scholarly journals The mic-mac connection inside stars: the interdependence of atomic diffusion and hydrodynamic instabilities

2019 ◽  
Vol 82 ◽  
pp. 175-182
Author(s):  
S. Vauclair
Keyword(s):  
Observational Data ◽  
Diffusion Processes ◽  
Stellar Structure ◽  
Atomic Diffusion ◽  
Hydrodynamic Instabilities ◽  
Stellar Rotation ◽  
Gravitational Settling ◽  
Jean Paul ◽  
Chemical Stratification ◽  
Hydrodynamic Processes

The interdependence of microscopic (atomic) and macroscopic (hydrodynamic) processes inside stars and their consequences for stellar structure and evolution were recognized by Jean-Paul Zahn several decades ago. He was a pioneer in that respect, discussing the importance of the macroscopic motions related to stellar rotation, in competition with the chemical stratification induced by gravitational settling and radiative accelerations. This has been much developed in recent years, in connection with the improvement of observational data, including asteroseismology. Morover, it has been recently discovered that the microscopic atomic diffusion processes can lead to macroscopic results which may infuence in a non negligible way the internal stellar structure, independently of the abundances observed at the surface.

scholarly journals Metamorphism of Air Bubbles in a Snow Crystal

1967 ◽  
Vol 6 (46) ◽  
pp. 561-564
Author(s):  
Norikazu Maeno ◽  
Daisuke Kuroiwa
Keyword(s):  
Activation Energy ◽  
Diffusion Processes ◽  
Atomic Diffusion ◽  
Mechanical Relaxation ◽  
Air Bubbles ◽  
A Value ◽  
Snow Crystal ◽  
Activation Energy For Diffusion ◽  
Snow Crystals ◽  
Natural Snow

RésuméObservations have been made of the modification produced by a temperature gradient in the shape of air bubbles in natural snow crystals, and also of the shrinkage of the bubbles with time. The rate of shrinkage is governed by a constant which is strongly temperature dependent with an activation energy of about 15.1 kcal./mole, a value sufficiently similar to the activation energy for diffusion of tritium, dielectric relaxation and mechanical relaxation to suggest that atomic diffusion processes may be responsible for all of these phenomena.

scholarly journals The importance of helium and metals diffusion in stars

1997 ◽  
Vol 189 ◽  
pp. 355-360
Author(s):  
Charles R. Proffitt
Keyword(s):  
Sound Speed ◽  
Globular Clusters ◽  
Main Sequence ◽  
Chemical Elements ◽  
Substantial Reduction ◽  
Solar Interior ◽  
Relative Importance ◽  
Main Sequence Stars ◽  
Gravitational Settling ◽  
Hydrodynamic Processes

Comparisons between models of the solar interior and sound speed profiles derived from inversions of helioseismic data have demonstrated that it is essential to include the effects of gravitational settling when calculating the structure and evolution of the Sun. Including settling should also be necessary for models of metal poor main-sequence stars and results in a substantial reduction in the ages derived for globular clusters.In many cases it is clear that competing hydrodynamic processes, such as mass loss or rotationally driven mixing, will limit the effectiveness of gravitational separation of chemical elements. However, the quantitative details and even the relative importance of the different processes in various types of stars remains poorly understood.

scholarly journals Combined asteroseismology, spectroscopy, and astrometry of the CoRoT B2V target HD 170580

2019 ◽  
Vol 624 ◽  
pp. A75 ◽  
Author(s):  
C. Aerts ◽  
M. G. Pedersen ◽  
E. Vermeyen ◽  
L. Hendriks ◽  
C. Johnston ◽  
...  
Keyword(s):  
Time Series ◽  
Massive Star ◽  
Rotation Period ◽  
Stellar Structure ◽  
Atomic Diffusion ◽  
High Mass ◽  
High Signal ◽  
Hydrogen Burning ◽  
Average Rotation ◽  
Low Order

