Helium abundance anomalies and radiative forces in stellar envelopes

Michaud, G.; Montemerle, T.; Cox, A. N.; Magee, N. H.; Hodson, S. W.; Martel, A. R.

doi:10.1086/157489

Cited by 36 publications

(38 citation statements)

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“…The observed abundance peculiarities can in principle be explained within the framework of atomic diffusion (Michaud et al 1979). Diffusion theory predicts that the photospheres of all HgMn stars should be deficient in helium, which was confirmed for instance in a dedicated study by Dworetsky (2004).…”

Section: Introductionmentioning

confidence: 60%

A non-LTE spectral analysis of the³He and⁴He isotopes in the HgMn starκCancri

Maza

Nieva

Przybilla

2014

A&A

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Aims. We present a pilot study on non-local thermodynamic equilibrium (NLTE) line-formation computations for the isotopes 3 He and 4 He in the mercury-manganese star κ Cancri. The impact of NLTE effects on the determination of isotopic abundances and the vertical stratification of helium in the atmosphere is investigated. Methods. Modern NLTE line-formation computations were employed to analyse a high-resolution and high signal-to-noise ratio ESO-VLT/UVES spectrum of κ Cnc. The atmospheric parameters were determined from fitting the hydrogen Balmer lines and the spectral energy distribution. Multiple He i lines were investigated, including He i λ4921 Å and λ6678 Å, which show the widest isotopic splits.Results. Half of the observed He i lines in the spectrum of κ Cnc show significant NLTE strengthening, the effects are strongest in the red lines He i λ5875 Å and He i λ6678 Å. NLTE abundances from individual He i lines are up to a factor of ∼3 lower than LTE values. Helium is found to be stratified in the atmosphere of κ Cnc. While the LTE analysis indicates a step-like profile of the helium abundance, a gradual decrease with height is indicated by the NLTE analysis. A 3 He/ 4 He ratio of ∼0.25-0.30 is found. With the available data it cannot be decided whether the two isotopes follow the same stratification profile, or not. Conclusions. This work implies that NLTE effects may be ubiquitous in the atmospheres of HgMn stars and may have a significant impact on abundance determinations and the interpretation of the vertical abundance stratification of elements.

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Section: Introductionmentioning

confidence: 60%

A non-LTE spectral analysis of the³He and⁴He isotopes in the HgMn starκCancri

Maza

Nieva

Przybilla

2014

A&A

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“…Among these stars, He-strong and He-weak stars present sometimes signs of magnetically controlled winds (Brown et al 1985;Shore & Brown 1990) which have been interpreted as the signature of mass loss near one magnetic pole. For Herich stars, a stellar wind has also been invoked by Vauclair (1975) to account for their observed helium enrichment (by a factor 2 or 3): this enrichment cannot be explained by diffusion alone as the upward radiative acceleration on helium is always smaller that the gravity (Montmerle & Michaud 1976;Michaud et al 1979). Vauclair (1975) showed that an overall macroscopic mass loss flux could advect helium up to the observed layers.…”

Section: Mass Lossmentioning

confidence: 99%

“…In the case of helium the radiative acceleration is always negligible because the helium cosmic abundance is very high. Moreover, in the considered stars, the thermal diffusion is also usually negligible (Michaud et al 1979). Taking into account the electrical field induced by ionelectron separation, the gravitational settling term can be written:…”

Section: Microscopic Diffusionmentioning

confidence: 99%

Helium settling and mass loss in magnetic Ap stars

Théado

Vauclair

Cunha

2005

A&A

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The chemical stratification is likely to play a predominant role in the excitation mechanism of high order modes in rapidly oscillating Ap stars. In this paper we discuss some processes (convection, He settling, mass loss and turbulent mixing) which may strongly affect the chemical composition of Ap stars. We discuss their interplay with a dipolar magnetic field in both magnetic polar and equatorial regions, where the magnetic lines are respectively mainly radially and horizontally oriented. We also present stellar evolutionary computations for magnetic Ap stars. As the magnetic geometry strongly breaks the sphericity of the star, we modeled and computed the polar and equatorial regions separately. We examine the effects of various chemical transport processes on the chemical stratification of both regions. In a forthcoming paper, we will study the stability of these models against pulsations.

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“…Since underabundances of Ca and Sc are essential observational characteristics of most Am stars, the disappearance of the He II convection zone is essential if Am stars are to be explained by this diffusion model. As the results of Michaud et al (1976) show, most of the elements for which g R > g below the H convection zone are observed to be overabundant and those for which g R < g are underabundant.…”

Section: Upper Main Sequence and Am Starsmentioning

confidence: 88%

“…This is the simplest assumption but may not be justified. Michaud et al (1987) evaluated that there could be separation in the wind (i. e. above the photosphere) for mass loss rates as high as 10" 12 M 0 yr" 1 but this rested on approximations for the wind model that maximize the separation for a given mass loss rate. They concluded that the separation was more likely to occur in the photosphere than above.…”

Section: A and F Starsmentioning

confidence: 99%

Diffusion, Meridional Circulation and Mass Loss in Main Sequence and Horizontal Branch Stars

Michaud

1991

Symp. - Int. Astron. Union

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. In many objects, the turbulent particle diffusion coefficient is at most 10 times larger than the atomic diffusion coefficient. ContextOver the last few years the quality of observational material has improved to the point that much finer tests of stellar evolution models have become possible. One example is the dip in Li abundance at T eff = 6700 Κ reviewed by Rebolo elsewhere in this volume. Demonstrating the presence of the Li gap has been made possible by the high signal to noise data available on relatively faint objects.We develop tests of the physical processes using the observed abundance anomalies. In addition to "standard" stellar physics, we shall consider particle transport by atomic diffusion, meridional circulation, mass loss and turbulence. Atomic diffusion is a basic physical process that must be included unless there are more efficient transport processes that compete with it and wipe out its effects. Meridional circulation is calculated from first principles without arbitrary parameters (Tassoul and Tassoul 1982); it defines a model for particle transport in rotating stars whose effects can be tested. Mass loss is known to be present in the Sun at a rate of 10" 14 M 0 yr" 1 and it is appropriate to determine what would be its effect in other stars at that level. While mass loss is a natural phenomenon that may well play a role, its introduction currently requires at least one arbitrary parameter. As to turbulence, it is discussed elsewhere in this volume and we shall mention it only in so far as it is possible to put interesting upper limits on it.

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Helium abundance anomalies and radiative forces in stellar envelopes

Cited by 36 publications

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A non-LTE spectral analysis of the³He and⁴He isotopes in the HgMn starκCancri

A non-LTE spectral analysis of the³He and⁴He isotopes in the HgMn starκCancri

Helium settling and mass loss in magnetic Ap stars

Diffusion, Meridional Circulation and Mass Loss in Main Sequence and Horizontal Branch Stars

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Helium abundance anomalies and radiative forces in stellar envelopes

Cited by 36 publications

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A non-LTE spectral analysis of the3He and4He isotopes in the HgMn starκCancri

A non-LTE spectral analysis of the3He and4He isotopes in the HgMn starκCancri

Helium settling and mass loss in magnetic Ap stars

Diffusion, Meridional Circulation and Mass Loss in Main Sequence and Horizontal Branch Stars

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A non-LTE spectral analysis of the³He and⁴He isotopes in the HgMn starκCancri

A non-LTE spectral analysis of the³He and⁴He isotopes in the HgMn starκCancri