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C Charbonnel - One of the best experts on this subject based on the ideXlab platform.
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Lithium depletion and angular Momentum Transport in solar-type stars
'EDP Sciences', 2021Co-Authors: T. Dumont, C Charbonnel, A. Palacios, O. Richard, L. Amard, K. Augustson, S. MathisAbstract:Context. Transport processes occurring in the radiative interior of solar-type stars are evidenced by the surface variation of light elements, in particular 7Li, and the evolution of their rotation rates. For the Sun, inversions of helioseismic data indicate that the radial profile of angular velocity in its radiative zone is nearly uniform, which implies the existence of angular Momentum Transport mechanisms that are efficient over evolutionary timescales. While there are many independent Transport models for angular Momentum and chemical species, there is a lack of self-consistent theories that permit stellar evolution models to simultaneously match the present-day observations of solar lithium abundances and radial rotation profiles. Aims. We explore how additional Transport processes can improve the agreement between evolutionary models of rotating stars and observations for 7Li depletion, the rotation evolution of solar-type stars, and the solar rotation profile. Methods. Models of solar-type stars are computed including atomic diffusion and rotation-induced mixing with the code STAREVOL. We explore different additional Transport processes for chemicals and for angular Momentum such as penetrative convection, tachocline mixing, and additional turbulence. We constrain the resulting models by simultaneously using the evolution of the surface rotation rate and 7Li abundance in the solar-type stars of open clusters with different ages, and the solar surface and internal rotation profile as inverted from helioseismology when our models reach the age of the Sun. Results. We show the relevance of penetrative convection for the depletion of 7Li in pre-main sequence and early main sequence stars. The rotational dependence of the depth of penetrative convection yields an anti-correlation between the initial rotation rate and 7Li depletion in our models of solar-type stars that is in agreement with the observed trend. Simultaneously, the addition of an ad hoc vertical viscosity νadd leads to efficient Transport of angular Momentum between the core and the envelope during the main sequence evolution and to solar-type models that match the observed profile of the Sun. We also self-consistently compute for the first time the thickness of the tachocline and find that it is compatible with helioseismic estimations at the age of the Sun, but we highlight that the associated turbulence does not allow the observed 7Li depletion to be reproduced. The main sequence depletion of 7Li in solar-type stars is only reproduced when adding a parametric turbulent mixing below the convective envelope. Conclusions. The need for additional Transport processes in stellar evolution models for both chemicals and angular Momentum in addition to atomic diffusion, meridional circulation, and turbulent shear is confirmed. We identify the rotational dependence of the penetrative convection as a key process. Two additional and distinct parametric turbulent mixing processes (one for angular Momentum and one for chemicals) are required to simultaneously explain the observed surface 7Li depletion and the solar internal rotation profile. We highlight the need of additional constraints for the internal rotation of young solar-type stars and also for the beryllium abundances of open clusters in order to test our predictions
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angular Momentum Transport by internal gravity waves iv wave generation by surface convection zone from the pre main sequence to the early agb in intermediate mass stars
Astronomy and Astrophysics, 2008Co-Authors: Suzanne Talon, C CharbonnelAbstract:Context. This is the fourth in a series of papers that deal with angular Momentum Transport by internal gravity waves in stellar interiors. Aims. Here, we want to examine the potential role of waves in other evolutionary phases than the main sequence. Methods. We study the evolution of a $3\,M_\odot$ Population I model from the pre-main sequence to the early-AGB phase and examine whether waves can lead to angular Momentum redistribution and/or element diffusion at the external convection zone boundary. Results. We find that, although waves produced by the surface convection zone can be ignored safely for such a star during the main sequence, it is not the case for later evolutionary stages. In particular, angular Momentum Transport by internal waves could be quite important at the end of the sub-giant branch and during the early-AGB phase. Wave-induced mixing of chemicals is expected during the early-AGB phase.
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angular Momentum Transport by internal gravity waves iv wave generation by surface convection zone from the pre main sequence to the early agb in intermediate mass stars
arXiv: Astrophysics, 2008Co-Authors: Suzanne Talon, C CharbonnelAbstract:This is the fourth in a series of papers that deal with angular Momentum Transport by internal gravity waves in stellar interiors. Here, we want to examine the potential role of waves in other evolutionary phases than the main sequence. We study the evolution of a 3Msun Population I model from the pre-main sequence to the early-AGB phase and examine whether waves can lead to angular Momentum redistribution and/or element diffusion at the external convection zone boundary. We find that, although waves produced by the surface convection zone can be ignored safely for such a star during the main sequence, it is not the case for later evolutionary stages. In particular, angular Momentum Transport by internal waves could be quite important at the end of the sub-giant branch and during the early-AGB phase. Wave-induced mixing of chemicals is expected during the early-AGB phase.
