scholarly journals Non-LTE line blanketed atmospheres for hot stars

1997 ◽  
Vol 189 ◽  
pp. 209-216
Author(s):  
D. J. Hillier
Keyword(s):  
Atomic Data ◽  
Computational Techniques ◽  
Stellar Winds ◽  
Hot Stars ◽  
Computing Power ◽  
Plane Parallel ◽  
Spectroscopic Analyses ◽  
Recent Advances ◽  
Parallel Approximation

The modeling of hot star atmospheres falls into two broad classes: those where the plane parallel approximation can be used, and those where the effects of spherical extension and stellar winds are important. In both cases non-LTE modeling is a necessity for reliable spectroscopic analyses.While simple ions (e.g., H, He I, and He II) have been treated routinely in non-LTE for many years it is only recently that advances in computing power, computational techniques, and the availability of atomic data have made it feasible to perform non-LTE line blanketing calculations. Present models, with varying degrees of approximation and sophistication, are now capable of treating the effects of tens of thousands of lines. We review the latest efforts in incorporating non-LTE line blanketing, highlighting recent advances in the modeling of 0 stars, hot sub-dwarfs, Wolf-Rayet stars, novae, and supernovae.

scholarly journals Atmospheric parameters and accelerations in the outer parts of luminous hot stars

1989 ◽  
Vol 113 ◽  
pp. 287-288
Author(s):  
Hans Nieuwenhuijzen ◽  
Cornells de Jager
Keyword(s):  
Radiation Pressure ◽  
Dynamic Pressure ◽  
Great Part ◽  
Hydrostatic Equilibrium ◽  
Stellar Winds ◽  
Gravitational Acceleration ◽  
Atmospheric Parameters ◽  
Hot Stars ◽  
Plane Parallel ◽  
Turbulent Pressure

In the atmospheres of the most extreme luminous stars, close to the Humphreys-Davidson limit, the inward gravitational acceleration is for a great part compensated by outward accelerations due to radiation pressure, turbulent pressure and dynamic pressure of the stellar winds. As a result the effective acceleration is very small, resulting in blown-up atmospheres that can no longer be considered plane-parallel or in hydrostatic equilibrium.

scholarly journals Atmospheres and Winds of Hot Stars - Impacts of New Opacity Calculations and Continuing Needs

1995 ◽  
Vol 10 ◽  
pp. 585-587
Author(s):  
Keith Butler
Keyword(s):  
Linearization Method ◽  
Spectral Lines ◽  
The Other ◽  
Atomic Data ◽  
Stellar Winds ◽  
Function Concept ◽  
Hot Stars ◽  
Other Hand ◽  
Recent Developments ◽  
On Line

In this paper I review some recent advances in the use of large amounts of atomic data in the modelling of atmospheres and winds of hot stars. The review is highly selective but representative of current developments. A more general overview is to be found in Kudritzki and Hummer (1990) although the field is changing so rapidly that much has happened since then. The paper breaks down into three parts: work on line formation, in which the atmospheric structure is known and held fixed, is described first, then follows a description of the inclusion of line opacities in non-LTE in the atmosphere problem itself, and finally recent developments in the theory of radiatively driven stellar winds are summarized. Here special emphasis is given to a novel distance determination method based entirely on spectroscopie quantities. I close with a brief shopping list.In a series of papers, Becker and Butler (1992,1994a, b,c) have investigated iron and nickel spectra in sub-dwarfs using the complete linearization method of Auer and Heasley (1976). The method scales linearly with the number of frequency points so they were able to use well over 10000 frequencies to adequately describe the line opacities. Several thousand lines were treated explicitly and the resultant computed spectra gave execellent fits to observed Hubble spectra in the wavelength ranges dominated by the ions concerned.The different ionization stages gave consistent results for the iron and nickel abundances but only after line-blocking from millions of spectral lines in the far UV had been included. This was done using the Kurucz (1988) line lists coupled with line grouping as suggested by Anderson (1989) and described briefly in the next section.The line-blanketed atmospheres of Kurucz (1991) are the best available up to about 30000K, where non-LTE effects start to become important. Non-LTE line-blanketed atmospheres have become feasible because the computational requirements of the accelerated lambda iteration (ALI) method (Werner and Husfeld, 1985) also scale linearly with the number of frequency points. On the other hand, Anderson (1989) suggested grouping energetically adjacent atomic levels together to form pseudo-levels on the basis that although they might, as a group, be in non-LTE, they should be in LTE with respect to one another due to the large number of collisions between them. This greatly reduces the number of levels to be considered but instead gives rise to highly complicated pseudo line-profiles. Grigsby et al (1992), who did not use ALI, constructed the first grid of line-blanketed non-LTE models by using a variation on the Opacity Distribution Function concept to group line opacities into blocks thereby reducing the number of frequency points required. Dreizler and Werner (1993) on the other hand were able to sample the opacity as they used ALI in their models.