Context. Space asteroseismology reveals that stellar structure and evolution models of intermediate- and high-mass stars are in need of improvement in terms of angular momentum and chemical element transport. Aims. We aim to probe the interior structure of a hot, massive star in the core-hydrogen-burning phase of its evolution. Methods. We analysed CoRoT space photometry, Gaia DR2 space astrometry, and high-resolution high signal-to-noise HERMES and HARPS time-series spectroscopy of the slowly rotating B2V star HD 170580. Results. From the time-series spectroscopy, we derive v sin i = 4 ± 2 km s−1, where the uncertainty results from the complex pulsational line-profile variability that has been so far ignored in the literature. We detect 42 frequencies with amplitudes above five times the local noise level. Amongst these we identify five rotationally split triplets and one quintuplet. Asteroseismic modelling based on CoRoT, Gaia DR2, and spectroscopic data leads to a star of M ∼ 8 M⊙ near core-hydrogen exhaustion and an extended overshoot zone. The detected low-order pressure-mode frequencies cannot be fit within the uncertainties of the CoRoT data by models without atomic diffusion. Irrespective of this limitation, the low-order gravity modes reveal HD 170580 to be a slow rotator with an average rotation period between 73 and 98 d and a hint of small differential rotation. Conclusions. Future Gaia DR3 data taking into account the multiplicity of the star, along with long-term TESS photometry would allow us to put better observational constraints on the asteroseismic models of this blue evolved massive star. Improved modelling with atomic diffusion, including radiative levitation, is needed to achieve compliance with the low helium surface abundance of the star. This poses immense computational challenges but is required to derive the interior rotation and mixing profiles of this star.

scholarly journals 3D Modeling of the Structure and Dynamics of a Main-Sequence F-type Star

2019 ◽  
Vol 15 (S354) ◽  
pp. 86-93
Author(s):  
Irina N. Kitiashvili ◽  
Alan A. Wray
Keyword(s):  
Main Sequence ◽  
Initial Approximation ◽  
Stellar Structure ◽  
Computational Domain ◽  
Mixing Length ◽  
Stellar Rotation ◽  
Solar Mass ◽  
Turbulent Layer ◽  
Near Surface ◽  
Structure And Dynamics

AbstractCurrent state-of-the-art computational modeling makes it possible to build realistic models of stellar convection zones and atmospheres that take into account chemical composition, radiative effects, ionization, and turbulence. The standard 1D mixing-length-based evolutionary models are not able to capture many physical processes of the stellar interior dynamics. Mixing-length models provide an initial approximation of stellar structure that can be used to initialize 3D radiative hydrodynamics simulations which include realistic modeling of turbulence, radiation, and other phenomena.In this paper, we present 3D radiative hydrodynamic simulations of an F-type main-sequence star with 1.47 solar mass. The computational domain includes the upper layers of the radiation zone, the entire convection zone, and the photosphere. The effects of stellar rotation is modeled in the f-plane approximation. These simulations provide new insight into the properties of the convective overshoot region, the dynamics of the near-surface, highly turbulent layer, and the structure and dynamics of granulation. They reveal solar-type differential rotation and latitudinal dependence of the tachocline location.

scholarly journals Modeling the global oscillations of Epsilon Eridani

1988 ◽  
Vol 123 ◽  
pp. 281-284
Author(s):  
David R. Soderblom ◽  
Werner Däppen
Keyword(s):  
Observational Data ◽  
Stellar Structure ◽  
Global Oscillation ◽  
Oscillation Frequencies ◽  
Best Estimates

We have reviewed the observational data for ∊ Eri to derive our best estimates of luminosity, radius, temperature, composition, and age, as well as the uncertainties associated with each of these. These quantities are then used in stellar structure models to try to reproduce the global oscillation frequencies observed by Noyes et al. We find that we can reproduce the observed frequencies and splittings at least as well as the computations of Guenther and Demarque, yet for significantly different stellar parameters, most notably the age.

scholarly journals The impact of Jean-Paul Zahn discoveries on rotation and evolution

2019 ◽  
Vol 82 ◽  
pp. 137-151
Author(s):  
A. Maeder ◽  
G. Meynet
Keyword(s):  
Binary Stars ◽  
Dominant Role ◽  
Close Binaries ◽  
Stellar Rotation ◽  
Jean Paul ◽  
Low Metallicity ◽  
Main Effects ◽  
The Impact ◽  
Rotational Coupling ◽  
Internal Coupling