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angular Momentum Transport by internal gravity waves iii wave excitation by core convection and the coriolis effect
Astronomy and Astrophysics, 2007Co-Authors: Suzanne Talon, Florian Pantillon, C CharbonnelAbstract:Context. This is the third in a series of papers that deal with angular Momentum Transport by internal gravity waves. We concentrate on the waves excited by core convection in a 3 M� , Pop I main sequence star. Aims. Here, we want to examine the role of the Coriolis acceleration in the equations of motion that describe the behavior of waves and to evaluate its impact on angular Momentum Transport. Methods. We use the so-called traditional approximation of geophysics, which allows variable separation in radial and horizontal components. In the presence of rotation, the horizontal structure is described by Hough functions instead of spherical harmonics. Results. The Coriolis acceleration has two main effects on waves. It transforms pure gravity waves into gravito-inertial waves that have a larger amplitude closer to the equator, and it introduces new waves whose restoring force is mainly the conservation of vorticity. Conclusions. Taking the Coriolis acceleration into account changes the subtle balance between prograde and retrograde waves in nonrotating stars. It also introduces new types of waves that are either purely prograde or retrograde. We show in this paper where the local deposition of angular Momentum by such waves is important.
A V Bridgwater - One of the best experts on this subject based on the ideXlab platform.
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a cfd approach on the effect of particle size on char entrainment in bubbling fluidised bed reactors
Biomass & Bioenergy, 2010Co-Authors: K Papadikis, Sai Gu, A V BridgwaterAbstract:The fluid – particle interaction inside a 41.7 mg s-1 fluidised bed reactor is modelled. Three char particles of sizes 500 µm, 250 µm, and 100 µm are injected into the fluidised bed and the Momentum Transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions and reactor design the char particles will either be entrained from the reactor or remain inside the bubbling bed. The particle size is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. A 3-Dimensional simulation has been performed with a completele revised Momentum Transport model for bubble three-phase flow according to the literature as an extension to the commercial finite volume code FLUENT 6.2.
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CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors, Part A: Eulerian computation of Momentum Transport in bubbling fluidised beds
Chemical Engineering Science, 2008Co-Authors: K Papadikis, A V BridgwaterAbstract:The fluid–particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the Momentum Transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase, according to the literature. FLUENT 6.2 has been used as the modelling framework of the simulations with a completely revised drag model, in the form of user defined function (UDF), to calculate the forces exerted on the particle as well as its velocity components. 2-D and 3-D simulations are tested and compared. The study is the first part of a complete pyrolysis model in fluidised bed reactors.
D N C Lin - One of the best experts on this subject based on the ideXlab platform.
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internal gravity waves in massive stars angular Momentum Transport
The Astrophysical Journal, 2013Co-Authors: T M Rogers, D N C Lin, J N Mcelwaine, Herbert H B LauAbstract:We present numerical simulations of internal gravity waves (IGW) in a star with a convective core and extended radiative envelope. We report on amplitudes, spectra, dissipation, and consequent angular Momentum Transport by such waves. We find that these waves are generated efficiently and Transport angular Momentum on short timescales over large distances. We show that, as in Earth's atmosphere, IGW drive equatorial flows which change magnitude and direction on short timescales. These results have profound consequences for the observational inferences of massive stars, as well as their long term angular Momentum evolution. We suggest IGW angular Momentum Transport may explain many observational mysteries, such as: the misalignment of hot Jupiters around hot stars, the Be class of stars, Ni enrichment anomalies in massive stars, and the non-synchronous orbits of interacting binaries.
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internal gravity waves in massive stars angular Momentum Transport
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: T M Rogers, D N C Lin, J N Mcelwaine, Herbert H B LauAbstract:We present numerical simulations of internal gravity waves (IGW) in a star with a convective core and extended radiative envelope. We report on amplitudes, spectra, dissipation and consequent angular Momentum Transport by such waves. We find that these waves are generated efficiently and Transport angular Momentum on short timescales over large distances. We show that, as in the Earth's atmosphere, IGW drive equatorial flows which change magnitude and direction on short timescales. These results have profound consequences for the observational inferences of massive stars, as well as their long term angular Momentum evolution. We suggest IGW angular Momentum Transport may explain many observational mysteries, such as: the misalignment of hot Jupiters around hot stars, the Be class of stars, Ni enrichment anomalies in massive stars and the non-synchronous orbits of interacting binaries.
