scholarly journals Chemical Separation vs Rotation in a and F-Stars

1993 ◽  
Vol 138 ◽  
pp. 474-489
Author(s):  
Paul Charbonneau
Keyword(s):  
Mass Loss ◽  
Convection Zone ◽  
Chemical Separation ◽  
Chemical Abundances ◽  
Loss Model ◽  
Peculiar Stars ◽  
Thermally Driven ◽  
Chemically Peculiar Stars ◽  
F Stars ◽  
Diffusion Mass

AbstractThe role played by rotationally-induced mixing in the diffusion-based models for non-magnetic chemically peculiar stars is investigated. This paper focuses on one specific rotationally controlled mixing mechanism, namely thermally-driven meridional circulation. Its effects on the time evolution of chemical abundances are illustrated by means of three specific examples. The first two concern the diffusion model for FmAm stars, where it is shown that while circulation has a determining influence on the settling of Helium, it has no significant effect on the diffusion of heavier metals once the He superficial convection zone has disappeared. The third example is concerned with the diffusion/mass loss model for λBootis stars. It is shown that the inclusion of circulation prevents the appearance of generalized underabundances at any epoch of the evolution, indicating that the diffusion/mass loss model for these objects must be abandoned.

scholarly journals A Magnetic Snapshot Survey of F-Type Stars

2020 ◽  
Author(s):  
J M Seach ◽  
S C Marsden ◽  
B D Carter ◽  
C Neiner ◽  
C P Folsom ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Longitudinal Magnetic Field ◽  
Convection Zone ◽  
Stellar Radius ◽  
Spectral Class ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
The Mean ◽  
Chemically Peculiar Stars ◽  
F Stars

Abstract We present a spectropolarimetric magnetic snapshot survey of 55 stars which includes 53 F-type stars ranging from spectral types F0 to F9 plus 2 chemically peculiar stars β CrB, and δ Cap. We look for magnetic fields in stars spanning a range of effective temperatures where the transition from fossil to dynamo magnetic fields is believed to occur. High-resolution spectropolarimetry using circularly polarized spectra is used to look for a magnetic detection in the Stokes V profile, determine the mean longitudinal magnetic field (Bl), and to look for correlations with stellar parameters. Surface magnetic fields are detected on 14 F-stars, and present in every spectral class from F3V-F9V ranging in strength from 0.3 ± 0.1 G (36 UMa, F8V) to 8.3 ± 0.9 G (h Dra, F8V). Thus we find photospheric magnetic fields are present in stars as early as spectral type F3V with an outer convection zone thickness less than a few per cent of the stellar radius.

scholarly journals Stellar evolution models with mass loss and turbulence

2008 ◽  
Vol 4 (S252) ◽  
pp. 289-295
Author(s):  
M. Vick ◽  
G. Michaud ◽  
O. Richard
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
Mass Loss Rate ◽  
Chemical Separation ◽  
Atomic Diffusion ◽  
Anomalous Surface ◽  
F Stars ◽  
Surface Convection ◽  
Diffusion Mass ◽  
Do So

AbstractAlthough chemical separation is generally accepted as the main physical process responsible for the anomalous surface abundances of AmFm stars, its exact behavior within the interior of these stars is still uncertain. We will explore two hydrodynamical processes which could compete with atomic diffusion: mass loss and turbulence. We will also discuss the extent to which separation occurs immediately below the surface convection zone as well as the extent to which separation occurs below 200,000 K. To do so, self-consistent stellar models with mass loss and turbulence where calculated using the Montreal stellar evolution code and compared to observations of A and F stars. It is shown that to the precision of observations available for F stars, a mass loss rate of 2×10−14M⊙· yr−1, is compatible with observations and that no turbulence is then required.

scholarly journals Diamagnetic Abundance Differentiation in the Solar System

1992 ◽  
Vol 150 ◽  
pp. 425-426
Author(s):  
Raphael Steinitz ◽  
Estelle Kunoff
Keyword(s):  
Solar Wind ◽  
Solar System ◽  
Solar Corona ◽  
Ionization Potential ◽  
Chemical Abundances ◽  
Magnetic Structures ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Chemically Peculiar Stars ◽  
First Ionization Potential

Chemical abundances in the solar corona or solar wind compared to those in the photosphere differentiate according to first ionization potential (FIP). We suggest that the effect is the result of diamagnetic diffusion pumps operating in the presence of gravitation and diverging magnetic structures. We then comment briefly on implications concerning abundances in the solar system and chemically peculiar stars.

scholarly journals Diffusion Models for Magnetic Ap Stars

1993 ◽  
Vol 138 ◽  
pp. 458-472
Author(s):  
J. Babel
Keyword(s):  
Mass Loss ◽  
Diffusion Theory ◽  
Transport Processes ◽  
Line Profiles ◽  
Loss Model ◽  
Spectrum Synthesis ◽  
On Line ◽  
Ap Stars ◽  
Diffusion Mass ◽  
Made In

AbstractProgress made in spectroscopy and in the diffusion theory permits now to make severe comparisons, based on line profiles, between theory and observation.We first review transport processes which are present in the atmospheric layers of Ap stars and discuss their relative importance. We then show that mass loss could play a key role for the creation of abundance maps. A mass loss model is proposed for 53 Cam and is compared, by spectrum synthesis, with visible and IUE high resolutions observations. The model accounts well for the line profiles of several elements with an exception for Ti. Furthermore, the abundance stratification predicted by this model gives close agreement with the large variation of the abundances of Cr and Fe found between the visible and UV domains. The diffusion-mass loss model finally permits to give a simple interpretation of the peculiar Ca II K lines observed in many Ap SrCrEu stars and in particular in 53 Cam, β CrB and HD 191742.

