scholarly journals On The Seismic Signature of the HeII Ionization Zone in Stellar Envelopes

1998 ◽  
Vol 185 ◽  
pp. 317-318
Author(s):  
Mário J.P.F.G. Monteiro ◽  
Michael J. Thompson
Keyword(s):  
Equation Of State ◽  
Stellar Structure ◽  
Ionization Zone ◽  
Characteristic Signal ◽  
Oscillation Frequencies ◽  
Stellar Envelopes ◽  
Seismic Signature ◽  
Better Than

Sharp variations of the structure of the star create a characteristic signal in its frequencies of oscillation (e.g. [3]). The zone of the second ionization of helium is such a localized feature of the structure whose properties depend mainly on the abundance of helium and the equation of state. Considering that such a signal should easily be detectable provided the frequencies are measured to rather better than 1μHz accuracy (the COROT project should measure oscillation frequencies with an accuracy of 0.1μHz), we present here a tool to study this aspect of stellar structure.

scholarly journals Asteroseismic analysis of 15 solar-like oscillating evolved stars

2021 ◽  
Vol 503 (3) ◽  
pp. 4529-4536
Author(s):  
Z Çelik Orhan ◽  
M Yıldız ◽  
C Kayhan
Keyword(s):  
Stellar Structure ◽  
Scaling Relations ◽  
Space Telescopes ◽  
Ionization Zone ◽  
Fundamental Parameters ◽  
Evolved Stars ◽  
Large Numbers ◽  
Analyse Data ◽  
Oscillation Frequencies

ABSTRACT Asteroseismology using space-based telescopes is vital to our understanding of stellar structure and evolution. CoRoT, Kepler, and TESS space telescopes have detected large numbers of solar-like oscillating evolved stars. Solar-like oscillation frequencies have an important role in the determination of fundamental stellar parameters; in the literature, the relations between the two is established by the so-called scaling relations. In this study, we analyse data obtained from the observation of 15 evolved solar-like oscillating stars using the Kepler and ground-based telescopes. The main purpose of the study is to determine very precisely the fundamental parameters of evolved stars by constructing interior models using asteroseismic parameters. We also fit the reference frequencies of models to the observational reference frequencies caused by the He ii ionization zone. The 15 evolved stars are found to have masses and radii within ranges of 0.79–1.47 M⊙ and 1.60–3.15 R⊙, respectively. Their model ages range from 2.19 to 12.75 Gyr. It is revealed that fitting reference frequencies typically increase the accuracy of asteroseismic radius, mass, and age. The typical uncertainties of mass and radius are ∼3–6 and ∼1–2 per cent, respectively. Accordingly, the differences between the model and literature ages are generally only a few Gyr.

scholarly journals Diffusion in CP Stars: The Quest for Accuracy

1998 ◽  
Vol 11 (2) ◽  
pp. 671-673
Author(s):  
G. Alecian
Keyword(s):  
Computational Methods ◽  
Stellar Evolution ◽  
Diffusion Processes ◽  
Stellar Atmospheres ◽  
Time Dependent ◽  
Data Bases ◽  
Self Consistent ◽  
Stellar Envelopes ◽  
Better Than

We present a brief review about recent progresses concerning the study of diffusion processes in CP stars. The most spectacular of them concerns the calculation of radiative accelerations in stellar envelopes for which an accuracy better than 30% can now be reached for a large number of ions. This improvement is mainly due to huge and accurate atomic and opacity data bases available since the beginning of the 90’s. Developments of efficient computational methods have been carried out to take advantage of these new data. These progresses have, in turn, led to a better understanding of how the element stratification is building up with time. A computation of self-consistent stellar evolution models, including time-dependent diffusion, can now be within the scope of the next few years. However, the progresses previously mentioned do not apply for stellar atmospheres and upper layers of envelopes.

OF CHARGED STARS AND CHARGED BLACK HOLES

2004 ◽  
Vol 13 (07) ◽  
pp. 1375-1379 ◽  
Author(s):  
MANUEL MALHEIRO ◽  
RODRIGO PICANÇO ◽  
SUBHARTHI RAY ◽  
JOSÉ P. S. LEMOS ◽  
VILSON T. ZANCHIN
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Charged Particle ◽  
Equation Of State ◽  
Mass Density ◽  
Compact Star ◽  
Maximum Amount ◽  
Stellar Structure ◽  
Charged Black Holes ◽  
Polytropic Equation

Effect of maximum amount of charge a compact star can hold, is studied here. We analyze the different features in the renewed stellar structure and discuss the reasons why such huge charge is possible inside a compact star. We studied a particular case of a polytropic equation of state (EOS) assuming the charge density is proportional to the mass density. Although the global balance of force allows a huge charge, the electric repulsion faced by each charged particle forces it to leave the star, resulting in the secondary collapse of the system to form a charged black hole.

The Equation of State and the Temperature, Pressure, and Shear Dependence of Viscosity for a Highly Viscous Reference Liquid, Dipentaerythritol Hexaisononanoate

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Scott Bair ◽  
Tsuyoshi Yamaguchi
Keyword(s):  
Molecular Weight ◽  
Shear Stress ◽  
Equation Of State ◽  
High Molecular Weight ◽  
Free Volume ◽  
Newtonian Limit ◽  
Scaling Models ◽  
The Difference ◽  
Better Than ◽  
Low Shear

