▪ Abstract The information on the chemical compositions of stars encoded in their spectra plays a central role in contemporary astrophysics. Stellar element abundances are, however, not observed: to decipher the spectral fingerprints in terms of abundances requires realistic models for the stellar atmospheres and the line-formation processes. Still today, the vast majority of abundance analyses of late-type stars rely on one-dimensional (1D), hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). In this review possible systematic errors in studies of F-, G- and K-type stars introduced by these questionable approximations are discussed. Departures from LTE are commonplace and often quite severe, in particular for low surface gravities or metallicities, with minority species and low-excitation transitions being the most vulnerable. Recently, time-dependent, 3D, hydrodynamical model atmospheres have started to be employed for stellar abundance purposes, with large differe...

New light on stellar abundance analyses: Departures from LTE and homogeneity.

The effect of neutral nonresonant collisions on the shape, shift, and intensity of atomic spectral-line profiles is reviewed. A general treatment for the study of an atomic spectral line is developed by establishing reasonable assumptions about the relevant collision processes and by finding an expression for the Fourier transform of the line profile. The authors look at parallel developments of other methods of calculation, consider special limits of practical interest, and illustrate numerical evaluations of complete line profiles. Interatomic potentials for use in line-profile calculations are described, and the problems imposed by nonadditivity and nonadiabaticity are also noted. The observation of line profiles by both conventional and tunable-laser techniques is surveyed. Representative experimental measurements of the widths and shifts of collision-broadened spectral line cores are tabulated, and the phenomena of satellites, oscillations, and power-law behavior of line wings are compared with theoretical expectations. The use of experimental results for the determination of excited atom-atom interactions, the prediction of collision broadening in stellar atmospheres, and the effect of foreign gases on laboratory standard wavelengths are also discussed.

The effect of neutral nonresonant collisions on atomic spectral lines

We demonstrate that a previously proposed model opens the route for the inclusion of refined non-Voigt profiles in spectroscopic databases and atmospheric radiative transfer codes. Indeed, this model fulfills many essential requirements: (i) it takes both velocity changes and the speed dependences of the pressure-broadening and -shifting coefficients into account. (ii) It leads to accurate descriptions of the line shapes of very different molecular systems. Tests made for pure H2, CO2 and O2 and for H2O diluted in N2 show that residuals are down to ≃ 0.2 % of the peak absorption, (except for the untypical system of H2 where a maximum residual of ±3% is reached), thus fulfilling the precision requirements of the most demanding remote sensing experiments. (iii) It is based on a limited set of parameters for each absorption line that have known dependences on pressure and can thus be stored in databases. (iv) Its calculation requires very reasonable computer costs, only a few times higher than that of a usual Voigt profile. Its inclusion in radiative transfer codes will thus induce bearable CPU time increases. (v) It can be extended in order to take line-mixing effects into account, at least within the so-called first-order approximation.

An isolated line-shape model to go beyond the Voigt profile in spectroscopic databases and radiative transfer codes

At the low temperatures achieved in cool brown dwarf and hot giant planet atmospheres, the less refractory neutral alkali metals assume an uncharacteristically prominent role in spectrum formation In particular, the wings of the Na-D (5890 A) and K I (7700 A) resonance lines come to define the continuum and dominate the spectrum of T dwarfs from 04 to 10 μm Whereas in standard stellar atmospheres the strengths and shapes of the wings of atomic spectral lines are rarely needed beyond 25 A from a line center, in brown dwarfs the far wings of the Na and K resonance lines out to thousands of angstroms detunings are important Using standard quantum chemical codes and the unified Franck-Condon model for line profiles in the quasi-static limit, we calculate the interaction potentials and the wing line shapes for the dominant Na and K resonance lines in H2- and helium-rich atmospheres Our theory has natural absorption profile cutoffs, has no free parameters, and is readily adapted to spectral synthesis calculations for stars, brown dwarfs, and planets with effective temperatures below 2000 K

http://iopscience.iop.org/article/10.1086/345412/pdf

Calculations of the Far-Wing Line Profiles of Sodium and Potassium in the Atmospheres of Substellar-Mass Objects

As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using arch ...

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Non-LTE line formation of Fe in late-type stars – III. 3D non-LTE analysis of metal-poor stars

The quantum-mechanical theory of pressure broadening of spectral lines is discussed in the framework of the adiabatic approximation, and a “Unified Franck-Condon” (UFC) line shape is derived. The UFC formula permits computation of the entire line profile and yields both the impact and the quasistatic approximations in the appropriate limits. The JWKB approximation is applied in order to simplify the general UFC formula, and Sando's theory of satellite bands is shown to be a limiting case. A simple extension of Sando's theory allows any number of Condon points to be treated in a way which includes pairwise interference effects and yields the appropriate quasistatic contribution for isolated points but yet, for an initial Maxwellian energy distribution, is almost as easy to apply as the binary quasistatic theory. The possibility of real and complex Condon points is discussed and sample calculations are given of corresponding wing profiles. Finally, satellite-band shapes are compared in various approximations for Lennard-Jones potentials.

Unified Franck-Condon treatment of pressure broadening of spectral lines

The influence of speed-changing collisions and the correlation between Doppler and collisional broadening on the spectral line profiles is considered in a unified way using the Anderson-Talman classical phase-shift theory for collisional broadening and the Galatry diffusion model for the motion of radiating particles. A general formula for the correlation function is derived in the impact limit, which yields the well known speed-dependent Voigt profile in the case when the speed-changing collisions are neglected, but the Doppler-collision correlations are taken into account. In the opposite case, when the Doppler-collision correlations are omitted, but the speed-changing collisions are included this formula becomes identical to that derived by Galatry. This formula is shown to lead to the results close to those calculated from the line profile formula recently derived by Duggan et al [ Phys. Rev. A 51 , 218 (1995)].

Speed-dependent pressure broadening and shift in the soft collision approximation

Profiles of line wings and rainbow satellites associated with optical and radiative collisions

We have generalized the combined strong and weak Dicke-narrowed spectral profile of Rautian and Sobel'man (Sov Phys Usp 1967;9:701–16) for speed-dependent broadening and shifts partially correlated with hard and/or soft velocity-changing collisions. The proposed line shape model is tested on Ar-broadened HF spectra.

A generalized speed-dependent line profile combining soft and hard partially correlated Dicke-narrowing collisions

We discuss spectral profiles from the point of view of a Boltzmann transport relaxation equation. We present an exact solution to the Fokker-Planck-type equation in the soft collision limit for the special case of a quadratic speed-dependent collisional width and shift of the line. The resultant spectral profile is compared to several related, more approximate expressions reported previously.

Józef Szudy

Papers

Unified Franck-Condon treatment of pressure broadening of spectral lines

Speed-dependent pressure broadening and shift in the soft collision approximation

Profiles of line wings and rainbow satellites associated with optical and radiative collisions

A generalized speed-dependent line profile combining soft and hard partially correlated Dicke-narrowing collisions

Spectral line shapes modeled by a quadratic speed-dependent Galatry profile