scholarly journals A Diffusion Mass-Loss Model for the Ap Star 53 Cam

1993 ◽  
Vol 137 ◽  
pp. 275-277
Author(s):  
J. Babel
Keyword(s):  
Mass Loss ◽  
Time Variation ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Spectral Features ◽  
Loss Model ◽  
A Value ◽  
Upper Limits ◽  
Ap Stars ◽  
Diffusion Mass

AbstractThe mechanism and properties of mass loss are poorly known for Ap stars. Present upper limits on the mass loss rate are of 10−10 M⊙yr−1, a value which does not permit any element separation. Abundance maps could be a very powerful tool to constrain the mass loss rate and the wind geometry of Ap stars, as surface abundances are sensitive to rates as small as 10−15 M⊙yr−1. We here propose a diffusion-mass loss model for 53 Cam and compute abundance distributions in the photosphere of 53 Cam. The mass loss geometry is determined from the Ca II K line profile and its time variation. We obtain that the diffusion-mass loss model explains many spectral features of 53 Cam, both in the UV and visible domains.

Main Sequence Abundances: Observational Aspects

1988 ◽  
Vol 108 ◽  
pp. 2-2
Author(s):  
Jun Jugaku
Keyword(s):  
Chemical Composition ◽  
Effective Temperature ◽  
Main Sequence ◽  
Theoretical Models ◽  
Current Data ◽  
Main Sequence Stars ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Chemically Peculiar Stars ◽  
F Stars

AbstractAlthough once it was thought that main-sequence stars are remarkably homogeneous with respect to their chemical composition, the upper main-sequence stars (30000 > Te > 7000) show a variaety of chemically peculiar stars besides the so-called normal stars. Those include the Am, Ap, λ Bootis, He-deficient, and He-rich stars. This review summarizes the current data, which are necessary to construct and test the theoretical models of these stars. In the second half of the review we concentrate on Li. In the lower main-sequecnce stars abundances of Li have been determined in hundreds of stars. Some of the remarkable results are: (1) A uniform upper abundance value irrespective of stellar effective temperature, (2) abundance gap in the F stars of the Hyades, and (3) increasing depletion with smaller stellar mass for the Hyades.

scholarly journals Beryllium in Lithium–Deficient F Stars: Constraints on Stellar Evolution

1993 ◽  
Vol 137 ◽  
pp. 174-176 ◽  
Author(s):  
Constantine P. Deliyannis ◽  
M.H. Pinsonneault
Keyword(s):  
Angular Momentum ◽  
Mass Loss ◽  
Stellar Evolution ◽  
Meridional Circulation ◽  
Initial Surface ◽  
Physical Mechanisms ◽  
Vital Interest ◽  
Angular Momentum Loss ◽  
F Stars ◽  
Diffusion Mass

AbstractStandard stellar evolution predicts that F stars should retain their initial surface lithium (Li) abundance because their convection zones are too shallow to destroy it at their base. Yet, observations reveal a severe Li depletion (the “Boesgaard Gap”), perhaps by as much as about two orders of magnitude, in a narrow Teff range. Several physical mechanisms, not usually included in stellar evolution calculations, have been proposed to account for this Li deficiency. These include diffusion, mass loss, meridional circulation, and rotationally-induced mixing driven by angular momentum loss. Identifying which of these (if any) might really be at work is not only of vital interest to stellar evolution, but may also have serious implications elsewhere (e.g. cosmology, Deliyannis et al. 1991). We bring attention to beryllium (Be) observations in F stars, which are crucial for discriminating between scenarios. Particularly important is the star 110 Her, which is depleted in Be by about a factor of 5 -10, but still has a detectable Li abundance (depleted by a factor of 100 - 200). Depleting surface Be without having depleted nearly all of the surface Li requires specific circumstances; we discuss how this depletion property severely constrains or eliminates most of the proposed mechanisms. One mechanism, rotationally-induced mixing, predicts relative depletions for these elements that agree well with what is observed.

scholarly journals On-line database of photometric observations of magnetic chemically peculiar stars

2007 ◽  
Vol 328 (1) ◽  
pp. 10-15 ◽  
Author(s):  
Z. Mikulášek ◽  
J. Janík ◽  
J. Zverko ◽  
J. Žižňovský ◽  
M. Zejda ◽  
...  
Keyword(s):  
Chemically Peculiar ◽  
Photometric Observations ◽  
Peculiar Stars ◽  
On Line ◽  
Chemically Peculiar Stars

scholarly journals Advanced test of the model stellar atmospheres: the nature of the light variability of magnetic chemically peculiar stars

2008 ◽  
Vol 4 (S252) ◽  
pp. 347-348
Author(s):  
J. Krtička ◽  
Z. Mikulášek ◽  
J. Zverko ◽  
J. Žižňovský ◽  
P. Zvěřina
Keyword(s):  
Chemical Elements ◽  
Horizontal Distribution ◽  
Stellar Atmospheres ◽  
Silicon Iron ◽  
Surface Distribution ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Distribution Of Elements ◽  
Chemically Peculiar Stars ◽  
Light Variability

AbstractThe magnetic chemically peculiar stars exhibit both inhomogeneous horizontal distribution of chemical elements on their surfaces and the light variability. We show that the observed light variability of these stars can be successfully simulated using models of their stellar atmospheres and adopting the observed surface distribution of elements. The most important elements that influence the light variability are silicon, iron, and helium.

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