The Experts below are selected from a list of 264 Experts worldwide ranked by ideXlab platform
John B Lester - One of the best experts on this subject based on the ideXlab platform.
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Stellar Atmospheres atmospheric extension and fundamental parameters weighing stars using the Stellar mass index
The Astrophysical Journal, 2016Co-Authors: Hilding R Neilson, Fabien Baron, R P Norris, Brian Kloppenborg, John B LesterAbstract:One of the great challenges of?understanding stars is measuring their masses. The best methods for measuring Stellar masses include binary interaction, asteroseismology, and Stellar evolution models, but these methods are not ideal for red giant and supergiant stars. In this work, we propose a novel method for inferring Stellar masses of evolved red giant and supergiant stars using interferometric and spectrophotometric observations combined with spherical model Stellar Atmospheres to measure what we call the Stellar mass index, defined as the ratio between the Stellar radius and mass. The method is based on the correlation between different measurements of angular diameter, used as a proxy for atmospheric extension, and fundamental Stellar parameters. For a given star, spectrophotometry measures the Rosseland angular diameter while interferometric observations generally probe a larger limb-darkened angular diameter. The ratio of these two angular diameters is proportional to the relative extension of the Stellar atmosphere, which is strongly correlated to the star's effective temperature, radius, and mass. We show that these correlations are strong and can lead to precise measurements of Stellar masses.
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Spherically symmetric model Stellar Atmospheres and limb darkening
Astronomy & Astrophysics, 2013Co-Authors: Hilding R Neilson, John B LesterAbstract:Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model Stellar Atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model Stellar Atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model Stellar Atmospheres. We find that sphericity is important even for dwarf model Atmospheres, leading to significant differences in the predicted coefficients
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spherically symmetric model Stellar Atmospheres and limb darkening i limb darkening laws gravity darkening coefficients and angular diameter corrections for red giant stars
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Hilding R Neilson, John B LesterAbstract:Model Stellar Atmospheres are fundamental tools for understanding Stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model Atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model Atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.
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indicators of mass in spherical Stellar Atmospheres
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: John B Lester, Rayomond Dinshaw, Hilding R NeilsonAbstract:Mass is the most important Stellar parameter, but it is not directly observable for a single star. Spherical model Stellar Atmospheres are explicitly characterized by their luminosity ($L_\star$), mass ($M_\star$) and radius ($R_\star$), and observations can now determine directly $L_\star$ and $R_\star$. We computed spherical model Atmospheres for red giants and for red supergiants holding $L_\star$ and $R_\star$ constant at characteristic values for each type of star but varying $M_\star$, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both Stellar classes we found similar signatures of the star's mass in both the surface-brightness distribution and the flux spectrum. The spectral features have been use previously to determine $\log_{10} (g)$, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model Stellar Atmospheres can be used to determine the star's mass from currently achievable spectroscopy. The surface-brightness variations with mass are slightly smaller than can be resolved by current Stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere.
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indicators of mass in spherical Stellar Atmospheres
Publications of the Astronomical Society of the Pacific, 2013Co-Authors: John B Lester, Rayomond Dinshaw, Hilding R NeilsonAbstract:Mass is the most important Stellar parameter, but it is not directly observable for a single star. Spherical model Stellar Atmospheres are explicitly characterized by their luminosity (L⋆), mass (M⋆), and radius (R⋆), and observations can now determine directly L⋆ and R⋆. We computed spherical model Atmospheres for red giants and for red supergiants holding L⋆ and R⋆ constant at characteristic values for each type of star but varying M⋆, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both Stellar classes we found similar signatures of the stars' mass in both the surface-brightness distri- bution and the flux spectrum. The spectral features have been use previously to determine log10ðgÞ, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model Stellar atmo- spheres can be used to determine a star's mass from currently achievable spectroscopy. The surface-brightness variations of mass are slightly smaller than can be resolved by current Stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere.
Yeisson Osorio - One of the best experts on this subject based on the ideXlab platform.
