scholarly journals A pure geometric approach to stellar structure

Open Physics ◽  
2013 ◽  
Vol 11 (7) ◽  
Author(s):  
Mamdouh Wanas ◽  
Samah Ammar
Keyword(s):  
Main Sequence ◽  
Convection Zone ◽  
Geometric Approach ◽  
Stellar Structure ◽  
Interior Solution ◽  
Main Sequence Stars ◽  
Field Equations ◽  
Exterior Field ◽  
Symmetric Configuration ◽  
Physical Information

AbstractThe present work represents a step in dealing with stellar structure using a pure geometric approach. Geometric field theory is used to construct a model for a spherically symmetric configuration. In this case, two solutions have been obtained for the field equations. The first represents an interior solution which may be considered as a pure geometric one in the sense that the tensor describing the material distributions is not a phenomenological object, but a part of the geometric structure used. A general equation of state for a perfect fluid, is obtained from, and not imposed on, the model. The second solution gives rise to Schwarzschild exterior field in its isotropic form. The two solutions are matched, at a certain boundary, to evaluate the constants of integration. The interior solution obtained shows that there are different zones characterizing the configuration: a central radiation dominant zone, a probable convection zone as a physical interpretation of the singularity of the model, and a corona like zone. The model may represent a type of main sequence stars. The present work shows that Einstein’s geometerization scheme can be extended to gain more physical information within material distribution, with some advantages.

A pure geometric approach to stellar structure: Mass–radius relation

2018 ◽  
Vol 15 (08) ◽  
pp. 1850134
Author(s):  
M. I. Wanas ◽  
Samah A. Ammar ◽  
Mona M. Foda
Keyword(s):  
Binary Systems ◽  
Geometric Approach ◽  
Stellar Model ◽  
Stellar Structure ◽  
Theoretical Expression ◽  
Energy Tensor ◽  
Main Sequence Stars ◽  
Field Equations ◽  
Material Energy ◽  
Stellar Configuration

This paper represents the second step towards understanding stellar structure using pure geometric tools. It is an attempt to get a theoretical expression for a mass–radius relation. The stellar model used has been obtained as an analytic solution of the field equations of a pure geometric field theory. The method suggested to get this relation is very simple. It depends mainly on a set of differential equations implying the vanishing of all components of a geometric material-energy tensor on a boundary of stellar configuration. The theoretical relation obtained is a linear one [Formula: see text] with one free parameter [Formula: see text] Comparison with observation, using a sample of lower main-sequence stars, members of binary systems, is given. For the primary members [Formula: see text], we get [Formula: see text]. It is worthy of mention that the model obtained is a simple one. Rotation, magnetic field, etc. are not considered in the present treatment. So, the model is far from being complete. It is just a step to show that pure geometric consideration objects can be used to treat problems of stellar structure.

scholarly journals Modelling of an Eclipsing RS CVn Binary: V405 And

2011 ◽  
Vol 7 (S282) ◽  
pp. 199-200
Author(s):  
Krisztián Vida ◽  
Katalin Oláh ◽  
Zsolt Kővári
Keyword(s):  
Light Curve ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Light Curves ◽  
Stellar Structure ◽  
Main Sequence Stars ◽  
M Dwarfs ◽  
Primary Star ◽  
Convective Envelope ◽  
Eclipsing Binary

AbstractV405 And is an ultrafast-rotating (Prot ≈ 0.46 days) eclipsing binary. The system consists of a primary star with radiative core and convective envelope, and a fully convective secondary. Theories have shown that stellar structure can depend on magnetic activity, i.e., magnetically active M-dwarfs should have larger radii. Earlier light curve modelling of V405 And indeed showed this behaviour: we found that the radius of the primary is significantly larger than the theoretically predicted value for inactive main sequence stars (the discrepancy is the largest of all known objects), while the secondary fits well to the mass-radius relation. By modelling our recently obtained light curves, which show significant changes of the spotted surface of the primary, we can find further proof for this phenomenon.

scholarly journals 2-D and 3-D models of convective turbulence and oscillations in intermediate-mass main-sequence stars

2015 ◽  
Vol 11 (A29B) ◽  
pp. 540-543
Author(s):  
Joyce A. Guzik ◽  
T. H. Morgan ◽  
N. J. Nelson ◽  
C. Lovekin ◽  
K. Kosak ◽  
...  
Keyword(s):  
Main Sequence ◽  
Convection Zone ◽  
Low Frequency ◽  
Main Sequence Stars ◽  
Low Frequencies ◽  
Rotation Rates ◽  
Multidimensional Modeling ◽  
Core Convection ◽  
Radiative Zone

