Line Formation In Stellar Atmospheres Research Articles

Abundance of beryllium in the Sun and stars: The role of non-local thermodynamic equilibrium effects

Context. Earlier studies have suggested that deviations from the local thermodynamic equilibrium (LTE) play a minor role in the formation of Be II 313 nm resonance lines in solar and stellar atmospheres. Recent improvements in the atomic data allow a more complete model atom of Be to be constructed and the validity of these claims to be reassessed using more up-to-date atomic physics. Aims. The main goal of this study therefore is to refocus on the role of non-local thermodynamic equilibrium (NLTE) effects in the formation of Be II 313.04 and 313.11 nm resonance lines in solar and stellar atmospheres. Methods. For this, we constructed a model atom of Be using new atomic data that recently became available. The model atom contains 98 levels and 383 radiative transitions of Be I and Be II and uses the most up-to-date collision rates with electrons and hydrogen. This makes it the most complete model atom used to determine 1D NLTE solar Be abundance and to study the role of NLTE effects in the formation of Be II 313 nm resonance lines. Results. We find that deviations from LTE have a significant influence on the strength of the Be II 313 nm line in solar and stellar atmospheres. For the Sun, we obtained the 1D NLTE Be abundance of A(Be)NLTE = 1.32 ± 0.05, which is in excellent agreement with the meteoritic value of A(Be) = 1.31 ± 0.04. Importantly, we find that NLTE effects become significant in FGK stars. Moreover, there is a pronounced variation in 1D NLTE–LTE abundance corrections with the effective temperature and metallicity. Therefore, contrary to our previous understanding, the obtained results indicate that NLTE effects play an important role in Be line formation in stellar atmospheres and have to be properly taken into account in Be abundance studies, especially in metal-poor stars.

On line contribution functions and examining spectral line formation in 3D model stellar atmospheres

Line contribution functions are useful diagnostics for studying spectral line formation in stellar atmospheres. I derive an expression for the contribution function to the abso- lute flux depression that emerges from three-dimensional box-in-a-star model stellar atmospheres. I illustrate the result by comparing the local thermodynamic equilibrium (LTE) spectral line formation of the high-excitation permitted OI777nm lines with the non-LTE case.

The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

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.

The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

Chemical abundances as measured from stellar spectral lines are often subject to uncertainties due to lack of accurate data for inelastic collisions, which is needed for non-local thermodynamic equilibrium (non-LTE) modelling. For cool stars, understanding of collision processes with electrons and hydrogen atoms is required to achieve high precision measurements. In this thesis, I have investigated the role of these collisions on the non-LTE formation of Li and Mg spectral lines in late-type stars.In the case of Li, electron impact excitation processes were calculated using the R-matrix with pseudo states method and the results found to agree well with recent calculations using the convergent close-coupling technique. These modern data were employed in non-LTE calculations by updating an existing model atom, which already included modern data for hydrogen collision processes. Our results were compared with calculations using older semi-empirical approximation calculations and only small differences were found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence of uncertainties in the electron collision data on non-LTE calculations is negligible. Indeed, together with the collision data for the charge transfer process Li + 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.In the case of Mg, electron impact excitation processes were again calculated with the Rmatrix with pseudo states method, and used together with recent hydrogen collision calculations to build and test a model atom, without free parameters, for non-LTE modelling. Both electron and hydrogen collision processes, including charge transfer and excitation, are found to be important thermalising agents in various cases. The modelled spectra agree well with observed spectra from benchmark stars in the optical and infrared. The modelling predicts non-LTE abundance corrections ∆A(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. Results of calculations in a large grid of 1D model atmospheres are presented, and the implications for studies of Mg discussed. The propagation of uncertainties in the inelastic collision data to those in stellar abundances is investigated, and found to lead to small uncertainties, once again typically less than 0.01 dex (2%), although for few stellar models in specific lines (e.g., metal-poor suns, in the 7691 A line) uncertainties can be as large as 0.03 dex (7%).