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Mg line formation in late-type Stellar Atmospheres : II. Calculations in a grid of 1D models
Astronomy and Astrophysics, 2016Co-Authors: Yeisson Osorio, Paul BarklemAbstract:Mg line formation in late-type Stellar Atmospheres : II. Calculations in a grid of 1D models
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mg line formation in late type Stellar Atmospheres i the model atom
Astronomy and Astrophysics, 2015Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Context. Magnesium is an element of significant astrophysical importance, often traced in late-type stars using lines of neutral magnesium, which is expected to be subject to departures from local thermodynamic equilibrium (LTE). The importance of Mg, together with the unique range of spectral features in late-type stars probing different parts of the atom, as well as its relative simplicity from an atomic physics point of view, makes it a prime target and test bed for detailed ab initio non-LTE modelling in Stellar Atmospheres. Previous non-LTE modelling of spectral line formation has, however, been subject to uncertainties due to lack of accurate data for inelastic collisions with electrons and hydrogen atoms. Aims. In this paper we build and test a Mg model atom for spectral line formation in late-type stars with new or recent inelastic collision data and no associated free parameters. We aim to reduce these uncertainties and thereby improve the accuracy of Mg non-LTE modelling in late-type stars. Methods. For the low-lying states of Mg i, electron collision data were calculated using the R-matrix method. Hydrogen collision data, including charge transfer processes, were taken from recent calculations by some of us. Calculations for collisional broadening by neutral hydrogen were also performed where data were missing. These calculations, together with data from the literature, were used to build a model atom. This model was then employed in the context of standard non-LTE modelling in 1D (including average 3D) model Atmospheres in a small set of Stellar atmosphere models. First, the modelling was tested by comparisons with observed spectra of benchmark stars with well-known parameters. Second, the spectral line behaviour and uncertainties were explored by extensive experiments in which sets of collisional data were changed or removed. Results. The modelled spectra agree well with observed spectra from benchmark stars, showing much better agreement with line profile shapes than with LTE modelling. The line-to-line scatter in the derived abundances shows some improvements compared to LTE (where the cores of strong lines must often be ignored), particularly when coupled with averaged 3D models. The observed Mg emission features at 7 and 12 μm in the spectra of the Sun and Arcturus, which are sensitive to the collision data, are reasonably well reproduced. Charge transfer with H is generally important as a thermalising mechanism in dwarfs, but less so in giants. Excitation due to collisions with H is found to be quite important in both giants and dwarfs. The R-matrix calculations for electron collisions also lead to significant differences compared to when approximate formulas are employed. The modelling predicts non-LTE abundance corrections ΔA(Mg) NLTE−LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Conclusions. Our results emphasise the need for accurate data of Mg collisions with both electrons and H atoms for precise non-LTE predictions of Stellar spectra, but demonstrate that such data can be calculated and that ab initio non-LTE modelling without resort to free parameters is possible. In contrast to Li and Na, where only the introduction of charge transfer processes has led to differences with respect to earlier non-LTE modelling, the more complex case of Mg finds changes due to improvements in the data for collisional excitation by electrons and hydrogen atoms, as well as due to the charge transfer processes. Grids of departure coefficients and abundance corrections for a range of Stellar parameters are planned for a forthcoming paper.
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the influence of electron collisions on non lte li line formation in Stellar Atmospheres
27th International Conference on Photonic Electronic and Atomic Collisions ICPEAC 2011 27 July 2011 through 2 August 2011 Belfast, 2012Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, M. AsplundAbstract:The influence of the uncertainties in the rate coefficient data for electron-impact excitation and ionization on non-LTE Li line formation in cool Stellar Atmospheres is investigated. We examine the electron collision data used in previous non-LTE calculations and compare them to our own calculations using the R-matrix with pseudostates (RMPS) method and to other calculations found in the literature.
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the influence of electron collisions on non lte li line formation in Stellar Atmospheres
arXiv: Solar and Stellar Astrophysics, 2011Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, M. AsplundAbstract:The influence of the uncertainties in the rate coefficient data for electron-impact excitation and ionization on non-LTE Li line formation in cool Stellar Atmospheres is investigated. We examine the electron collision data used in previous non-LTE calculations and compare them to recent calculations that use convergent close-coupling (CCC) techniques and to our own calculations using the R-matrix with pseudostates (RMPS) method. We find excellent agreement between rate coefficients from the CCC and RMPS calculations, and reasonable agreement between these data and the semi-empirical data used in non-LTE calculations up to now. The results of non-LTE calculations using the old and new data sets are compared and only small differences found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence on non-LTE calculations of uncertainties in the electron collision data is negligible. Indeed, together with the collision data for the charge exchange process Li(3s) + H Li^+ + H^- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.
Hilding R Neilson - One of the best experts on this subject based on the ideXlab platform.
