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.

Competition Between Diffusion Processes and Hydrodynamical Instabilities in Stellar Envelopes

1977 ◽  
Vol 4 (2) ◽  
pp. 193-203
Author(s):  
Gérard ◽  
Sylvie Vauclair
Keyword(s):  
Diffusion Processes ◽  
Stellar Atmospheres ◽  
Isotopic Ratios ◽  
Absolute Abundance ◽  
It Diffusion ◽  
Quantitative Results ◽  
Abundance Anomalies ◽  
Macroscopic Motion ◽  
Ap Stars ◽  
Stellar Envelopes

Since the work of Michaud (1970), the abundance anomalies observed in the peculiar Ap and Am stars are increasingly believed to be a consequence of diffusion processes in stellar atmospheres or stellar envelopes. A number of the problems that seemed at first sight insoluble within the framework of diffusion processes have now been solved by it. Diffusion processes can, for example, account for anonalous helium isotopic ratios (Vauclair et al, 1974 (b)) and mercury isotopic ratios (Michaud et al, 1974). Quantitative results on abundance variations due to diffusion processes have been obtained (Michaud et al, 1976; Michaud, this conference; Alecian, 1976). They show that, in general, the relative abundance anomalies obtained from computation are close to the observed ones. It is now well established that the largest abundance anomalies observed in Ap stars (for rare earths) can be interpreted by diffusion processes with a satisfactory time scale, in a completely stable atmosphere. However, the predicted absolute abundance variations often exceed the observed ones, as in the case of Am stars. This suggests that the assumption of stability is not completely valid for the stellar gas: some kind of macroscopic motion, such as a meridional circulation or turbulence or both, must be at work and slow down the diffusion.

Meridional Circulation, Turbulence, and Diffusion Processes in Stars

1976 ◽  
Vol 32 ◽  
pp. 109-116 ◽  
Author(s):  
S. Vauclair
Keyword(s):  
Convection Zone ◽  
Diffusion Processes ◽  
Meridional Circulation ◽  
Diffusion Time ◽  
Work In Progress ◽  
First Results ◽  
Stellar Envelopes ◽  
And Diffusion ◽  
Turbulence And Diffusion ◽  
Hr Diagram

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.

scholarly journals Inclusion of a time-dependent, non-local convection theory in a stellar evolution code

2006 ◽  
Vol 2 (S239) ◽  
pp. 314-316 ◽  
Author(s):  
Achim Weiss ◽  
Martin Flaskamp
Keyword(s):  
Velocity Field ◽  
Local Time ◽  
Stellar Evolution ◽  
Time Dependent ◽  
Test Cases ◽  
The Sun ◽  
Specific Test ◽  
Non Local ◽  
Convective Boundaries

AbstractThe non-local, time-dependent convection theory of Kuhfuß (1986) in both its one- and three-equation form has been implemented in the Garching stellar evolution code. We present details of the implementation and the difficulties encountered. Specific test cases have been calculated, among them a 5 M⊙ star and the Sun. These cases point out deficits of the theory. In particular, the assumption of an isotropic velocity field leads to too extensive overshooting and has to be modified at convective boundaries. Some encouraging aspects are indicated as well.

scholarly journals Numerical Boundary Conditions for Solar Magnetic Hydrodynamic Flows Using the Method of Characteristics

1996 ◽  
Vol 154 ◽  
pp. 149-153
Author(s):  
S. T. Wu ◽  
A. H. Wang ◽  
W. P. Guo
Keyword(s):  
Boundary Conditions ◽  
Method Of Characteristics ◽  
Numerical Solutions ◽  
The Self ◽  
Time Dependent ◽  
Solar Plasma ◽  
The Method Of Characteristics ◽  
Mhd Simulations ◽  
Self Consistent ◽  
Dynamic Structures

AbstractWe discuss the self-consistent time-dependent numerical boundary conditions on the basis of theory of characteristics for magnetohydrodynamics (MHD) simulations of solar plasma flows. The importance of using self-consistent boundary conditions is demonstrated by using an example of modeling coronal dynamic structures. This example demonstrates that the self-consistent boundary conditions assure the correctness of the numerical solutions. Otherwise, erroneous numerical solutions will appear.

scholarly journals Josephson oscillations in split one-dimensional Bose gases

2021 ◽  
Vol 10 (4) ◽  
Author(s):  
Yuri Daniel van Nieuwkerk ◽  
Jörg Schmiedmayer ◽  
Fabian Essler
Keyword(s):  
Three Dimensional ◽  
Time Dependent ◽  
Dimensional Theory ◽  
Bose Gases ◽  
Microscopic Description ◽  
One Dimensional ◽  
Hartree Fock ◽  
Equilibrium Dynamics ◽  
Self Consistent ◽  
Double Well Potential

We consider the non-equilibrium dynamics of a weakly interacting Bose gas tightly confined to a highly elongated double well potential. We use a self-consistent time-dependent Hartree--Fock approximation in combination with a projection of the full three-dimensional theory to several coupled one-dimensional channels. This allows us to model the time-dependent splitting and phase imprinting of a gas initially confined to a single quasi one-dimensional potential well and obtain a microscopic description of the ensuing damped Josephson oscillations.

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.

A catalog of 159,238 white dwarf ages

2019 ◽  
Vol 15 (S357) ◽  
pp. 188-191
Author(s):  
Ted von Hippel ◽  
Adam Moss ◽  
Isabelle Kloc ◽  
Natalie Moticska ◽  
Jimmy Sargent ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Stellar Mass ◽  
Posterior Distributions ◽  
Trigonometric Parallaxes ◽  
On Line ◽  
Better Than

AbstractWe employ Pan-STARRS photometry, Gaia trigonometric parallaxes, modern stellar evolution and atmosphere models, and our Bayesian fitting approach to determine cooling and total ages for 159,238 white dwarfs. In many cases we are able to derive precise ages (better than 5%) for individual white dwarfs. These results are meant for broad use within the white dwarf and stellar astrophysics communities and we plan to make available on-line the posterior distributions for cooling age, total age, initial stellar mass, and other parameters.

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