scholarly journals Tomography of cool giant and supergiant star atmospheres

2018 ◽  
Vol 610 ◽  
pp. A29 ◽  
Author(s):  
K. Kravchenko ◽  
S. Van Eck ◽  
A. Chiavassa ◽  
A. Jorissen ◽  
B. Freytag ◽  
...  
Keyword(s):  
Spectral Line ◽  
Three Dimensional ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Line Of Sight ◽  
Line Formation ◽  
Tomographic Method ◽  
1D Model ◽  
Supergiant Star

Context. Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. Aim. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths. Methods. The tomographic method is applied to one-dimensional (1D) model atmospheres and to a realistic three-dimensional (3D) radiative hydrodynamics simulation performed with CO5BOLD in order to compare their spectral line formation depths and velocity fields. Results. In 1D model atmospheres, each spectral line forms in a restricted range of optical depths. On the other hand, in 3D simulations, the line formation depths are spread in the atmosphere mainly because of temperature and density inhomogeneities. Comparison of cross-correlation function profiles obtained from 3D synthetic spectra with velocities from the 3D simulation shows that the tomographic method correctly recovers the distribution of the velocity component projected on the line of sight in the atmosphere.

scholarly journals Formation Depths of Spectral Lines in Cepheids

1995 ◽  
Vol 155 ◽  
pp. 373-374
Author(s):  
Michael D. Albrow ◽  
P. L. Cottrell
Keyword(s):  
Spectral Line ◽  
Velocity Fields ◽  
Radial Velocities ◽  
Spectral Lines ◽  
Line Formation ◽  
Ionisation Potential ◽  
Velocity Gradients ◽  
Different Levels ◽  
Line Depth

There has been a number of observational programmes that have endeavoured to investigate the atmospheric velocity fields in Cepheids (e.g., Sanford 1956, Wallerstein et al. 1992, Butler 1993). These studies measured the radial velocities of lines of different strength, excitation and ionisation potential as these provide an indication of line formation at different levels in the atmosphere. From these measurements, the presence of velocity gradients can be inferred, but determination of the magnitude of such gradients requires knowledge of the spectral line depth of formation. Through dynamical modelling we are endeavouring to ascertain what is actually being measured in the above observational programmes.

scholarly journals Three-dimensional simulations of scattering polarization and the Hanle effect in MHD chromospheric models

2014 ◽  
Vol 10 (S305) ◽  
pp. 360-367 ◽  
Author(s):  
J. Štěpán
Keyword(s):  
Recent Progress ◽  
Spectral Synthesis ◽  
Three Dimensional ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Line Formation ◽  
Spatially Resolved ◽  
Synthetic Spectra ◽  
Hanle Effect ◽  
Three Dimensional Simulations

AbstractScattering line polarization and the Hanle effect are among the most important mechanisms for diagnostics of the solar and stellar atmospheres. The fact that real stellar atmospheres are horizontally inhomogeneous makes the spectral synthesis and interpretation very challenging because the effect of thermodynamic fluctuations on spectral line polarization is entangled with the action of magnetic fields. This applies to the spatially resolved as well as to the averaged spectra. The necessary step towards the interpretation of such spectra is to study the line formation in sufficiently realistic 3D MHD models and compare the synthetic spectra with observations. This paper gives an overview of recent progress in the field of 3D NLTE synthesis of polarized spectral lines resulting from investigations with the radiative transfer code PORTA.

Tomography of the red supergiant star μ Cep

2018 ◽  
Vol 14 (S343) ◽  
pp. 441-442
Author(s):  
K. Kravchenko ◽  
A. Chiavassa ◽  
S. Van Eck ◽  
A. Jorissen ◽  
T. Merle ◽  
...  
Keyword(s):  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Photometric Variability ◽  
Tomographic Method ◽  
Supergiant Star ◽  
Atmospheric Motions

AbstractA tomographic method, aiming at probing velocity fields at depth in stellar atmospheres, is applied to the red supergiant star μ Cep and to snapshots of 3D radiative-hydrodynamics simulation in order to constrain atmospheric motions and relate them to photometric variability.

scholarly journals The relationship between photometric and spectroscopic oscillation amplitudes from 3D stellar atmosphere simulations

2021 ◽  
Author(s):  
Yixiao Zhou ◽  
Thomas Nordlander ◽  
Luca Casagrande ◽  
Meridith Joyce ◽  
Yaguang Li ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Three Dimensional ◽  
Quantitative Relationship ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Wavelength Shift ◽  
The Sun ◽  
Theoretical Derivation ◽  
Velocity Amplitude ◽  
Good Agreement