AbstractWe present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid γ Dor/δ Sct variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models are calculated using the 2-D pulsation code NRO. We present new insights into pulsations of 1-2 M⊙ stars gained by multidimensional modeling.

scholarly journals The thermodynamics of collapsars

2016 ◽  
Author(s):  
Trevor W. Marshall
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Neutron Stars ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Field Equations ◽  
Gravitational Radius ◽  
Final Density ◽  
Shell Models ◽  
Independent Case

This article argues that there is a consistent description of gravitationally collapsed bodies, including neutron stars above the Tolman-Oppenheimer-Volkoff mass and also supermassive galactic centres, according to which collapse stops before the object reaches its gravitational radius, the density reaching a maximum close to the surface and then decreasing towards the centre. Models for such shell-like objects have been constructed using classic formulations found in the 1939 articles of Oppenheimer-Volkoff and Oppenheimer-Snyder. It was possible to modify the conclusions of the first article by changing the authors’ boundary conditions at r = 0. In the second case we find that the authors’ solution of the field equations needs no changes, but that the choice of their article’s title led many of their successors to believe that it supports the black-hole hypothesis. However, it is easily demonstrated that their final density distribution accords with the shell models found in our articles. Because black holes, according to many formulations, "have no hair", their thermodynamics is rather simple. The kind of collapsar which our models describe are more like main-sequence stars; they have spatiotemporal distributions of pressure, density and temperature, that is they have hair. In this article we shall concentrate on the dynamics of the Oppenheimer-Snyder collapsar; both pressure and temperature are everywhere zero, so there can be no thermodynamics. Only in the time independent case of Oppenheimer-Volkoff type models is it currently feasible to consider some thermodynamic implications; here some valuable new insights are obtained through the incorporation of the Oppenheimer-Snyder dynamics.

scholarly journals Stellar models with self-consistent Rosseland opacities

2021 ◽  
Vol 646 ◽  
pp. L6
Author(s):  
A. Hui-Bon-Hoa
Keyword(s):  
Cross Sections ◽  
Metal Content ◽  
Main Sequence ◽  
Relative Difference ◽  
Stellar Structure ◽  
Main Sequence Stars ◽  
Metal Mixture ◽  
Consistent Model ◽  
Self Consistent ◽  
Structural Differences

Context. The building of a stellar structure requires knowing the Rosseland mean opacity at each layer of the model. This mean opacity is very often interpolated in pre-computed tables due to the overwhelming time to compute it from monochromatic cross sections. The main drawback to using tables is that the opacities can be inconsistent with the actual local chemical composition, for instance in the regions of the star where nucleosynthesis occurs. Aims. We study the effects of self-consistent Rosseland mean opacity calculations on the stellar structure and evolution, in comparison with models where the metal mixture remains equal to the initial one. Methods. We developed a strategy that allows very fast calculations of Rosseland opacities from monochromatic cross sections. We are then able to compute evolutionary tracks with models whose Rosseland opacities are fully consistent with the chemical mix everywhere in the star. This method has been implemented in the Toulouse-Geneva evolution code. Results. Our self-consistent models show very small structural differences compared to models where the Rosseland opacity is computed with a fixed metal mixture. As a consequence, the main-sequence evolutionary tracks are almost the same for models of mass ranging from 2 to 8 M⊙. At a given surface gravity the relative difference in age is lower than 2% and generally below 1% between the two kinds of calculations, the self-consistent model being younger most of the time. Unless such a precision in age is sought out, the use of tabulated Rosseland opacities with a metal content defined globally is still acceptable, at least in main-sequence stars where the chemical mix changes only through nucleosynthesis.

scholarly journals Theoretical properties of regularities in the oscillation spectra of A-F main-sequence stars

2013 ◽  
Vol 9 (S301) ◽  
pp. 89-92 ◽  
Author(s):  
Juan Carlos Suárez ◽  
Antonio García Hernández ◽  
Andrés Moya ◽  
Carlos Rodrigo ◽  
Enrique Solano ◽  
...  
Keyword(s):  
Main Sequence ◽  
Low Frequency ◽  
Average Density ◽  
Stellar Structure ◽  
Main Sequence Stars ◽  
Mode Identification ◽  
Pulsating Stars ◽  
The Mean ◽  
Scuti Stars ◽  
Δ Scuti Stars

AbstractWe study the theoretical properties of the regular spacings found in the oscillation spectra of δ Scuti stars. A linear relation between the large separation and the mean density is predicted to be found in the low-frequency domain (i.e. radial orders spanning from 1 to 8, approximately) of the main-sequence δ Scuti stars' oscillation spectrum. This implies an independent direct measure of the average density of δ Scuti stars, analogous to that of the Sun, and places tight constraints on the mode identification and hence on the stellar internal structure and dynamics, and allows a determination the radii of planets orbiting around δ Scuti stars with unprecedented precision. This opens the way for studying the evolution of regular patterns in pulsating stars, and its relation to stellar structure and evolution.

