Search results for "Radiative Transfer"
showing 10 items of 551 documents
Radiative decays in bottomonium beyond the long wavelength approximation
2019
We revisit the nonrelativistic quark model description of electromagnetic radiative decays in bottomonium. We show that even for the simplest spectroscopic quark model the calculated widths can be in good agreement with data once the experimental masses of bottomonium states and the photon energy are properly implemented in the calculation. For transitions involving the lower lying spectral states this implementation can be easily done via the Long Wave Length approximation. For transitions where this approximation does not apply we develop a new method of implementing the experimental energy dependencies.
The role of radiative losses in the late evolution of pulse-heated coronal loops/strands
2012
Radiative losses from optically thin plasma are an important ingredient for modeling plasma confined in the solar corona. Spectral models are continuously updated to include the emission from more spectral lines, with significant effects on radiative losses, especially around 1 MK. We investigate the effect of changing the radiative losses temperature dependence due to upgrading of spectral codes on predictions obtained from modeling plasma confined in the solar corona. The hydrodynamic simulation of a pulse-heated loop strand is revisited comparing results using an old and a recent radiative losses function. We find significant changes in the plasma evolution during the late phases of plas…
The excitation of circumstellar C2in carbon-rich post-AGB object IRAS 22272+5435
2013
Context. Observations of the circumstellar envelopes of post-asymptotic giant branch (post-AGB) stars provide information about their mass-loss history during the AGB phase and about the chemistry inside envelopes after that phase. Aims. We analyze visual observations of C2 molecule in absorption and millimeter and sub-millimeter observations of CO molecule in emission with the aim of determining the physical and chemical conditions in the circumstellar shell of the carbon-rich post-AGB star IRAS 22272+5435. Methods. We determined the column densities and excitation structure of C2 from equivalent width analysis of the molecular absorptions. The thermal and density structure of post-AGB she…
Fine-Scale Droplet Clustering in Atmospheric Clouds: 3D Radial Distribution Function from Airborne Digital Holography
2018
The extent of droplet clustering in turbulent clouds has remained largely unquantified, and yet is of possible relevance to precipitation formation and radiative transfer. To that end, data gathered by an airborne holographic instrument are used to explore the three-dimensional spatial statistics of cloud droplet positions in homogeneous stratiform boundary-layer clouds. The three-dimensional radial distribution functions g(r) reveal unambiguous evidence of droplet clustering. Three key theoretical predictions are observed: the existence of positive correlations, onset of correlation in the turbulence dissipation range, and monotonic increase of g(r) with decreasing r. This implies that cur…
3D numerical modeling of YSO accretion shocks
2013
International audience; The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field. A quasi-periodic emission signature is theoretically expected to be observed, but observations do not confirm any such pattern. In this work, we assume a uniform background field, in the regime of optically thin energy losses, and we study the multi-dimensional shock evolution in the presence of perturbations, i.e. clumps in the stream and an acoustic energy flux flowing at the base of the chromosphere. We perform 3D MHD simulations using the PLUTO code, modeling locally the impact of the infalling gas onto the chromosphere. We find t…
X-ray emission from dense plasma in classical T Tauri stars: hydrodynamic modeling of the accretion shock
2008
Context: High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T~2-3×10^6 K and density n_e~10^11-10^13 cm^-3, which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere. Aims: We investigate the dynamics and stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region. Methods: We perform one-dimensional hydrodynamic simulation…
A Simple Model of Radiative Emission in M87
2005
We present a simple physical model of the central source emission in the M87 galaxy. It is well known that the observed X-ray luminosity from this galactic nucleus is much lower than the predicted one, if a standard radiative efficiency is assumed. Up to now the main model invoked to explain such a luminosity is the ADAF (Advection-Dominated-Accretion-Flow) model. Our approach supposes only a simple axis-symmetric adiabatic accretion with a low angular momentum together with the bremsstrahlung emission process in the accreting gas. With no other special hypothesis on the dynamics of the system, this model agrees well enough with the luminosity value measured by Chandra.
Optical and near-infrared recombination lines of oxygen ions from Cassiopeia A knots
2008
Context. Fast-moving knots (FMK) in the Galactic supernova remnant Cassiopeia A consist mainly of metals and allow to study element production in supernovae and shock physics in great detail. Aims. We work out theoretically and suggest to observe previously unexplored class of spectral lines -- metal recombination lines in optical and near-infrared bands -- emitted by the cold ionized and cooling plasma in the fast-moving knots. Methods. By tracing ion radiative and dielectronic recombination, collisional $l$-redistribution and radiative cascade processes, we compute resulting oxygen, silicon and sulphur recombination line emissivities. It allows us to determine the oxygen recombination lin…
On the origin of the X-ray emission from Herbig Ae/Be stars
2006
We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. These stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Their X-ray emission has remained unexplained. The superior spatial resolution of Chandra with respect to previous X-ray instrumentation has allowed us to examine the possible role of late-type companions in generating the observed X-rays. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10". The detection fraction of Herbig Ae/Be stars is 76 %, but lowers to 35 …
Stellar X-ray heating of planet atmospheres
2006
Aims. To investigate the effects of the stellar X-ray irradiation on planet atmospheres, we study the X-ray transfer and energy deposition in a hydrogen rich gas. Methods. We construct an accurate X-ray transfer model taking both photoionization and Compton scattering into account; the electron energy deposition is followed by tracking the discrete exchange processes between electrons and the gas mixture. Results. Exospheric heating rates are derived as functions of the pressure in model atmospheres using a wide range of X-ray luminosity, spectral hardness representative of different stellar ages, and distances from the parent star. The computed heating rates suggest that X-ray irradiation …