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Stellar Atmospheres atmospheric extension and fundamental parameters weighing stars using the Stellar mass index
The Astrophysical Journal, 2016Co-Authors: Hilding R Neilson, Fabien Baron, R P Norris, Brian Kloppenborg, John B LesterAbstract:One of the great challenges of?understanding stars is measuring their masses. The best methods for measuring Stellar masses include binary interaction, asteroseismology, and Stellar evolution models, but these methods are not ideal for red giant and supergiant stars. In this work, we propose a novel method for inferring Stellar masses of evolved red giant and supergiant stars using interferometric and spectrophotometric observations combined with spherical model Stellar Atmospheres to measure what we call the Stellar mass index, defined as the ratio between the Stellar radius and mass. The method is based on the correlation between different measurements of angular diameter, used as a proxy for atmospheric extension, and fundamental Stellar parameters. For a given star, spectrophotometry measures the Rosseland angular diameter while interferometric observations generally probe a larger limb-darkened angular diameter. The ratio of these two angular diameters is proportional to the relative extension of the Stellar atmosphere, which is strongly correlated to the star's effective temperature, radius, and mass. We show that these correlations are strong and can lead to precise measurements of Stellar masses.
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Spherically symmetric model Stellar Atmospheres and limb darkening
Astronomy & Astrophysics, 2013Co-Authors: Hilding R Neilson, John B LesterAbstract:Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model Stellar Atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model Stellar Atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model Stellar Atmospheres. We find that sphericity is important even for dwarf model Atmospheres, leading to significant differences in the predicted coefficients
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spherically symmetric model Stellar Atmospheres and limb darkening i limb darkening laws gravity darkening coefficients and angular diameter corrections for red giant stars
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Hilding R Neilson, John B LesterAbstract:Model Stellar Atmospheres are fundamental tools for understanding Stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model Atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model Atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.
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indicators of mass in spherical Stellar Atmospheres
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: John B Lester, Rayomond Dinshaw, Hilding R NeilsonAbstract:Mass is the most important Stellar parameter, but it is not directly observable for a single star. Spherical model Stellar Atmospheres are explicitly characterized by their luminosity ($L_\star$), mass ($M_\star$) and radius ($R_\star$), and observations can now determine directly $L_\star$ and $R_\star$. We computed spherical model Atmospheres for red giants and for red supergiants holding $L_\star$ and $R_\star$ constant at characteristic values for each type of star but varying $M_\star$, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both Stellar classes we found similar signatures of the star's mass in both the surface-brightness distribution and the flux spectrum. The spectral features have been use previously to determine $\log_{10} (g)$, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model Stellar Atmospheres can be used to determine the star's mass from currently achievable spectroscopy. The surface-brightness variations with mass are slightly smaller than can be resolved by current Stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere.
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indicators of mass in spherical Stellar Atmospheres
Publications of the Astronomical Society of the Pacific, 2013Co-Authors: John B Lester, Rayomond Dinshaw, Hilding R NeilsonAbstract:Mass is the most important Stellar parameter, but it is not directly observable for a single star. Spherical model Stellar Atmospheres are explicitly characterized by their luminosity (L⋆), mass (M⋆), and radius (R⋆), and observations can now determine directly L⋆ and R⋆. We computed spherical model Atmospheres for red giants and for red supergiants holding L⋆ and R⋆ constant at characteristic values for each type of star but varying M⋆, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both Stellar classes we found similar signatures of the stars' mass in both the surface-brightness distri- bution and the flux spectrum. The spectral features have been use previously to determine log10ðgÞ, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model Stellar atmo- spheres can be used to determine a star's mass from currently achievable spectroscopy. The surface-brightness variations of mass are slightly smaller than can be resolved by current Stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere.
P S Barklem - One of the best experts on this subject based on the ideXlab platform.