Measurements are reported for dipentaerythritol hexaisononanoate (DiPEiC9) of pressure–volume–temperature (pVT) response to pressures to 400 MPa and temperatures to 100 °C, and of viscosity at pressures to 700 MPa and temperatures to 90 °C and shear stress to 18 MPa. These data complement the low-shear viscosities published by Harris to pressures to 200 MPa and the compressions by Fandiño et al. to 70 MPa. The improved Yasutomi correlation reproduces all viscosity measurements with accuracy better than the Doolittle free volume and the Bair and Casalini thermodynamic scaling models which require an equation of state (EoS). The interaction parameter for thermodynamic scaling, γ = 3.6, is less than that reported by Harris (γ = 4.2) and the difference is primarily in the choice of EoS. The shear stress at the Newtonian limit, about 6 MPa, is exceptionally large given the high molecular weight of DiPEiC9. The large Newtonian limit is also seen in the oscillatory shear response.

scholarly journals Fundamental properties of Kepler and CoRoT targets – IV. Masses and radii from frequencies of minimum Δν and their implications

2019 ◽  
Vol 489 (2) ◽  
pp. 1753-1769 ◽  
Author(s):  
M Yıldız ◽  
Z Çelik Orhan ◽  
C Kayhan
Keyword(s):  
Rotational Velocity ◽  
Galactic Disc ◽  
Ionization Zone ◽  
Chemical Enrichment ◽  
Fundamental Properties ◽  
Diagnostic Potential ◽  
The Difference ◽  
The Masses ◽  
Oscillation Frequencies

ABSTRACT Recently, by analysing the oscillation frequencies of 90 stars, Yıldız, Çelik Orhan & Kayhan have shown that the reference frequencies (νmin0, νmin1, and νmin2) derived from glitches due to He ii ionization zone have very strong diagnostic potential for the determination of their effective temperatures. In this study, we continue to analyse the same stars and compute their mass, radius, and age from different scaling relations including relations based on νmin0, νmin1, and νmin2. For most of the stars, the masses computed using νmin0 and νmin1 are very close to each other. For 38 stars, the difference between these masses is less than 0.024 M$\odot$. The radii of these stars from νmin0 and νmin1 are even closer, with differences of less than 0.007 R$\odot$. These stars may be the most well known solar-like oscillating stars and deserve to be studied in detail. The asteroseismic expressions we derive for mass and radius show slight dependence on metallicity. We therefore develop a new method for computing initial metallicity from this surface metallicity by taking into account the effect of microscopic diffusion. The time dependence of initial metallicity shows some very interesting features that may be important for our understanding of chemical enrichment of Galactic Disc. According to our findings, every epoch of the disc has its own lowest and highest values for metallicity. It seems that rotational velocity is inversely proportional to 1/2 power of age as given by the Skumanich relation.

scholarly journals Modeling the global oscillations of Epsilon Eridani

1988 ◽  
Vol 123 ◽  
pp. 281-284
Author(s):  
David R. Soderblom ◽  
Werner Däppen
Keyword(s):  
Observational Data ◽  
Stellar Structure ◽  
Global Oscillation ◽  
Oscillation Frequencies ◽  
Best Estimates

We have reviewed the observational data for ∊ Eri to derive our best estimates of luminosity, radius, temperature, composition, and age, as well as the uncertainties associated with each of these. These quantities are then used in stellar structure models to try to reproduce the global oscillation frequencies observed by Noyes et al. We find that we can reproduce the observed frequencies and splittings at least as well as the computations of Guenther and Demarque, yet for significantly different stellar parameters, most notably the age.

Equation of state of water based on the SCAN meta-GGA density functional

2020 ◽  
Vol 22 (8) ◽  
pp. 4626-4631 ◽  
Author(s):  
Gang Zhao ◽  
Shuyi Shi ◽  
Huijuan Xie ◽  
Qiushuang Xu ◽  
Mingcui Ding ◽  
...  
Keyword(s):  
Molecular Dynamics ◽  
Equation Of State ◽  
Ab Initio ◽  
Molecular Dynamics Simulations ◽  
Density Functional ◽  
State Of Water ◽  
Water Based ◽  
Dynamics Simulations ◽  
Better Than

By ab initio molecular dynamics simulations, the newly developed SCAN meta-GGA functional is proved better than the widely used PBE-GGA functional in describing the equation of state of water.

STELLAR STRUCTURE FROM QCD

2012 ◽  
Vol 18 ◽  
pp. 91-95
Author(s):  
BRUNO FRANZON ◽  
F. S. NAVARRA ◽  
DAVID FOGAÇA
Keyword(s):  
Equation Of State ◽  
Quark Gluon Plasma ◽  
Mean Field ◽  
Gluon Plasma ◽  
Field Approximation ◽  
Compact Stars ◽  
Stellar Structure ◽  
Mean Field Approximation

Using an equation of state based on a mean-field approximation for QCD (MQCD) to describe the cold quark gluon plasma we study the stellar structure of compact stars.

A modification of the Redlich-Kwong-Soave equation of state and the determination of its parameters on the basis of saturated vapour pressures and second virial coefficients of pure substances

1989 ◽  
Vol 54 (6) ◽  
pp. 1446-1463 ◽  
Author(s):  
Petr Voňka ◽  
Pavel Dittrich ◽  
Josef P. Novák
Keyword(s):  
Equation Of State ◽  
Virial Coefficients ◽  
Effective Algorithm ◽  
Liquid Equilibrium ◽  
Second Virial Coefficients ◽  
Saturated Vapour ◽  
Pure Substances ◽  
Better Than ◽  
Wagner Equation

The temperature dependence of parameter a = a(Tr) of the Redlich-Kwong-Soave equation of state was modified. To calculate the corresponding individual parameters, an effective algorithm applying the Newton method was proposed. The parameters were determined for 60 substances, and the new modification correlates saturated vapour pressures from the values of pr = 0.001 to pr = 1.0 with the accuracy which is usually better than 0.2% and is comparable with that attained in terms of the Wagner equation. The modification proposed is utilized above all when applying the equation of state to the calculation of vapour-liquid equilibrium.

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