Abstract We establish a quantitative relationship between photometric and spectroscopic detections of solar-like oscillations using ab initio, three-dimensional (3D), hydrodynamical numerical simulations of stellar atmospheres. We present a theoretical derivation as proof of concept for our method. We perform realistic spectral line formation calculations to quantify the ratio between luminosity and radial velocity amplitude for two case studies: the Sun and the red giant ε Tau. Luminosity amplitudes are computed based on the bolometric flux predicted by 3D simulations with granulation background modelled the same way as asteroseismic observations. Radial velocity amplitudes are determined from the wavelength shift of synthesized spectral lines with methods closely resembling those used in BiSON and SONG observations. Consequently, the theoretical luminosity to radial velocity amplitude ratios are directly comparable with corresponding observations. For the Sun, we predict theoretical ratios of 21.0 and 23.7 ppm/[m s−1] from BiSON and SONG respectively, in good agreement with observations 19.1 and 21.6 ppm/[m s−1]. For ε Tau, we predict K2 and SONG ratios of 48.4 ppm/[m s−1], again in good agreement with observations 42.2 ppm/[m s−1], and much improved over the result from conventional empirical scaling relations which gives 23.2 ppm/[m s−1]. This study thus opens the path towards a quantitative understanding of solar-like oscillations, via detailed modelling of 3D stellar atmospheres.

scholarly journals FBILI method for the two-level atom line formation in media with low velocity fields

2014 ◽  
pp. 53-67
Author(s):  
I. Pirkovic ◽  
O. Atanackovic
Keyword(s):  
Finite Thickness ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Benchmark Problems ◽  
Line Formation ◽  
Convergence Properties ◽  
Low Velocity ◽  
Level Atom ◽  
Line Transfer ◽  
Transfer Problems

In this paper we generalized the fast convergent Forth-and-Back Implicit Lambda Iteration (FBILI) method to the solution of the two-level atom line transfer problems in media with low velocity fields using the observer?s reference frame. In order to test the accuracy and the convergence properties of the method we solved several astrophysically important benchmark problems of the NLTE line formation: in a plan-parallel differentially expanding medium of finite thickness, and in spherically symmetric stellar atmospheres, both static and expanding. We compared our solutions with those obtained by other authors using different numerical methods.

scholarly journals Observations of velocity fields in WN and Of stars

1979 ◽  
Vol 83 ◽  
pp. 475-478
Author(s):  
Virpi S. Niemelä
Keyword(s):  
Velocity Field ◽  
Large Scale ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Emission Lines ◽  
Temperature Structure ◽  
Early Type ◽  
Velocity Gradients ◽  
Early Type Stars

Systematic wavelength shifts of series of spectral line centers observed in many early type stars, generally interpreted as due to large scale motions, can give us information about the velocity gradients in stellar atmospheres. However, it should be borne in mind that the velocity gradients inferred from the observed displacements of spectral lines may not correspond to a unique alternative (e.g. see Karp 1978). Also, and especially when we are dealing with stars which have emission lines in their spectra, the structure of the velocity field depends on the assumed temperature structure of the atmosphere, i.e. in which atmospheric region do the lines originate.

scholarly journals Formation of polarized spectral lines in atmospheres with horizontal inhomogeneities

2014 ◽  
Vol 10 (S305) ◽  
pp. 401-406
Author(s):  
A. Tichý ◽  
J. Štěpán ◽  
J. Trujillo Bueno ◽  
J. Kubát
Keyword(s):  
Spectral Line ◽  
Linear Polarization ◽  
Transfer Problem ◽  
Resonant Scattering ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Spectral Line Intensity ◽  
Line Radiation ◽  
Radiative Transfer Problem ◽  
3D Geometry

AbstractWe study the problem of the generation and transfer of spectral line intensity and polarization in models of stellar atmospheres with horizontal plasma inhomogeneities. We solve the non-LTE radiative transfer problem in full 3D geometry taking into account resonant scattering polarization and its modification by magnetic fields via the Hanle effect. We show that horizontal fluctuations of the thermodynamical conditions of stellar atmospheres can have a significant impact on the linear polarization of the emergent spectral line radiation and its center-to-limb variation.

scholarly journals Velocity fields in and around sunspots at the highest resolution

2010 ◽  
Vol 6 (S273) ◽  
pp. 204-211
Author(s):  
Carsten Denker ◽  
Meetu Verma
Keyword(s):  
Feature Tracking ◽  
Three Dimensional ◽  
Velocity Fields ◽  
Dimensional Structure ◽  
Line Of Sight ◽  
Proper Motions ◽  
Magnetic Shear ◽  
Flow Channels ◽  
Review Reports

AbstractThe flows in and around sunspots are rich in detail. Starting with the Evershed flow along low-lying flow channels, which are cospatial with the horizontal penumbral magnetic fields, Evershed clouds may continue this motion at the periphery of the sunspot as moving magnetic features in the sunspot moat. Besides these well-ordered flows, peculiar motions are found in complex sunspots, where they contribute to the build-up or relaxation of magnetic shear. In principle, the three-dimensional structure of these velocity fields can be captured. The line-of-sight component of the velocity vector is accessible with spectroscopic measurements, whereas local correlation or feature tracking techniques provide the means to assess horizontal proper motions. The next generation of ground-based solar telescopes will provide spectropolarimetric data resolving solar fine structure with sizes below 50 km. Thus, these new telescopes with advanced post-focus instruments act as a ‘zoom lens’ to study the intricate surface flows associated with sunspots. Accompanied by ‘wide-angle’ observations from space, we have now the opportunity to describe sunspots as a system. This review reports recent findings related to flows in and around sunpots and highlights the role of advanced instrumentation in the discovery process.

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