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mg line formation in late type Stellar Atmospheres i the model atom
Astronomy and Astrophysics, 2015Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Context. Magnesium is an element of significant astrophysical importance, often traced in late-type stars using lines of neutral magnesium, which is expected to be subject to departures from local thermodynamic equilibrium (LTE). The importance of Mg, together with the unique range of spectral features in late-type stars probing different parts of the atom, as well as its relative simplicity from an atomic physics point of view, makes it a prime target and test bed for detailed ab initio non-LTE modelling in Stellar Atmospheres. Previous non-LTE modelling of spectral line formation has, however, been subject to uncertainties due to lack of accurate data for inelastic collisions with electrons and hydrogen atoms. Aims. In this paper we build and test a Mg model atom for spectral line formation in late-type stars with new or recent inelastic collision data and no associated free parameters. We aim to reduce these uncertainties and thereby improve the accuracy of Mg non-LTE modelling in late-type stars. Methods. For the low-lying states of Mg i, electron collision data were calculated using the R-matrix method. Hydrogen collision data, including charge transfer processes, were taken from recent calculations by some of us. Calculations for collisional broadening by neutral hydrogen were also performed where data were missing. These calculations, together with data from the literature, were used to build a model atom. This model was then employed in the context of standard non-LTE modelling in 1D (including average 3D) model Atmospheres in a small set of Stellar atmosphere models. First, the modelling was tested by comparisons with observed spectra of benchmark stars with well-known parameters. Second, the spectral line behaviour and uncertainties were explored by extensive experiments in which sets of collisional data were changed or removed. Results. The modelled spectra agree well with observed spectra from benchmark stars, showing much better agreement with line profile shapes than with LTE modelling. The line-to-line scatter in the derived abundances shows some improvements compared to LTE (where the cores of strong lines must often be ignored), particularly when coupled with averaged 3D models. The observed Mg emission features at 7 and 12 μm in the spectra of the Sun and Arcturus, which are sensitive to the collision data, are reasonably well reproduced. Charge transfer with H is generally important as a thermalising mechanism in dwarfs, but less so in giants. Excitation due to collisions with H is found to be quite important in both giants and dwarfs. The R-matrix calculations for electron collisions also lead to significant differences compared to when approximate formulas are employed. The modelling predicts non-LTE abundance corrections ΔA(Mg) NLTE−LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Conclusions. Our results emphasise the need for accurate data of Mg collisions with both electrons and H atoms for precise non-LTE predictions of Stellar spectra, but demonstrate that such data can be calculated and that ab initio non-LTE modelling without resort to free parameters is possible. In contrast to Li and Na, where only the introduction of charge transfer processes has led to differences with respect to earlier non-LTE modelling, the more complex case of Mg finds changes due to improvements in the data for collisional excitation by electrons and hydrogen atoms, as well as due to the charge transfer processes. Grids of departure coefficients and abundance corrections for a range of Stellar parameters are planned for a forthcoming paper.
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the influence of electron collisions on non lte li line formation in Stellar Atmospheres
27th International Conference on Photonic Electronic and Atomic Collisions ICPEAC 2011 27 July 2011 through 2 August 2011 Belfast, 2012Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, M. AsplundAbstract:The influence of the uncertainties in the rate coefficient data for electron-impact excitation and ionization on non-LTE Li line formation in cool Stellar Atmospheres is investigated. We examine the electron collision data used in previous non-LTE calculations and compare them to our own calculations using the R-matrix with pseudostates (RMPS) method and to other calculations found in the literature.
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Inelastic Mg plus H collision data for non-LTE applications in Stellar Atmospheres
Astronomy and Astrophysics - A&A, 2012Co-Authors: P S Barklem, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s(2) S-1, 3s3p P-3, 3s3p P-1, 3s4s S-3, 3s4s S-1, 3s3d D-1, 3s4p P-3)+H(1s) and Mg+(3s S-2)+H-. The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool Stellar Atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.
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inelastic mg h collision data for non lte applications in Stellar Atmospheres
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: P S Barklem, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer). The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool Stellar Atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.
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the influence of electron collisions on non lte li line formation in Stellar Atmospheres
arXiv: Solar and Stellar Astrophysics, 2011Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, M. AsplundAbstract:The influence of the uncertainties in the rate coefficient data for electron-impact excitation and ionization on non-LTE Li line formation in cool Stellar Atmospheres is investigated. We examine the electron collision data used in previous non-LTE calculations and compare them to recent calculations that use convergent close-coupling (CCC) techniques and to our own calculations using the R-matrix with pseudostates (RMPS) method. We find excellent agreement between rate coefficients from the CCC and RMPS calculations, and reasonable agreement between these data and the semi-empirical data used in non-LTE calculations up to now. The results of non-LTE calculations using the old and new data sets are compared and only small differences found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence on non-LTE calculations of uncertainties in the electron collision data is negligible. Indeed, together with the collision data for the charge exchange process Li(3s) + H Li^+ + H^- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.
N Feautrier - One of the best experts on this subject based on the ideXlab platform.
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Model estimates of inelastic calcium-hydrogen collision data for non-LTE Stellar Atmospheres modeling
Astronomy and Astrophysics - A&A, 2016Co-Authors: A. Belyaev, A Spielfiedel, Marie Guitou, A. Yakovleva, A. Mitrushchenkov, N FeautrierAbstract:Aims. Inelastic processes in low-energy Ca + H and Ca+ + H- collisions are treated for the states from the ground state up to the ionic state with the aim to provide rate coeffcients needed for non-LTE modeling of Ca in cool Stellar Atmospheres. Methods. The electronic molecular structure was determined using a recently proposed model approach that is based on an asymptotic method. Nonadiabatic nuclear dynamics were treated by means of multichannel formulas, based on the Landau-Zener model for nonadiabatic transition probabilities. Results. The cross sections and rate coeffcients for inelastic processes in Ca + H and Ca+ + H- collisions were calculated for all transitions between 17 low-lying covalent states plus the ionic state. It is shown that the highest rate coeffcient values correspond to the excitation, de-excitation, ion-pair formation, and mutual neutralization processes involving the Ca(4s5s 1;3S) and the ionic Ca+ + H- states. The next group with the second highest rate coeffcients includes the processes involving the Ca(4s5p 1;3P), Ca(4s4d 1;3D), and Ca(4s4p 1P) states. The processes from these two groups are likely to be important for non-LTE modeling.