scholarly journals Theory and observations of non-thermal phenomena in hot massive binaries

1995 ◽  
Vol 163 ◽  
pp. 438-449
Author(s):  
Richard L. White ◽  
Wan Chen
Keyword(s):  
Thermal Radiation ◽  
Binary Systems ◽  
Theoretical Work ◽  
Stellar Winds ◽  
Hot Stars ◽  
Thermal Emissions ◽  
Γ Rays ◽  
Physical Effects ◽  
Thermal Phenomena ◽  
Colliding Winds

The shock between the colliding winds in binary systems containing two massive stars accelerates particles to relativistic energies. These energetic particles can produce observable non-thermal radiation from the radio to γ-rays. The important physical processes in such systems are very similar to those we have proposed for non-thermal emissions from single hot stars, which have shocks generated by instabilities in the radiatively driven stellar winds. This paper discusses the theory and observations of non-thermal radiation in the radio, X-ray, and γ-ray regions from both single stars and massive binaries. Similarities and differences between the two types of systems are outlined. We discuss two important physical effects that apparently have been neglected in previous theoretical work on colliding wind binaries.

scholarly journals Sphericity Effects of Extended Atmospheres of Late-Type Giants in the HR Diagram

1978 ◽  
Vol 80 ◽  
pp. 387-390
Author(s):  
Keiichi Kodaira
Keyword(s):  
Stellar Evolution ◽  
Stellar Atmospheres ◽  
Late Type ◽  
Stellar Radius ◽  
Know How ◽  
Atmospheric Structure ◽  
The Third ◽  
Plane Parallel ◽  
Parallel Approximation ◽  
Hr Diagram

In the late phases of stellar evolution, evolutionary tracks of stars with different masses come together along the Hayashi line in the HR diagram. The theoretical HR diagram (log L, log Teff) is accordingly partially degenerate in the domain of late-type giants and supergiants, with respect to the third parameter, the stellar mass M. The stellar radius, R, being determined by log L and log Teff, the mass determines the surface gravity log g at the radius R. These parameters enable us to transform a point in the theoretical HR diagram to the corresponding point in the empirical HR diagram MV, (R-I) or spectral type. This transformation is conventionally carried out within the framework of the plane-parallel approximation in stellar atmospheres, and the parameters for the abscissa of the empirical HR diagram are dependant upon Teffand log g alone, irrespective of the mass itself. In this case, the parameter M indirectly affects the observable quantities through log g, but the effects of a variation by Δlog g=±0.5, corresponding to Δlog M=±0.5, are almost insignificant (cf. Tsuji 1976). The transformation between the theoretical and the empirical HR diagram is, therefore, almost one-to-one, within the framework of the plane-parallel approximation. Late-type giants and supergiants, however, have moderately extended atmospheres in general (cf. Schmid-Burgk and Scholz 1975), and their photometric colors and spectra are expected to be influenced by the sphericity of the atmospheric structure. Consequently, in comparing empirical HR diagrams with theoretical ones, it is important to know how atmospheric sphericity affects the transformation in the degenerate domains of the theoretical diagram.

