually "bumpless" profiles, can be described by the present model. We prove that a mixture of a relatively small number (54 species in 4 charge states each) of polycyclic aromatic hydrocarbons can reproduce the features of the extinction curve in the ultraviolet, dismissing an old objection to the contribution of polycyclic aromatic hydrocarbons to the interstellar extinction curve. Despite the large number of free parameters (at most the 54 × 4 column densities of each species in each ionization state included in the molecular ensemble plus the 9 parameters defining the physical properties of classical particles), we can strongly constrain some physically relevant properties such as the total number of C atoms in all species and the mean charge of the mixture. Such properties are found to be largely independent of the adopted dust model whose variation provides effects that are orthogonal to those brought about by the molecular component. Finally, the fitting procedure, together with some physical sense, suggests (but does not require) the presence of an additional component of chemically different very small carbonaceous grains
Observational evidence of dust evolution in galactic extinction curves
No full textAlthough structural and optical properties of hydrogenated amorphous carbons are known to respond to varying physical conditions, most conventional extinction models are basically curve fits with modest predictive power. We compare an evolutionary model of the physical properties of carbonaceous grain mantles with their determination by homogeneously fitting observationally derived Galactic extinction curves with the same physically well-defined dust model. We find that a large sample of observed Galactic extinction curves are compatible with the evolutionary scenario underlying such a model, requiring physical conditions fully consistent with standard density, temperature, radiation field intensity, and average age of diffuse interstellar clouds. Hence, through the study of interstellar extinction we may, in principle, understand the evolutionary history of the diffuse interstellar clouds
Rotational and vibrational excitation of CO molecules by collisions with He-4 atoms
No full textFull close-coupled calculations are carried out of the cross sections for energy transfer between rotational levels of carbon monoxide in collision with He-4 atoms with energies between 5 and 600 cm(-1). At low energies, the cross sections are dominated by contributions from shape resonances. The calculated cross sections are in satisfactory agreement with the experimental data measured at an energy of 570 cm(-1). Calculations using the infinite order sudden approximation are carried out of cross sections for energy transfer between vibrational levels of CO. Vibrational energy transfer is dominated by transitions in which the vibrational quantum number changes by unity
Modeling Galactic Extinction with dust and "real" PAHs
No full textWe elaborated an interstellar dust model assuming a distribution of core–mantle
grains and a collection of single polyciclic aromatic hydrocarbons. Exploiting this model, we
are able to reproduce a very large sample of galactic extinction profiles with very different
flavours, proving that a polyciclic aromatic hydrocarbons population can reproduce extinction
curve features in the ultraviolet range. Dust grains are composed by an hollow silicatic core and
a carbonaceous mantle (this description is mutuated by the cycle of carbon in the interstellar
medium); molecular population is represented by 54 molecules in four charged states and, despite
the large number of free parameters (we have 9 parameters to represent grain distribution and
54 × 4 column densities to reproduce the molecular contribution to the extiction), we are able
to determine some global properties for molecular ensemble and we found that these properties
are indipendent by specific species which we use in our model
Modeling peculiar extinction curves
No full textWe investigate the remarkable variety of IR-through-UV extinction curves by modelling
extinction profiles with core-mantle grains and a collection of single and stacked polycyclic
aromatic hydrocarbons. Such a model can closely reproduce the observed curves, by simply
assuming a different ratio of the molecular component to classical dust, while leaving relatively
unmodified the underlying global picture. In particular, we find that for extinction curve
morphologies differing significantly from the average Galactic extinction, dust grain sizes
exhibit a low-end cut-off at approximately 100 nm. An additional component of very small
grains of sizes around 10 nm is required to model lines of sight whose extinctions are similar to
the average Galactic curve. We conclude that to be accurately described, the so-called peculiar
extinction curves do not need dust grains with exotic properties
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