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mg line formation in late type Stellar Atmospheres i the model atom
Astronomy and Astrophysics, 2015Co-Authors: Yeisson Osorio, P S Barklem, Karin Lind, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Context. Magnesium is an element of significant astrophysical importance, often traced in late-type stars using lines of neutral magnesium, which is expected to be subject to departures from local thermodynamic equilibrium (LTE). The importance of Mg, together with the unique range of spectral features in late-type stars probing different parts of the atom, as well as its relative simplicity from an atomic physics point of view, makes it a prime target and test bed for detailed ab initio non-LTE modelling in Stellar Atmospheres. Previous non-LTE modelling of spectral line formation has, however, been subject to uncertainties due to lack of accurate data for inelastic collisions with electrons and hydrogen atoms. Aims. In this paper we build and test a Mg model atom for spectral line formation in late-type stars with new or recent inelastic collision data and no associated free parameters. We aim to reduce these uncertainties and thereby improve the accuracy of Mg non-LTE modelling in late-type stars. Methods. For the low-lying states of Mg i, electron collision data were calculated using the R-matrix method. Hydrogen collision data, including charge transfer processes, were taken from recent calculations by some of us. Calculations for collisional broadening by neutral hydrogen were also performed where data were missing. These calculations, together with data from the literature, were used to build a model atom. This model was then employed in the context of standard non-LTE modelling in 1D (including average 3D) model Atmospheres in a small set of Stellar atmosphere models. First, the modelling was tested by comparisons with observed spectra of benchmark stars with well-known parameters. Second, the spectral line behaviour and uncertainties were explored by extensive experiments in which sets of collisional data were changed or removed. Results. The modelled spectra agree well with observed spectra from benchmark stars, showing much better agreement with line profile shapes than with LTE modelling. The line-to-line scatter in the derived abundances shows some improvements compared to LTE (where the cores of strong lines must often be ignored), particularly when coupled with averaged 3D models. The observed Mg emission features at 7 and 12 μm in the spectra of the Sun and Arcturus, which are sensitive to the collision data, are reasonably well reproduced. Charge transfer with H is generally important as a thermalising mechanism in dwarfs, but less so in giants. Excitation due to collisions with H is found to be quite important in both giants and dwarfs. The R-matrix calculations for electron collisions also lead to significant differences compared to when approximate formulas are employed. The modelling predicts non-LTE abundance corrections ΔA(Mg) NLTE−LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Conclusions. Our results emphasise the need for accurate data of Mg collisions with both electrons and H atoms for precise non-LTE predictions of Stellar spectra, but demonstrate that such data can be calculated and that ab initio non-LTE modelling without resort to free parameters is possible. In contrast to Li and Na, where only the introduction of charge transfer processes has led to differences with respect to earlier non-LTE modelling, the more complex case of Mg finds changes due to improvements in the data for collisional excitation by electrons and hydrogen atoms, as well as due to the charge transfer processes. Grids of departure coefficients and abundance corrections for a range of Stellar parameters are planned for a forthcoming paper.
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Inelastic Mg plus H collision data for non-LTE applications in Stellar Atmospheres
Astronomy and Astrophysics - A&A, 2012Co-Authors: P S Barklem, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s(2) S-1, 3s3p P-3, 3s3p P-1, 3s4s S-3, 3s4s S-1, 3s3d D-1, 3s4p P-3)+H(1s) and Mg+(3s S-2)+H-. The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool Stellar Atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.
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inelastic mg h collision data for non lte applications in Stellar Atmospheres
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: P S Barklem, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer). The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool Stellar Atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.
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inelastic mg h collision data for non lte applications in Stellar Atmospheres
Astronomy and Astrophysics, 2010Co-Authors: P S Barklem, A K Belyaev, A Spielfiedel, Marie Guitou, N FeautrierAbstract:Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s(2) S-1, 3s3p P-3, 3s3p P-1, 3s4s S-3, 3s4s S-1, 3s3d D-1, 3s4p P-3)+H(1s) and Mg+(3s S-2)+H-. The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool Stellar Atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.
