scholarly journals Surface convection in Population II stars

2008 ◽  
Vol 4 (S252) ◽  
pp. 253-254
Author(s):  
Laurent Piau ◽  
Robert F. Stein
Keyword(s):  
Convection Zone ◽  
Initial Surface ◽  
Preliminary Results ◽  
Hydrodynamical Simulations ◽  
Surface Convection ◽  
And Diffusion

AbstractThe initial surface abundances of Population II stars have been altered by the interplay between convection, rotational mixing and diffusion. In particular the shallower the outer convection zone the stronger the diffusion impact. We present preliminary results on constraining the extension of the convection zones of Population II stars thanks to 3D hydrodynamical simulations.

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 Fractional Stellar Convection Zone Depth and the Generation of Magnetic Flux: An Empirical Approach

1983 ◽  
Vol 102 ◽  
pp. 187-192
Author(s):  
Mark S. Giampapa
Keyword(s):  
Magnetic Flux ◽  
Convection Zone ◽  
Line Emission ◽  
Dwarf Stars ◽  
Preliminary Results ◽  
Telescope Facility ◽  
Zone Depth ◽  
M Dwarf Stars ◽  
M Dwarf ◽  
Mirror Telescope

I present preliminary results from an observational investigation of very late M dwarf stars utilizing the Multiple Mirror Telescope facility. I find that dwarf stars later than spectral type M5 do not necessarily exhibit Hα line emission, contrary to the assertion by Joy and Abt (1974). The preliminary results I discuss herein tentatively suggest, but do not prove, that the generation of significant magnetic fields and magnetic flux is severely inhibited in fully convective stars.

Convection and the solar abundances: Does the sun have a sub-solar metallicity?

2006 ◽  
Vol 2 (S239) ◽  
pp. 122-129
Author(s):  
Martin Asplund
Keyword(s):  
Convection Zone ◽  
Model Atmosphere ◽  
Solar Photosphere ◽  
The Sun ◽  
Line Formation ◽  
Solar Convection ◽  
Solar Metallicity ◽  
Solar Abundances ◽  
Hydrodynamical Simulations ◽  
Mixing Length Theory

AbstractIn the Sun, the convection zone reaches up to the solar photosphere and can thus directly influence the emergent spectrum. Traditionally, the effects of convection has been modelled with the local mixing length theory in theoretical 1D hydrostatic model atmospheres. In a different approach, we have performed realistic time-dependent, 3D, radiative-hydrodynamical simulations of the outer layers of the solar convection zone, including the photosphere. Both the different mean stratification and the presence of atmospheric inhomogeneities in 3D impact the spectral line formation. In a series of papers, we have applied our 3D solar model atmosphere to the problem of the solar chemical composition. Furthermore, we have adopted the best possible atomic and molecular line data and taken into account departures from LTE in the line formation when necessary. The inferred C, N, O and Ne abundances are all significantly lower than estimated from previous 1D modelling by 0.2-0.3 dex. These results have significant implications for a range of topics in contemporary astrophysics, including causing a severe headache for helioseismology.

A Study of Permeability and Diffusion at the Agglomerate-Scale in Heap (Bio)Leaching Systems

2015 ◽  
Vol 1130 ◽  
pp. 316-320 ◽  
Author(s):  
Elaine Govender ◽  
Athanasios Kotsiopoulos ◽  
Sue T.L. Harrison
Keyword(s):  
Time Distribution ◽  
Initial Period ◽  
Unsteady State ◽  
Microbial Transport ◽  
Preliminary Results ◽  
Solution Exchange ◽  
Concentration Time ◽  
Liquid Phases ◽  
Bed Agglomeration ◽  
And Diffusion

Multiple mini-column reactors, loaded with identically constructed ore samples representing grab samples of a larger heap, were used to study the behaviour of solution tracers to elucidate solution diffusion, dispersion and transport. The tracers were either introduced to the ore bed as a pulse, included during agglomeration of the ore or introduced to the system by submerging the ore bed. These methods of tracer introduction allowed for the characterisation of flow interchange in unsteady state systems. The resulting concentration-time distribution curves were analysed to allow characterisation of flow dispersion and diffusion, which facilitates exchange between the fast flowing and largely stagnant liquid phases. Preliminary results support the presence of distinct stagnant and flowing regions within the agglomerated ore bed. Agglomeration with the tracer promotes increased retention on the ore; potentially enhancing microbial transport via increased solution exchange after the initial period of attachment.

scholarly journals Pulsations of White Dwarf Stars with Thick Hydrogen or Helium Surface Layers

1988 ◽  
Vol 123 ◽  
pp. 333-337
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield ◽  
Russell B. Kidman ◽  
W. Dean Pesnell
Keyword(s):  
Stellar Evolution ◽  
White Dwarf ◽  
Convection Zone ◽  
Helium Surface ◽  
Surface Layers ◽  
Dwarf Stars ◽  
Surface Mass ◽  
White Dwarf Stars ◽  
Surface Convection ◽  
Nonradial Pulsation

In order to see if there could be agreement between results of stellar evolution theory and those of nonradial pulsation theory, calculations of white dwarf models have been made for hydrogen surface masses of 10−4M⊙. Earlier results by Winget et al. (1982) indicated that surface masses greater than 10−8M⊙ would not allow nonradial pulsations, even though all the driving and damping is in surface layers only 10−12 of the mass thick. We show that the surface mass of hydrogen in the pulsating white dwarfs (ZZ Ceti variables) can be any value as long as it is thick enough to contain the surface convection zone.

scholarly journals Velocities Observed in Supergranules

1976 ◽  
Vol 71 ◽  
pp. 121-134
Author(s):  
S. P. Worden ◽  
G. W. Simon
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Convection Zone ◽  
Convective Motion ◽  
Diode Array ◽  
Sacramento Peak ◽  
Sacramento Peak Observatory ◽  
And Diffusion ◽  
Time Sequences ◽  
Flux Redistribution

The evolution of the velocity and magnetic fields associated with supergranulation has been investigated using the Sacramento Peak Observatory Diode Array Magnetograph. The observations consist of time sequences of simultaneous velocity, magnetic field, and chromospheric network measurements. From these data it appears that the supergranular velocity cells have lifetimes in excess of 30 h. Magnetic field motions associated with supergranulation were infrequent and seem to be accompanied by changes in the velocity field. More prevalent was the slow dissipation and diffusion of stationary flux points. These observations suggest that surface motions do not exhibit the detailed flux redistribution expected in the random-walk diffusion of magnetic fields. It is suggested that the surface motions are only the reflection of magnetic field-convective motion interactions which occur deeper in the convection zone.

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