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S Deheuvels - One of the best experts on this subject based on the ideXlab platform.

  • angular momentum transport efficiency in post main sequence low mass stars
    Astronomy and Astrophysics, 2016
    Co-Authors: F Spada, M Gellert, Rainer Arlt, S Deheuvels
    Abstract:

    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.

C Charbonnel - One of the best experts on this subject based on the ideXlab platform.

  • Lithium in red giant stars: Constraining non-standard mixing with large surveys in the
    'EDP Sciences', 2020
    Co-Authors: C Charbonnel, N. Lagarde, G. Jasniewicz, P. L. North, M. Shetrone, Krugler J. Hollek, V. V. Smith, R. Smiljanic, A. Palacios, G. Ottoni
    Abstract:

    Context. Li is extensively known to be a good tracer of non-standard mixing processes occurring in Stellar Interiors. Aims. We present the results of a new large Li survey in red giant stars and combine it with surveys from the literature to probe the impact of rotation-induced mixing and thermohaline double-diffusive instability along Stellar evolution. Methods. We determined the surface Li abundance for a sample of 829 giant stars with accurate Gaia parallaxes for a large sub-sample (810 stars) complemented with accurate HIPPARCO

  • 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, 2008
    Co-Authors: Suzanne Talon, C Charbonnel
    Abstract:

    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.

  • 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, 2008
    Co-Authors: Suzanne Talon, C Charbonnel
    Abstract:

    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.

D M Bowman - One of the best experts on this subject based on the ideXlab platform.

  • asteroseismology of high mass stars new insights of Stellar Interiors with space telescopes
    Frontiers in Astronomy and Space Sciences, 2020
    Co-Authors: D M Bowman
    Abstract:

    Massive stars are important metal factories in the Universe. They have short and energetic lives, and many of them inevitably explode as a supernova and become a neutron star or black hole. In turn, the formation, evolution and explosive deaths of massive stars impact the surrounding interStellar medium and shape the evolution of their host galaxies. Yet the chemical and dynamical evolution of a massive star, including the chemical yield of the ultimate supernova and the remnant mass of the compact object, strongly depend on the interior physics of the progenitor star. We currently lack empirically calibrated prescriptions for various physical processes at work within massive stars, but this is now being remedied by asteroseismology. The study of Stellar structure and evolution using Stellar oscillations -- asteroseismology -- has undergone a revolution in the last two decades thanks to high-precision time series photometry from space telescopes. In particular, the long-term light curves provided by the MOST, CoRoT, BRITE, Kepler/K2 and TESS missions provided invaluable data sets in terms of photometric precision, duration and frequency resolution to successfully apply asteroseismology to massive stars and probe their interior physics. The observation and subsequent modelling of Stellar pulsations in massive stars has revealed key missing ingredients in Stellar structure and evolution models of these stars. Thus asteroseismology has opened a new window into calibrating Stellar physics within a highly degenerate part of the Hertzsprung--Russell diagram. In this review, I provide a historical overview of the progress made using ground-based and early space missions, and discuss more recent advances and breakthroughs in our understanding of massive star Interiors by means of asteroseismology with modern space telescopes.

  • Detecting axisymmetric magnetic fields using gravity modes in intermediate-mass stars
    'EDP Sciences', 2020
    Co-Authors: Van Beeck J., D M Bowman, Van Reeth T., Mathis S., Neiner C., Aerts C.
    Abstract:

    Context. Angular momentum (AM) transport models of Stellar Interiors require improvements to explain the strong extraction of AM from Stellar cores that is observed with asteroseismology. One of the frequently invoked mediators of AM transport are internal magnetic fields, even though their properties, observational signatures, and influence on Stellar evolution are largely unknown. Aims. We study how a fossil, axisymmetric internal magnetic field affects period spacing patterns of dipolar gravity mode oscillations in main sequence stars with 1.3, 2.0, and 3.0 solar masses . We assess the influence of fundamental Stellar parameters on the magnitude of pulsation mode frequency shifts. Methods. We computed dipolar gravity mode frequency shifts due to a fossil, axisymmetric poloidal-toroidal internal magnetic field for a grid of Stellar evolution models, varying Stellar fundamental parameters. Rigid rotation was taken into account using the traditional approximation of rotation, and the influence of the magnetic field was computed using a perturbative approach. Results. We find magnetic signatures for dipolar gravity mode oscillations in terminal-age main sequence stars that are measurable for a near-core field strength larger than 10⁵ G. The predicted signatures differ appreciably from those due to rotation. Conclusions. Our formalism demonstrates the potential for the future detection and characterization of strong fossil, axisymmetric internal magnetic fields in gravity-mode pulsators near the end of core-hydrogen burning from Kepler photometry, if such fields exist.status: publishe