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internal gravity waves modulate the apparent misalignment of exoplanets around hot stars
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: T M Rogers, D N C Lin, Herbert H B LauAbstract:We propose that the observed misalignment between extra-solar planets and their hot host stars can be explained by angular Momentum Transport within the host star. Observations have shown that this misalignment is preferentially around hot stars, which have convective cores and extended radiative envelopes. This situation is amenable to substantial angular Momentum Transport by internal gravity waves (IGW) generated at the convective-radiative interface. Here we present numerical simulations of this process and show that IGW can modulate the surface rotation of the star. With these two- dimensional simulations we show that IGW could explain the retrograde orbits observed in systems such as HAT-P-6 and HAT-P-7, however, extension to high obliquity objects will await future three- dimensional simulations. We note that these results also imply that individual massive stars should show temporal variations in their v sini measurements.
S Deheuvels - One of the best experts on this subject based on the ideXlab platform.
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angular Momentum Transport efficiency in post main sequence low mass stars
Astronomy and Astrophysics, 2016Co-Authors: F Spada, M Gellert, Rainer Arlt, S DeheuvelsAbstract:Context. Using asteroseismic techniques, it has recently become possible to probe the internal rotation profile of low-mass (≈1.1−1.5 M ⊙ ) subgiant and red giant stars. Under the assumption of local angular Momentum conservation, the core contraction and envelope expansion occurring at the end of the main sequence would result in a much larger internal differential rotation than observed. This suggests that angular Momentum redistribution must be taking place in the interior of these stars. Aims. We investigate the physical nature of the angular Momentum redistribution mechanisms operating in stellar interiors by constraining the efficiency of post-main sequence rotational coupling. Methods. We model the rotational evolution of a 1.25 M ⊙ star using the Yale Rotational stellar Evolution Code. Our models take into account the magnetic wind braking occurring at the surface of the star and the angular Momentum Transport in the interior, with an efficiency dependent on the degree of internal differential rotation. Results. We find that models including a dependence of the angular Momentum Transport efficiency on the radial rotational shear reproduce very well the observations. The best fit of the data is obtained with an angular Momentum Transport coefficient scaling with the ratio of the rotation rate of the radiative interior over that of the convective envelope of the star as a power law of exponent ≈3. This scaling is consistent with the predictions of recent numerical simulations of the Azimuthal Magneto-Rotational Instability. Conclusions. We show that an angular Momentum Transport process whose efficiency varies during the stellar evolution through a dependence on the level of internal differential rotation is required to explain the observed post-main sequence rotational evolution of low-mass stars.
Ben Brown - One of the best experts on this subject based on the ideXlab platform.
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angular Momentum Transport via internal gravity waves in evolving stars
The Astrophysical Journal, 2014Co-Authors: Jim Fuller, Daniel Lecoanet, Matteo Cantiello, Ben BrownAbstract:Recent asteroseismic advances have allowed for direct measurements of the internal rotation rates of many subgiant and red giant stars. Unlike the nearly rigidly rotating Sun, these evolved stars contain radiative cores that spin faster than their overlying convective envelopes, but slower than they would in the absence of internal angular Momentum Transport. We investigate the role of internal gravity waves in angular Momentum Transport in evolving low-mass stars. In agreement with previous results, we find that convectively excited gravity waves can prevent the development of strong differential rotation in the radiative cores of Sun-like stars. As stars evolve into subgiants, however, low-frequency gravity waves become strongly attenuated and cannot propagate below the hydrogen-burning shell, allowing the spin of the core to decouple from the convective envelope. This decoupling occurs at the base of the subgiant branch when stars have surface temperatures of T ≈ 5500 K. However, gravity waves can still spin down the upper radiative region, implying that the observed differential rotation is likely confined to the deep core near the hydrogen-burning shell. The torque on the upper radiative region may also prevent the core from accreting high angular Momentum material and slow the rate of core spin-up. The observed spin-down of cores on the red giant branch cannot be totally attributed to gravity waves, but the waves may enhance shear within the radiative region and thus increase the efficacy of viscous/magnetic torques.