scholarly journals Asteroseismology of solar-type stars

2019 ◽  
Vol 16 (1) ◽  
Author(s):  
Rafael A. García ◽  
Jérôme Ballot
Keyword(s):  
Main Sequence ◽  
Main Sequence Stars ◽  
Stellar Oscillations ◽  
Global Properties ◽  
Physical Information ◽  
Solar Type ◽  
Stellar Interiors ◽  
Surface Convection ◽  
Observational Techniques ◽  
External Characteristics

Abstract Until the last few decades, investigations of stellar interiors had been restricted to theoretical studies only constrained by observations of their global properties and external characteristics. However, in the last 30 years the field has been revolutionized by the ability to perform seismic investigations of stellar interiors. This revolution begun with the Sun, where helioseismology has been yielding information competing with what can be inferred about the Earth’s interior from geoseismology. The last two decades have witnessed the advent of asteroseismology of solar-like stars, thanks to a dramatic development of new observing facilities providing the first reliable results on the interiors of distant stars. The coming years will see a huge development in this field. In this review we focus on solar-type stars, i.e., cool main-sequence stars where oscillations are stochastically excited by surface convection. After a short introduction and a historical overview of the discipline, we review the observational techniques generally used, and we describe the theory behind stellar oscillations in cool main-sequence stars. We continue with a complete description of the normal mode analyses through which it is possible to extract the physical information about the structure and dynamics of the stars. We then summarize the lessons that we have learned and discuss unsolved issues and questions that are still unanswered.

scholarly journals Element Stratification in Main Sequence Stars and its Effect on Stellar Oscillations

2004 ◽  
Vol 193 ◽  
pp. 413-421
Author(s):  
Sylvie Vauclair
Keyword(s):  
Thermal Gradient ◽  
Main Sequence ◽  
Stellar Structure ◽  
Combined Effects ◽  
Main Sequence Stars ◽  
Stellar Oscillations ◽  
Peculiar Stars ◽  
Solar Type ◽  
Abundance Anomalies ◽  
Radiative Acceleration

AbstractElement settling due to the combined effects of gravity, thermal gradient, radiative acceleration and concentration gradient may lead to important abundance variations inside the stars that cannot be neglected in the computation of stellar structure. These processes were first introduced to account for abundance anomalies in “peculiar stars”, but their importance in the so-called “normal” stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. These microscopic processes work in competition with macroscopic motions, such as rotation-induced mixing or mass loss, which increase the settling timescales. We have recently obtained clear evidence that asteroseismology of main sequence solar-type stars can give signatures of the chemical variations inside the stars and provide a better understanding of these processes.

scholarly journals The Boundary between the Magnetic Fields of Ap Stars and the Fields of Solar-type Stars

1991 ◽  
Vol 130 ◽  
pp. 342-346
Author(s):  
John D. Landstreet
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Convection Zone ◽  
Frequency Of Occurrence ◽  
Main Sequence Stars ◽  
Chemically Peculiar ◽  
Solar Type ◽  
F Stars ◽  
Ap Stars

AbstractThe boundary between Ap-type magnetic fields and the magnetic fields of solar-type stars occurs near Te ~ 7000K, about where deep envelope convection develops in main sequence stars. This seems natural for solar-type stars, in which the field is generated by the convection zone. However, among magnetic Ap stars the frequency of occurrence declines from about 10% of all A stars near A0 to about 1% near F0. It is not clear what produces this decline in frequency, but the convection zone is probably not responsible. In fact, it seems likely that if global fossil fields occur in main sequence F stars, such fields should be detectable even if the stars having them are not chemically peculiar.

Chromospheric activity as a function of age in main-sequence stars

1966 ◽  
Vol 24 ◽  
pp. 40-43
Author(s):  
O. C. Wilson ◽  
A. Skumanich
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Main Sequence Stars ◽  
Tentative Conclusion ◽  
Chromospheric Activity ◽  
General Field ◽  
Large Clusters

Evidence previously presented by one of the authors (1) suggests strongly that chromospheric activity decreases with age in main sequence stars. This tentative conclusion rests principally upon a comparison of the members of large clusters (Hyades, Praesepe, Pleiades) with non-cluster objects in the general field, including the Sun. It is at least conceivable, however, that cluster and non-cluster stars might differ in some fundamental fashion which could influence the degree of chromospheric activity, and that the observed differences in chromospheric activity would then be attributable to the circumstances of stellar origin rather than to age.

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