COMPUTATIONAL APPROACH FOR ESTIMATING HYGRIC PROPERTIES OF HETEROGENEOUS MATERIALS IN LONG-TERM ASSESSMENT OF MOISTURE-INDUCED DAMAGE

2017 ◽  
Vol 4 (1) ◽  
Author(s):  
Václav Kočí ◽  
Jiří Maděra ◽  
Robert Černý
Keyword(s):  
Building Materials ◽  
Heterogeneous Materials ◽  
Computational Time ◽  
Computational Techniques ◽  
Computing Power ◽  
One Dimensional ◽  
Dimensional Modeling ◽  
Hygric Properties

Long-term assessment of degradation processes is a very useful tool for an analysis of building materials performance. Since computational techniques are mostly used for this purpose, hygric properties of involved materials are required as substantial input data. Unfortunately, some construction details or heterogeneous materials have to be solved by means of multi-dimensional modelling which is demanding on computing power and thus the calculations may take a lot of time. The presented paper aims at determination of effective hygric properties of heterogeneous materials which would allow one-dimensional transformation. The parameter identification process is carried out on the basis of results of multi-dimensional modeling, using genetic algorithms. The main objective is to find such effective global moisture transport and accumulation functions that provide in one-dimensional modeling as similar results to multidimensional modeling as possible. The obtained functions give a very good agreement; the investigated relative humidity profiles differ only by 1.48 percentage points in average. The correctness of obtained results is also verified using the Lichtenecker's mixing rule as homogenization technique. The transformation of the original multidimensional problem into one-dimensional is found to substantially contribute to minimization of computational time, which is reduced from weeks to minutes.

scholarly journals Computational materials design of crystalline solids

2016 ◽  
Vol 45 (22) ◽  
pp. 6138-6146 ◽  
Author(s):  
Keith T. Butler ◽  
Jarvist M. Frost ◽  
Jonathan M. Skelton ◽  
Katrine L. Svane ◽  
Aron Walsh
Keyword(s):  
Functional Materials ◽  
Materials Design ◽  
Computational Techniques ◽  
Crystalline Solids ◽  
Recent Advances ◽  
Computational Materials Design ◽  
Computational Materials

Recent advances in the computational techniques and procedures for the design of functional materials are reviewed.

scholarly journals X-ray spectroscopy of massive stellar winds: previous and ongoing observations of the hot star ζ Pup

2018 ◽  
Vol 14 (S346) ◽  
pp. 88-92
Author(s):  
N. Miller ◽  
W. Waldron ◽  
J. Nichols ◽  
D. Huenemoerder ◽  
M. Dahmer ◽  
...  
Keyword(s):  
Emission Line ◽  
Line Profile ◽  
Stellar Wind ◽  
Galactic Evolution ◽  
Stellar Winds ◽  
Hot Stars ◽  
X Ray ◽  
Wind Structure ◽  
Important Impact ◽  
Emission Line Profile

AbstractThe stellar winds of hot stars have an important impact on both stellar and galactic evolution, yet their structure and internal processes are not fully understood in detail. One of the best nearby laboratories for studying such massive stellar winds is the O4I(n)fp star ζ Pup. After briefly discussing existing X-ray observations from Chandra and XMM, we present a simulation of X-ray emission line profile measurements for the upcoming 840 kilosecond Chandra HETGS observation. This simulation indicates that the increased S/N of this new observation will allow several major steps forward in the understanding of massive stellar winds. By measuring X-ray emission line strengths and profiles, we should be able to differentiate between various stellar wind models and map the entire wind structure in temperature and density. This legacy X-ray spectrum of ζ Pup will be a useful benchmark for future X-ray missions.

scholarly journals Colliding stellar winds: recent advances in modelling

1999 ◽  
Vol 193 ◽  
pp. 289-297
Author(s):  
Ian R. Stevens ◽  
Julian M. Pittard
Keyword(s):  
Line Profile ◽  
Gas Dynamics ◽  
Interaction Region ◽  
Stellar Winds ◽  
Radiation Hydrodynamics ◽  
3D Simulations ◽  
X Ray ◽  
Recent Advances

We report on recent advances in the modelling of colliding stellar winds in WR binaries. Here, we concentrate on the modelling of X-ray observations using numerical gas-dynamics, in some cases including radiation hydrodynamics, and look in detail at a few systems, such as γ2 Velorum, η Carinae and HD 193793. We also report briefly on the modelling of infrared line-profile variability in a number of close WR+O-star systems, which can reveal further information on the dynamics of the interaction region. We discuss recent 3D-simulations and describe the exciting prospects that AXAF and XMM open up.

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