  • Fully compressible simulations of waves and core convection in main-sequence stars
    'EDP Sciences', 2020
    Co-Authors: L. Horst, D M Bowman, C Aerts, P. V. F. Edelmann, R. Andrassy, F. K. Röpke, R. P. Ratnasingam
    Abstract:

    Context. Recent, nonlinear simulations of wave generation and propagation in full-star models have been carried out in the anelastic approximation using spectral methods. Although it makes long time steps possible, this approach excludes the physics of sound waves completely and requires rather high artificial viscosity and thermal diffusivity for numerical stability. A direct comparison with observations is thus limited. Aims. We explore the capabilities of our compressible multidimensional Seven-League Hydro (SLH) code to simulate Stellar oscillations. Methods. We compare some fundamental properties of internal gravity and pressure waves in 2D SLH simulations to linear wave theory using two test cases: (1) an interval gravity wave packet in the Boussinesq limit and (2) a realistic 3 M⊙ Stellar model with a convective core and a radiative envelope. Oscillation properties of the Stellar model are also discussed in the context of observations. Results. Our tests show that specialized low-Mach techniques are necessary when simulating oscillations in Stellar Interiors. Basic properties of internal gravity and pressure waves in our simulations are in good agreement with linear wave theory. As compared to anelastic simulations of the same Stellar model, we can follow internal gravity waves of much lower frequencies. The temporal frequency spectra of velocity and temperature are flat and compatible with the observed spectra of massive stars. Conclusion. The low-Mach compressible approach to hydrodynamical simulations of Stellar oscillations is promising. Our simulations are less dissipative and require less luminosity boosting than comparable spectral simulations. The fully-compressible approach allows for the coupling of gravity and pressure waves in the outer convective envelopes of evolved stars to be studied in the future

  • low frequency gravity waves in blue supergiants revealed by high precision space photometry
    Nature Astronomy, 2019
    Co-Authors: D M Bowman, Siemen Burssens, M G Pedersen, C Johnston, C Aerts, B Buysschaert, Mathias Michielsen, A Tkachenko
    Abstract:

    Almost all massive stars explode as supernovae and form a black hole or neutron star. The remnant mass and the impact of the chemical yield on subsequent star formation and galactic evolution strongly depend on the internal physics of the progenitor star, which is currently not well understood. The theoretical uncertainties of Stellar Interiors accumulate with Stellar age, which is particularly pertinent for the blue supergiant phase. Stellar oscillations represent a unique method of probing Stellar Interiors, yet inference for blue supergiants is hampered by a dearth of observed pulsation modes. Here we report the detection of diverse variability in blue supergiants using the K2 and TESS space missions. The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives. Future asteroseismic modelling will provide constraints on ages, core masses, interior mixing, rotation and angular momentum transport. The discovery of variability in blue supergiants is a step towards a data-driven empirical calibration of theoretical evolution models for the most massive stars in the Universe. Leveraging the precision of K2 and TESS, Bowman et al. have detected variability in galactic and Magellanic blue supergiants that is due to low-frequency gravity waves in their Interiors.

  • period spacings of gravity modes in rapidly rotating magnetic stars i axisymmetric fossil field with poloidal and toroidal components
    Astronomy and Astrophysics, 2019
    Co-Authors: V Prat, D M Bowman, C Aerts, B Buysschaert, S Mathis, J Van Beeck, C Neiner
    Abstract:

    Context. Stellar magnetic fields are often invoked to explain the missing transport of angular momentum observed in models of Stellar Interiors. However, the properties of an internal magnetic field and the consequences of its presence on Stellar evolution are largely unknown. Aims. We study the effect of an axisymmetric internal magnetic field on the frequency of gravity modes in rapidly rotating stars to check whether gravity modes can be used to detect and probe such a field. Methods. Rotation is taken into account using the traditional approximation of rotation and the effect of the magnetic field is computed using a perturbative approach. As a proof of concept, we compute frequency shifts due to a mixed (i.e. with both poloidal and toroidal components) fossil magnetic field for a representative model of a known magnetic, rapidly rotating, slowly pulsating B-type star: HD 43317. Results. We find that frequency shifts induced by the magnetic field scale with the square of its amplitude. A magnetic field with a near-core strength of the order of 150 kG (which is consistent with the observed surface field strength of the order of 1 kG) leads to signatures that are detectable in period spacings for high-radial-order gravity modes. Conclusions. The predicted frequency shifts can be used to constrain internal magnetic fields and offer the potential for a significant step forward in our interpretation of the observed structure of gravity-mode period spacing patterns in rapidly rotating stars.

