Velocity fields in stellar atmospheres and the concept of microturbulence

1973 ◽  
Vol 15 ◽  
pp. 39-60 ◽  
Author(s):  
Gordon Worrall ◽  
Alistair M. Wilson
Keyword(s):  
Velocity Fields ◽  
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 Observing convection in stellar atmospheres

2006 ◽  
Vol 2 (S239) ◽  
pp. 103-112
Author(s):  
John D. Landstreet
Keyword(s):  
Velocity Field ◽  
Stellar Atmosphere ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Line Profiles ◽  
Line Centre ◽  
Solar Granulation ◽  
Voigt Profile ◽  
Microturbulent Velocity ◽  
Granulation Pattern

AbstractConvection occurs in the visible photospheric layers of most stars having Te less than about 10000 K, and in some hotter stars. The solar granulation pattern is a symptom of this, as is the non-zero microturbulent velocity often required in abundance analysis to make both weak and strong lines yield the same abundance.In very sharp-lined stars, the presence of a non-thermal velocity field in the visible stellar atmosphere leads to several other effects which may be detected in spectral line profiles. These include radial velocities that vary systematically with equivalent width, distortions of the line profile as compared to a profile computed with a Voigt profile and rotational broadening (“macroturbulence”), and asymmetries with respect to the line centre (“bisector curvature”).Detection and interpretation of these effects, with the goal of obtaining empirical information about a velocity field present in the visible layers, requires comparison with calculated synthetic spectra which incorporate model velocity fields. Thus, this review will summarize some of the observational clues concerning photospheric velocity fields, as well as modelling aimed at interpreting these data.

scholarly journals Summary-Introduction

1960 ◽  
Vol 12 ◽  
pp. 1-43
Author(s):  
J.C. Pecker ◽  
R. N. Thomas
Keyword(s):  
Radiation Field ◽  
Degrees Of Freedom ◽  
Energy Transport ◽  
Atmospheric Model ◽  
Relative Energy ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Primary Concern ◽  
Stellar Rotation ◽  
Thermodynamic State

This paper is an introduction to the astronomical material underlying the Varenna Symposium on Aerodynamical Phenomena in Stellar Atmospheres. The term « aerodynamical phenomena » rather than simply « velocity fields » is used in the title of the symposium to imply that primary concern centers as much on the physical phenomena and consequences associated with the presence of velocity fields as it does simply on the velocity fields themselves. To fully appreciate this distinction between aerodynamical phenomena and velocity fields from the astronomer's viewpoint, one must consider it against the background of the classical theory (*) of stellar atmospheres, which assumes that all the properties of the atmosphere are strictly controlled by the radiation field. The thermodynamic state of the classical atmosphere is fixed by the three conditions of radiative equilibrium (no energy transport other than by radiation), hydrostatic equilibrium (no mechanical momentum transport), and local thermodynamic equilibrium at a temperature fixed by the local energy-density of the radiation field (complete coupling between radiation field and atomic degrees of freedom). Analyses of stellar spectra under the framework of this classical atmospheric model take account of the presence of velocity fields (other than thermal) only in their effect upon the atomic absorption coefficient, not in their energetic or momentum coupling to the thermodynamic state of the atmosphere. Thus, if we become interested in aerodynamic phenomena in stellar atmospheres, we must investigate the possible perturbation these velocity fields may have upon the thermodynamic state of the atmosphere. We develop a primary concern with differential motions, velocity gradients, and dissipation mechanisms — all quantities which may produce a local non-relative energy source — rather than directing our attention only at stellar rotation and uniform expansion of an atmosphere. Thus, what we call aerodynamic phenomena embraces not only velocity fields but also their influence upon the thermodynamic state of the atmosphere.

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 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.

B. The Solar Atmospheric Convection Zone

1967 ◽  
Vol 28 ◽  
pp. 347-404 ◽  
Author(s):  
Edward A. Spiegel ◽  
Karl-Heinz Böhm
Keyword(s):  
Convective Instability ◽  
Convection Zone ◽  
Physical Structure ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Solar Photosphere ◽  
Theoretical Problem ◽  
General Area ◽  
Physical Insight ◽  
Made In

Observational progress has, in the last decade, greatly surpassed theoretical progress in our study of the non-thermal velocity fields in the solar photosphere and chromosphere. This differential progress has not been simply a matter of filling in details. Attempted formulation of the theoretical problem of the velocity fields arising from convective instability in stellar atmospheres has resulted in no really new conceptual developments, nor large clarifications in approach, to match the observational ones just summarized by Noyes. Discussion of the problem at Varenna consisted mainly in the expression by aerodynamicists of severe reservations on the likelihood of gaining deep physical insight, or reliable numerical results, from the mixing-length approach generally used in astrophysics, with essentially no suggestions of a better approach. What progress that has been made in the general area of convective problems similar to those of interest in stellar atmospheres lies in the exploration of the physical structure of simplified convective configurations, often by exploration of various intuitive approaches.

scholarly journals Nonradial Oscillations: The Cause of Macroturbulence in Late-Type Stars?

1980 ◽  
Vol 58 ◽  
pp. 307-312
Author(s):  
Myron A. Smith
Keyword(s):  
Fundamental Problem ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Line Formation ◽  
Line Broadening ◽  
Late Type ◽  
Solar Granulation ◽  
Short Period ◽  
The Many ◽  
Primary Velocity

A fundamental problem in contemporary stellar atmospheres research concerns the cause of what the spectroscoplst calls “macroturbulence.” Even in so well studied a star as the Sun, it is unclear as to which of the many resolved velocity fields is most responsible for the broadening of the disk-integrated spectrum. There are several uncertainties attached to the identification of this primary velocity field. To cite one, Beckers (1980) indicates in a recent review that the two principal contributors to macroturbulence, convective granulation and the five-minute nonradial oscilltion pattern, each add only an r.m. s. velocity of 1/2 km s at τ5000 = 0.1. According to him, even when they are put together with related unresolved patterns [e.g. subgranulation and short period (<30) oscilltions] the sum of all known velocities seems to fall short of macroturbulence obtained from line broadening studies [~ 3 km s-1; radial-tangential model (Gray 1977, Smith 1978)]. The most recent models of the solar granulation field (Keil 1980) suggest somewhat higher velocities, e.g. 1.1 km s-1 at τ5000 = 0.1, when revised corrections for terrestrial seeing are taken into account. Nonetheless, such corrections must be added to both the convection and oscillation amplitudes, so it is still not clear whether one of these fields dominates the line formation.

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.

Sign in / Sign up

Export Citation Format

Share Document