We first review the main effects of stellar rotation on evolution along the fundamental discoveries by Jean-Paul. Then, we examine some of the consequences of rotation in the evolution of single and binary stars. The proper account of meridional circulation in close binaries tends to increase the synchronization time because meridional currents always counteract the tidal interaction. We consider the case of the very low metallicity Z stars, in particular the CEMP-no stars, where rotational mixing may have played a dominant role in their strange chemical composition. Then, turning to “What are the mysteries?”, we emphasize that all over the evolution and for various masses the present models seem to still have a lack of rotational coupling between cores and envelopes. We suggest that magnetic fields may produce this missing internal coupling.

scholarly journals New and improved rotational periods of magnetic CP stars from ASAS-3, KELT, and MASCARA data

2020 ◽  
Vol 493 (3) ◽  
pp. 3293-3330 ◽  
Author(s):  
Klaus Bernhard ◽  
Stefan Hümmerich ◽  
Ernst Paunzen
Keyword(s):  
Binary Systems ◽  
Time Series Data ◽  
Eclipsing Binaries ◽  
Atomic Diffusion ◽  
Close Binary ◽  
Series Data ◽  
Rotational Period ◽  
Stellar Rotation ◽  
Apparent Lack ◽  
Final Sample

ABSTRACT Magnetic chemically peculiar (mCP) stars allow the investigation of such diverse phenomena as atomic diffusion, magnetic fields, and stellar rotation. The aim of the present investigation is to enhance our knowledge of the rotational properties of mCP stars by increasing the sample of objects with accurately determined rotational periods. To this end, archival photometric time-series data from the ASAS-3, KELT, and MASCARA surveys were employed to improve existing rotational period information and derive rotational periods for mCP stars hitherto not known to be photometric variables. Our final sample consists of 294 mCP stars, a considerable amount of which (more than 40 per cent) are presented here as photometric variables for the first time. In addition, we identified 24 mCP star candidates that show light variability in agreement with rotational modulation but lack spectroscopic confirmation. The rotational period distribution of our sample agrees well with the literature. Most stars are between 100 Myr and 1 Gyr old, with an apparent lack of very young stars. No objects were found on the zero-age main sequence; several stars seem to have evolved to the subgiant stage, albeit well before the first dredge-up. We identified four eclipsing binaries (HD 244391, HD 247441, HD 248784, and HD 252519), which potentially host an mCP star. This is of great interest because mCP stars are very rarely found in close binary systems, particularly eclipsing ones. Using archival spectra, we find strong evidence that the HD 252519 system indeed harbours an mCP star component.

On Atomic Diffusion and the Cosmological Lithium Abundance

2013 ◽  
Vol 9 (S298) ◽  
pp. 407-407
Author(s):  
Pieter Gruyters ◽  
Andreas J. Korn ◽  
Paul S. Barklem
Keyword(s):  
Globular Clusters ◽  
Ad Hoc ◽  
Background Radiation ◽  
Big Bang ◽  
Stellar Structure ◽  
Atomic Diffusion ◽  
Green Line ◽  
Microwave Background ◽  
Lithium Abundance ◽  
Microwave Background Radiation

When it comes to lithium in late-type stars, atomic diffusion (AD) refers to the slow gravitational settling below the convective zone. Richard, Michaud & Richter, J. (2005) computed the influence of diffusion on the lithium abundance with different additional mixing (AddMix) parameters, after 13.5 Gyr with an initial Li abundance compatible with BBN. Without AddMix the abundance of lithium would decrease when the temperature of the star increases. This is depicted by the dashed green line in the left panel of Fig. 1 and is in contradiction with the existence of a lithium plateau. But with a model including ad-hoc AddMix, where the AddMix diffusion coefficient is given by DT and is connected to DHe(AD) at a reference temperature of log T0=6.25, it is possible to reproduce the plateau as seen in the figure (solid green line). AD with AddMix has so far been shown to be at work in two globular clusters (GC) with different metallicities. Korn et al. (2007) showed the effects in NGC 6397 at [Fe/H] = −2.1. More recently Gruyters et al. (2013) have shown smaller effects, but similar in nature, in NGC 6752 at [Fe/H] = −1.6. The Li abundance for both clusters can be brought in to agreement with predictions from the cosmic microwave background radiation and Big Bang nucleosynthesis (CMB+BBN) by using stellar structure models including AD and AddMix, although with different efficiencies of AddMix. It seems there is an evolution of AddMix with metallicity which renders AD less efficient. As AddMix acts only in the outer regions, helium settling in the core is not affected, and so the overall evolution (e.g. Teff-age relation) will be similar regardless of this parameter.