C Aerts - One of the best experts on this subject based on the ideXlab platform.

  • Fully compressible simulations of waves and core convection in main-sequence stars
    'EDP Sciences', 2020
    Co-Authors: L. Horst, D M Bowman, C Aerts, P. V. F. Edelmann, R. Andrassy, F. K. Röpke, R. P. Ratnasingam
    Abstract:

    Context. Recent, nonlinear simulations of wave generation and propagation in full-star models have been carried out in the anelastic approximation using spectral methods. Although it makes long time steps possible, this approach excludes the physics of sound waves completely and requires rather high artificial viscosity and thermal diffusivity for numerical stability. A direct comparison with observations is thus limited. Aims. We explore the capabilities of our compressible multidimensional Seven-League Hydro (SLH) code to simulate Stellar oscillations. Methods. We compare some fundamental properties of internal gravity and pressure waves in 2D SLH simulations to linear wave theory using two test cases: (1) an interval gravity wave packet in the Boussinesq limit and (2) a realistic 3 M⊙ Stellar model with a convective core and a radiative envelope. Oscillation properties of the Stellar model are also discussed in the context of observations. Results. Our tests show that specialized low-Mach techniques are necessary when simulating oscillations in Stellar Interiors. Basic properties of internal gravity and pressure waves in our simulations are in good agreement with linear wave theory. As compared to anelastic simulations of the same Stellar model, we can follow internal gravity waves of much lower frequencies. The temporal frequency spectra of velocity and temperature are flat and compatible with the observed spectra of massive stars. Conclusion. The low-Mach compressible approach to hydrodynamical simulations of Stellar oscillations is promising. Our simulations are less dissipative and require less luminosity boosting than comparable spectral simulations. The fully-compressible approach allows for the coupling of gravity and pressure waves in the outer convective envelopes of evolved stars to be studied in the future

  • low frequency gravity waves in blue supergiants revealed by high precision space photometry
    Nature Astronomy, 2019
    Co-Authors: D M Bowman, Siemen Burssens, M G Pedersen, C Johnston, C Aerts, B Buysschaert, Mathias Michielsen, A Tkachenko
    Abstract:

    Almost all massive stars explode as supernovae and form a black hole or neutron star. The remnant mass and the impact of the chemical yield on subsequent star formation and galactic evolution strongly depend on the internal physics of the progenitor star, which is currently not well understood. The theoretical uncertainties of Stellar Interiors accumulate with Stellar age, which is particularly pertinent for the blue supergiant phase. Stellar oscillations represent a unique method of probing Stellar Interiors, yet inference for blue supergiants is hampered by a dearth of observed pulsation modes. Here we report the detection of diverse variability in blue supergiants using the K2 and TESS space missions. The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives. Future asteroseismic modelling will provide constraints on ages, core masses, interior mixing, rotation and angular momentum transport. The discovery of variability in blue supergiants is a step towards a data-driven empirical calibration of theoretical evolution models for the most massive stars in the Universe. Leveraging the precision of K2 and TESS, Bowman et al. have detected variability in galactic and Magellanic blue supergiants that is due to low-frequency gravity waves in their Interiors.

  • period spacings of gravity modes in rapidly rotating magnetic stars i axisymmetric fossil field with poloidal and toroidal components
    Astronomy and Astrophysics, 2019
    Co-Authors: V Prat, D M Bowman, C Aerts, B Buysschaert, S Mathis, J Van Beeck, C Neiner
    Abstract:

    Context. Stellar magnetic fields are often invoked to explain the missing transport of angular momentum observed in models of Stellar Interiors. However, the properties of an internal magnetic field and the consequences of its presence on Stellar evolution are largely unknown. Aims. We study the effect of an axisymmetric internal magnetic field on the frequency of gravity modes in rapidly rotating stars to check whether gravity modes can be used to detect and probe such a field. Methods. Rotation is taken into account using the traditional approximation of rotation and the effect of the magnetic field is computed using a perturbative approach. As a proof of concept, we compute frequency shifts due to a mixed (i.e. with both poloidal and toroidal components) fossil magnetic field for a representative model of a known magnetic, rapidly rotating, slowly pulsating B-type star: HD 43317. Results. We find that frequency shifts induced by the magnetic field scale with the square of its amplitude. A magnetic field with a near-core strength of the order of 150 kG (which is consistent with the observed surface field strength of the order of 1 kG) leads to signatures that are detectable in period spacings for high-radial-order gravity modes. Conclusions. The predicted frequency shifts can be used to constrain internal magnetic fields and offer the potential for a significant step forward in our interpretation of the observed structure of gravity-mode period spacing patterns in rapidly rotating stars.

