Structure of Radiative versus Convective Stellar Envelopes

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.

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Sakurai’s Object: Constraints from its Variability?

Highlights of Astronomy ◽

10.1017/s1539299600021110 ◽

1998 ◽

Vol 11 (1) ◽

pp. 360-360

Author(s):

T. Gautschy ◽

H.W. Duerbeck ◽

A.M. Van Genderen ◽

S. Benetti

Keyword(s):

Cooling Rate ◽

Nuclear Energy ◽

Stability Analyses ◽

The Past ◽

Stellar Pulsations ◽

First Results ◽

Effective Temperatures ◽

Stellar Envelopes ◽

Light Variability ◽

Hr Diagram

The peculiar outburst of the star baptized Sakurai’s Object (SO) is a conceivable example of a late He shell flash in a post-AGB object. The new source of nuclear energy forces such objects toward high luminosities and eventually low effective temperatures; they cross the HR diagram in a comparable fashion as FG Sge did in the past - i.e., they move noticeably on the HR diagram on human timescales. From monitoring campaigns of SO during the last year, first estimates of its cooling rate were derived and in particular cyclic light variability was established. We present first results from attempts to model stellar envelopes appropriate for SO. As we hypothesize the light variability to be attributable to stellar pulsations, we aim at constraining the basic stellar parameters based on stability analyses of our envelope models. Radial, nonadiabatic stability computations provided predictions of the modal content which should be observable as SO evolves. The particular components in such mode spectra of SO as they are to appear in the coming years should indeed help to constrain basic stellar parameters such as mass and luminosity.

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Diffusion in CP Stars: The Quest for Accuracy

Highlights of Astronomy ◽

10.1017/s1539299600018414 ◽

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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Radiation from a gas heated by a shock wave and its role in the emission of stellar envelopes of close binary systems

Astrophysics ◽

10.1007/bf01013131 ◽

1970 ◽

Vol 4 (4) ◽

pp. 209-214

Author(s):

V. G. Gorbatskii

Keyword(s):

Shock Wave ◽

Binary Systems ◽

Close Binary ◽

Close Binary Systems ◽

Stellar Envelopes

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Diffusion in Stellar Envelopes

Highlights of Astronomy ◽

10.1017/s1539299600018396 ◽

1998 ◽

Vol 11 (2) ◽

pp. 664-666

Author(s):

M.J. Seaton

Keyword(s):

Unit Area ◽

Effective Area ◽

Radiative Acceleration ◽

Stellar Envelopes ◽

Present Summary

In an attempt to make the present summary comprehensible yet sufficiently concise I will risk some over-simplification: further details are given in Seaton (1997, 1998 — Papers I and II).In a star let Fv dv be the outward flux of radiant and let an atom of element k present an effective area of σv(k) for absorption of radiation. Then the momentum absorbed per atom and per-unit area and unit time is G(k) = (1/c) ∫ Fvσv(k) dv which is just the force acting on the atom. The radiative acceleration is grad(k) = G(k)/M(k) where M(k) is the atom’s mass.

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