scholarly journals Stellar hydrodynamics caught in the act: Asteroseismology with CoRoT and Kepler

2010 ◽  
Vol 6 (S271) ◽  
pp. 32-61 ◽  
Author(s):  
Jørgen Christensen-Dalsgaard ◽  
Michael J. Thompson
Keyword(s):  
Indirect Effects ◽  
Dynamic Behaviour ◽  
Space Missions ◽  
Stellar Structure ◽  
Direct And Indirect Effects ◽  
Stellar Rotation ◽  
Stellar Oscillations ◽  
Stellar Interiors ◽  
Dynamic Phenomena

AbstractAsteroseismic investigations, particularly based on data on stellar oscillations from the CoRoT andKeplerspace missions, are providing unique possibilities for investigating the properties of stellar interiors. This constitutes entirely new ways to study the effects of dynamic phenomena on stellar structure and evolution. Important examples are the extent of convection zones and the associated mixing and the direct and indirect effects of stellar rotation. In addition, the stellar oscillations themselves show very interesting dynamic behaviour. Here we discuss examples of the results obtained from such investigations, across the Hertzsprung-Russell diagram.

scholarly journals Impacts of radiative accelerations on solar-like oscillating main-sequence stars

2018 ◽  
Vol 618 ◽  
pp. A10 ◽  
Author(s):  
M. Deal ◽  
G. Alecian ◽  
Y. Lebreton ◽  
M. J. Goupil ◽  
J. P. Marques ◽  
...  
Keyword(s):  
Seismic Data ◽  
Main Sequence ◽  
Chemical Elements ◽  
Mass Range ◽  
Companion Paper ◽  
Transport Processes ◽  
Atomic Diffusion ◽  
Parameter Method ◽  
Main Sequence Stars ◽  
Gravitational Settling

Context. Chemical element transport processes are among the crucial physical processes needed for precise stellar modelling. Atomic diffusion by gravitational settling is usually taken into account, and is essential for helioseismic studies. On the other hand, radiative accelerations are rarely accounted for, act differently on the various chemical elements, and can strongly counteract gravity in some stellar mass domains. The resulting variations in the abundance profiles may significantly affect the structure of the star. Aims. The aim of this study is to determine whether radiative accelerations impact the structure of solar-like oscillating main-sequence stars observed by asteroseismic space missions. Methods. We implemented the calculation of radiative accelerations operating on C, N, O, Ne, Na, Mg, Al, Si, S, Ca, and Fe in the CESTAM code using the single-valued parameter method. We built and compared several grids of stellar models including gravitational settling, some with and others without radiative accelerations. We considered masses in the range [0.9, 1.5] M⊙ and three values of the metallicity around the solar value. For each metallicity we determined the mass range where differences between models due to radiative accelerations exceed the uncertainties of global seismic parameters of the Kepler Legacy sample or expected for PLATO observations. Results. We found that radiative accelerations may not be neglected for stellar masses higher than 1.1 M⊙ at solar metallicity. The difference in age due to their inclusion in models can reach 9% for the more massive stars of our grids. We estimated that the percentage of the PLATO core program stars whose modelling would require radiative accelerations ranges between 33% and 58% depending on the precision of the seismic data. Conclusions. We conclude that in the context of Kepler, TESS, and PLATO missions which provide (or will provide) high-quality seismic data, radiative accelerations can have a significant effect when properly inferring the properties of solar-like oscillators. This is particularly important for age inferences. However, the net effect for each individual star results from the competition between atomic diffusion including radiative accelerations and other internal transport processes. Rotationally induced transport processes for instance are believed to reduce the effects of atomic diffusion. This will be investigated in a forthcoming companion paper.

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