On the line formation process in stellar atmospheres — I. Unpolarized light

The problem of line formation in stellar atmospheres is studied in terms of contribution functions. The correlation between the contribution from atmospheric layers to an emergent quantity and the line formation process is explored in the case of absorption lines for unpolarized light, while the case of polarized light will be considered in a forthcoming paper. We use four contribution functions derived from two (differential and integral) formal solutions to the radiative transfer equations for emergent specific intensity and line depression respectively. Two kinds of model atmosphere are employed to investigate the wavelength dependence of these contribution functions. One is artificial, and gives the restricted distribution of line absorbers in tenuous layers, while the other represents a more realistic stellar atmosphere in which line absorption and emission are ubiquitous. It is found in the artificial model that the line formation region is indicated only by the levels that provide a dominant contribution to the line core that is distinguishable from the contribution to the continuum, for the contribution functions that describe the contribution of the layer to the specific intensity at the surface. For those functions that denote the contribution to line depression, the formation layers can be more directly determined, but sometimes the original meaning of the contribution function is lost. Thus the depth distribution of the contribution and the line formation process should be carefully connected. For the contribution functions calculated in both types of model, they show a common feature: around the line centre there is a formation core the main contribution region of which delineates the line core formation region. Its width is related to a depth-averaged Doppler width, and all wavelength points within the core form in the same region. For the Mg I 5172.7 line forming in the solar umbral atmosphere, the formation core forms separately from the far wings, mainly as a result of the solar temperature stratification. Finally, the line core formation region is not influenced by the distribution of line-of-sight velocities, while the formation region of the line wings is affected by these velocities.

The Influence of Waves on Line Formation in Stellar Atmospheres

AbstractThe influence of acoustic waves on line formation is investigated. The effect of velocity model parameters, e.g. wave number and amplitude, and of the μ‐dependence on line formation are discussed by simplified model assumptions. The effect of waves and Gaussian stochastic turbulence is compared.

On an Exact Solution in the Theory of Line Formation in Stellar Atmospheres

On an Exact Solution in the Theory of Line Formation in Stellar Atmospheres

On the Radiative Equilibrium of a Stellar Atmosphere. VI.

view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Radiative Equilibrium of a Stellar Atmosphere. VI. Cesco, C. U. ; Chandrasekhar, S. ; Sahade, J. Abstract In this paper the solutions to the problem of line formation in stellar atmospheres, in the third approxi- mation, are tabulated for various values of the parameters which are involved and at close enough inter- vals to allow interpolation. Numerical forms of the solution for the problem of the radiative equilibrium of a planetary nebula are also obtained Publication: The Astrophysical Journal Pub Date: May 1945 DOI: 10.1086/144723 Bibcode: 1945ApJ...101..320C full text sources ADS | Related Materials (25) Erratum: 1945ApJ...102..137. Part 1: 1944ApJ....99..180C Part 2: 1944ApJ...100...76C Part 3: 1944ApJ...100..117C Part 4: 1944ApJ...100..355C Part 5: 1945ApJ...101...95C Part 7: 1945ApJ...101..328C Part 8: 1945ApJ...101..348C Part 9: 1946ApJ...103..165C Part 10: 1946ApJ...103..351C Part 11: 1946ApJ...104..110C Part 12: 1946ApJ...104..191C Part 13: 1947ApJ...105..151C Part 14: 1947ApJ...105..164C Part 15: 1947ApJ...105..424C Part 16: 1947ApJ...105..435C Part 17: 1947ApJ...105..441C Part 18: 1947ApJ...105..461C Part 19: 1947ApJ...106..143C Part 20: 1947ApJ...106..145C Part 21: 1947ApJ...106..152C Part 22: 1948ApJ...107...48C Part 23: 1948ApJ...107..188C Part 24: 1948ApJ...107..216C Part 25: 1948ApJ...108...92H