Helium settling and mass loss in magnetic Ap stars

Théado, S.; Vauclair, S.; Cunha, M. S.

doi:10.1051/0004-6361:20052933

Cited by 28 publications

(30 citation statements)

References 52 publications

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“…In single stars for which rotation cannot be slowed by tidal forces, strong magnetic fields may be the only process which sufficiently reduces rotation to allow chemical separation. Although it has to our knowledge never been shown that magnetic fields can completely eliminate convection, their topologies can induce horizontally dependent convection and abundance profiles (Babel & Michaud 1991), in addition to anisotropic mass loss and/or accretion (Théado et al 2005). If mass loss is in fact anisotropic, then strong mass loss rates, as suggested for young PMS stars by Boehm & Catala 1995, may be present only where field lines are vertical, and thus be compatible with anisotropic surface anomalies.…”

Section: General Discussion and Conclusionmentioning

confidence: 97%

Abundance anomalies in pre-main-sequence stars

Vick

Michaud

Richer

et al. 2010

A&A

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Aims. The effects of atomic diffusion on internal and surface abundances of A and F pre-main-sequence stars with mass loss are studied in order to determine at what age the effects materialize, as well as to further understand the processes at play in HAeBe and young ApBp stars. Methods. Self-consistent stellar evolution models of 1.5 to 2.8 M with atomic diffusion (including radiative accelerations) for all species within the OPAL opacity database were computed and compared to observations of HAeBe stars. Results. Atomic diffusion in the presence of weak mass loss can explain the observed abundance anomalies of pre-main-sequence stars, as well as the presence of binary systems with metal rich primaries and chemically normal secondaries such as V380 Ori and HD 72106. This is in contrast to turbulence models which do not allow for abundance anomalies to develop on the pre-main-sequence. The age at which anomalies can appear depends on stellar mass. Conclusions. For A and F stars, the effects of atomic diffusion can modify both the internal and surface abundances before the onset of the main-sequence. The appearance of important surface abundance anomalies on the pre-main-sequence does not require mass loss, though the mass loss rate affects their amplitude. Observational tests are suggested to decipher the effects of mass loss from those of turbulent mixing. If abundance anomalies are confirmed in pre-main-sequence stars they would severely limit the role of turbulence in these stars.

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Section: General Discussion and Conclusionmentioning

confidence: 97%

Abundance anomalies in pre-main-sequence stars

Vick

Michaud

Richer

et al. 2010

A&A

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“…The diffusion of He in stellar atmospheres with well-mixed winds (including H) has been studied in more detail by Vauclair et al (1991), Krticka & Kubát (2004) and Théado et al (2005); all groups confirm that without a mixed wind from the star, that He should diffuse downwards until quite low relative abundance is reached. However, we know of no published predictions of how low the He abundance should drop before an equilibrium distribution is achieved.…”

Section: Abundant Light Elements: He Omentioning

confidence: 90%

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

Bailey

Landstreet

Bagnulo

2014

A&A

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Context. The stars of the middle main sequence have relatively quiescent outer layers, and unusual chemical abundance patterns may develop in their atmospheres. The presence of chemical peculiarities reveal the action of such subsurface phenomena as gravitational settling and radiatively driven levitation of trace elements, and their competition with mixing processes such as turbulent diffusion. At present, little is known about the time evolution of these anomalous abundances, nor about the role that diffusion may play in maintaining them, during the main sequence lifetime of such a star. Aims. We want to establish whether abundance peculiarities change as stars evolve on the main sequence, and provide observational constraints to diffusion theory. Methods. We have performed spectral analysis of 15 magnetic Bp stars that are members of open clusters (and thus have well-known ages), with masses between about 3 and 4 M . For each star, we measured the abundances of He, O, Mg, Si, Ti, Cr, Fe, Pr and Nd. Results. We have discovered the systematic time evolution of trace elements through the main-sequence lifetime of magnetic chemically peculiar stars as their atmospheres cool and evolve towards lower gravity. During the main sequence lifetime, we observe clear and systematic variations in the atmospheric abundances of He, Ti, Cr, Fe, Pr and Nd. For all these elements, except He, the atmospheric abundances decrease with age. The abundances of Fe-peak elements converge towards solar values, while the rare-earth elements converge towards values at least 100 times more abundant than in the Sun. Helium is always underabundant compared to the Sun, evolving from about 1% up to 10% of the solar He abundance. We have attempted to interpret the observed abundance variations in the context of radiatively driven diffusion theory, which appears to provide a framework to understand some, but not all, of the anomalous abundance levels and variations that we observe.

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“…This hypothesis has been tested with reasonable success through nonadiabatic calculations (Cunha 2002). Moreover, microscopic diffusion is also supposed to have a significant impact on the excitation process (Théado et al 2005).…”

Section: Introductionmentioning

confidence: 89%

The fundamental parameters of the roAp star 10 Aquilae

Perraut

Borgniet

Cunha

et al. 2013

A&A

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Context. Owing to the strong magnetic field and related abnormal surface layers existing in rapidly oscillating Ap (roAp) stars, systematic errors are likely to be present when determining their effective temperatures, which potentially compromises asteroseismic studies of this class of pulsators. Aims. Using the unique angular resolution provided by long-baseline visible interferometry, our goal is to determine accurate angular diameters of a number of roAp targets, so as to derive unbiased effective temperatures (T eff ) and provide a T eff calibration for these stars. Methods. We obtained long-baseline interferometric observations of 10 Aql with the visible spectrograph VEGA at the combined focus of the CHARA array. We derived the limb-darkened diameter of this roAp star from our visibility measurements. Based on photometric and spectroscopic data available in the literature, we estimated the star's bolometric flux and used it, in combination with its parallax and angular diameter, to determine the star's luminosity and effective temperature. Results. We determined a limb-darkened angular diameter of 0.275 ± 0.009 mas and deduced a linear radius of R = 2.32 ± 0.09 R . For the bolometric flux we considered two datasets, leading to an effective temperature of T eff = 7800 ± 170 K and a luminosity of L/L = 18 ± 1 or T eff = 8000 ± 210 K and L/L = 19 ± 2. Finally we used these fundamental parameters together with the large frequency separation determined by asteroseismic observations to constrain the mass and the age of 10 Aql, using the CESAM stellar evolution code. Assuming a solar chemical composition and ignoring all kinds of diffusion and settling of elements, we obtained a mass M/M ∼ 1.92 and an age of ∼780 Gy or a mass M/M ∼ 1.95 and an age of ∼740 Gy, depending on the derived value of the bolometric flux. Conclusions. For the first time, thanks to the unique capabilities of VEGA, we managed to determine an accurate angular diameter for a star smaller than 0.3 mas and to derive its fundamental parameters. In particular, by only combining our interferometric data and the bolometric flux, we derived an effective temperature that can be compared to those derived from atmosphere models. Such fundamental parameters can help for testing the mechanism responsible for the excitation of the oscillations observed in the magnetic pulsating stars.

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Helium settling and mass loss in magnetic Ap stars

Cited by 28 publications

References 52 publications

Abundance anomalies in pre-main-sequence stars

Abundance anomalies in pre-main-sequence stars

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

The fundamental parameters of the roAp star 10 Aquilae

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