  • Period spacings of gravity modes in rapidly rotating magnetic stars
    Astronomy and Astrophysics - A&A, 2019
    Co-Authors: V Prat, D M Bowman, C Aerts, B Buysschaert, S Mathis, J Van Beeck, C Neiner
    Abstract:

    $Context$. Stellar magnetic fields are often invoked to explain the missing transport of angular momentum observed in models of Stellar Interiors. However, the properties of an internal magnetic field and the consequences of its presence on Stellar evolution are largely unknown.$Aims$. We study the effect of an axisymmetric internal magnetic field on the frequency of gravity modes in rapidly rotating stars to check whether gravity modes can be used to detect and probe such a field.$Methods$. Rotation is taken into account using the traditional approximation of rotation and the effect of the magnetic field is computed using a perturbative approach. As a proof of concept, we compute frequency shifts due to a mixed (i.e. with both poloidal and toroidal components) fossil magnetic field for a representative model of a known magnetic, rapidly rotating, slowly pulsating B-type star: HD 43317.$Results$. We find that frequency shifts induced by the magnetic field scale with the square of its amplitude. A magnetic field with a near-core strength of the order of 150 kG (which is consistent with the observed surface field strength of the order of 1 kG) leads to signatures that are detectable in period spacings for high-radial-order gravity modes.$Conclusions$. The predicted frequency shifts can be used to constrain internal magnetic fields and offer the potential for a significant step forward in our interpretation of the observed structure of gravity-mode period spacing patterns in rapidly rotating stars.

  • low frequency gravity waves in blue supergiants revealed by high precision space photometry
    arXiv: Solar and Stellar Astrophysics, 2019
    Co-Authors: D M Bowman, Siemen Burssens, M G Pedersen, C Johnston, C Aerts, B Buysschaert, Mathias Michielsen, A Tkachenko
    Abstract:

    Almost all massive stars explode as supernovae and form a black hole or neutron star. The remnant mass and the impact of the chemical yield on subsequent star formation and galactic evolution strongly depend on the internal physics of the progenitor star, which is currently not well understood. The theoretical uncertainties of Stellar Interiors accumulate with Stellar age, which is particularly pertinent for the blue supergiant phase. Stellar oscillations represent a unique method of probing Stellar Interiors, yet inference for blue supergiants is hampered by a dearth of observed pulsation modes. Here we report the detection of diverse variability in blue supergiants using the K2 and TESS space missions. The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives. Future asteroseismic modelling will provide constraints on ages, core masses, interior mixing, rotation and angular momentum transport. The discovery of variability in blue supergiants is a step towards a data-driven empirical calibration of theoretical evolution models for the most massive stars in the Universe.

H C Spruit - One of the best experts on this subject based on the ideXlab platform.

  • dynamo action by dierential rotation in a stably stratied Stellar interior
    2002
    Co-Authors: H C Spruit
    Abstract:

    Magnetic elds can be created in stably stratied (non-convective) layers in a dierentially rotating star. A magnetic instability in the toroidal eld (wound up by dierential rotation) replaces the role of convection in closing the eld amplication loop. Tayler instability is likely to be the most relevant magnetic instability. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in Stellar Interiors. It produces a predominantly horizontal eld. This dynamo process is found to be more eective in transporting angular momentum than the known hydrodynamic mechanisms. It might account for the observed pattern of rotation in the solar core.

  • dynamo action by differential rotation in a stably stratified Stellar interior
    Astronomy and Astrophysics, 2002
    Co-Authors: H C Spruit
    Abstract:

    Magnetic fields can be created in stably stratified (non-convective) layers in a differentially rotating star. A magnetic instability in the toroidal field (wound up by differential rotation) replaces the role of convection in closing the field amplification loop. Tayler instability is likely to be the most relevant magnetic instability. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in Stellar Interiors. It produces a predominantly horizontal field. This dynamo process is found to be more effective in transporting angular momentum than the known hydrodynamic mechanisms. It might account for the observed pattern of rotation in the solar core.

  • dynamo action by differential rotation in a stably stratified Stellar interior
    arXiv: Astrophysics, 2001
    Co-Authors: H C Spruit
    Abstract:

    Magnetic fields can be created in stably stratified (non-convective) layers in a differentially rotating star. A magnetic instability in the toroidal field (wound up by differential rotation) replaces the role of convection in closing the field amplification loop. Tayler instability is likely to be the most relevant magnetic instability. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in Stellar Interiors. It produces a prodominantly horizontal field. This dynamo process might account for the observed pattern of rotation in the solar core.