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 A Model of Time-Dependent and Non-Local Convection Included in a Stellar Evolution Code

2002 ◽  
Vol 185 ◽  
pp. 456-457
Author(s):  
Martin Flaskamp
Keyword(s):  
Time Dependence ◽  
Stellar Evolution ◽  
Diffusion Approximation ◽  
Time Dependent ◽  
Box Model ◽  
Hydrodynamic Equations ◽  
Convective Mixing ◽  
Present Test ◽  
Non Local ◽  
And Diffusion

AbstractThe Kuhfuß theory of convection is derived from the exact hydrodynamic equations by means of anelastic approximation and diffusion approximation. In the local and time-independent limit the result is identical to the MLT case. In the more general case the theory includes overshooting and convective mixing as well as time-dependence. We present test calculations in a simple box model and first steps toward the implementation in a stellar code.

scholarly journals New Theory of Stellar Convection without the mixing-length parameter: new stellar atmosphere models

2015 ◽  
Vol 11 (A29B) ◽  
pp. 154-155
Author(s):  
Stefano Pasetto ◽  
Cesare Chiosi ◽  
Mark Cropper
Keyword(s):  
Energy Transport ◽  
Surrounding Medium ◽  
Time Dependent ◽  
Length Scale ◽  
The Sun ◽  
Mixing Length ◽  
Local Pressure ◽  
Stellar Convection ◽  
Non Local ◽  
Stellar Models

AbstractStellar convection is customarily described by the mixing-length theory, which makes use of the mixing-length scale to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to suggest that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in the literature are hampered by this basic uncertainty.In a recent paper (Pasettoet al.2014) we presented a new theory that does not require the mixing length parameter. Our self-consistent analytical formulation of stellar convection determines all the properties of stellar convection as a function of the physical behavior of the convective elements themselves and the surrounding medium. The new theory of stellar convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of stellar convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time-dependent formalism.We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory are compared with those from the standard mixing-length paradigm with positive results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram.

scholarly journals Theory of stellar convection: removing the mixing-length parameter

2015 ◽  
Vol 11 (A29B) ◽  
pp. 608-613
Author(s):  
Stefano Pasetto ◽  
Cesare Chiosi ◽  
Mark Cropper ◽  
Eva K. Grebel
Keyword(s):  
Energy Transport ◽  
Surrounding Medium ◽  
Time Dependent ◽  
Length Scale ◽  
The Sun ◽  
Mixing Length ◽  
Local Pressure ◽  
Stellar Convection ◽  
Non Local ◽  
Stellar Models

AbstractStellar convection is customarily described by the mixing-length theory, which makes use of the mixing-length scale to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to suggest that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in the literature are hampered by this basic uncertainty.In a recent paper (Pasettoet al.2014) we presented a new theory that does not require the mixing length parameter. Our self-consistent analytical formulation of stellar convection determines all the properties of stellar convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium. The new theory of stellar convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of stellar convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time-dependent formalism.We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory are compared with those from the standard mixing-length paradigm with positive results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram.

scholarly journals Spectroscopic distances to late-type stars

2012 ◽  
Vol 8 (S289) ◽  
pp. 83-86
Author(s):  
Maria Bergemann ◽  
Aldo Serenelli ◽  
Gregory Ruchti
Keyword(s):  
Stellar Evolution ◽  
Large Scale ◽  
Local Thermodynamic Equilibrium ◽  
Systematic Errors ◽  
Spectroscopic Techniques ◽  
The Sun ◽  
Late Type ◽  
Non Local ◽  
Ionization Balance

AbstractA common approach to determining distances to stars without astrometric information is to compare stellar evolution models with parameters obtained from spectroscopic techniques. This method is routinely applied in the context of large-scale stellar surveys out to distances of several kpc. However, systematic errors may arise because of inaccurate spectroscopic parameters. We explore the effects of non-local thermodynamic equilibrium (NLTE) on the determination of surface gravities and metallicities for a large sample of metal-poor stars within approximately 10 kpc of the Sun. Using the improved Teff scale, we then show that stellar parameters estimated based on the widely used method of 1D LTE excitation-ionization balance of Fe results in distances which are systematically in error. For metal-poor giants, [Fe/H] ~ −2 dex, the distances can be overestimated by up to 70%. We compare the results with those from the Radial Velocity Experiment Survey catalogue (rave) for the stars in common, and find similar offsets.

scholarly journals Scale-free convection theory

2015 ◽  
Vol 11 (A29B) ◽  
pp. 747-747
Author(s):  
Stefano Pasetto ◽  
Cesare Chiosi ◽  
Mark Cropper ◽  
Eva K. Grebel
Keyword(s):  
Surrounding Medium ◽  
Stellar Structure ◽  
Time Dependent ◽  
Length Scale ◽  
The Sun ◽  
Mixing Length ◽  
Local Pressure ◽  
Scale Free ◽  
Non Local ◽  
Stellar Models

AbstractConvection is one of the fundamental mechanisms to transport energy, e.g., in planetology, oceanography, as well as in astrophysics where stellar structure is customarily described by the mixing-length theory, which makes use of the mixing-length scale parameter to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height of the star, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to claim that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in the literature are hampered by this basic uncertainty. In a recent paper (Pasetto et al. 2014) we presented the first fully analytical scale-free theory of convection that does not require the mixing-length parameter. Our self-consistent analytical formulation of convection determines all the properties of convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium (be it a star, an ocean, or a primordial planet). The new theory of convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time dependent formalism. We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory in astrophysical environment are compared with those from the standard mixing-length paradigm in stars with exceptional results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram.

The9Be Abundances of α Centauri A and B and the Sun: Implications for Stellar Evolution and Mixing

1997 ◽  
Vol 478 (2) ◽  
pp. 778-786 ◽  
Author(s):  
Jeremy R. King ◽  
Constantine P. Deliyannis ◽  
Ann Merchant Boesgaard
Keyword(s):  
Stellar Evolution ◽  
The Sun

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.

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