The model of magnetic braking of solar rotation considered by Charbonneau & MacGregor (1993) has ... more The model of magnetic braking of solar rotation considered by Charbonneau & MacGregor (1993) has been modified so that it is able to reproduce for the first time the rotational evolution of both the fastest and slowest rotators among solar-type stars in open clusters of different ages, without coming into conflict with other observational constraints, such as the time evolution of the atmospheric Li abundance in solar twins and the thinness of the solar tachocline. This new model assumes that rotation-driven turbulent diffusion, which is thought to amplify the viscosity and magnetic diffusivity in stellar radiative zones, is strongly anisotropic with the horizontal components of the transport coefficients strongly dominating over those in the vertical direction. Also taken into account is the poloidal field decay that helps to confine the width of the tachocline at the solar age. The model's properties are investigated by numerically solving the azimuthal components of the coupled momentum and magnetic induction equations in two dimensions using a finite element method
Angular momentum transport by internal magnetic fields is an important ingredient for stellar int... more Angular momentum transport by internal magnetic fields is an important ingredient for stellar interior models. In this paper we critically examine the basic heuristic assumptions in the model of the Tayler-Spruit dynamo, which describes how a pinch-type instability of a toroidal magnetic field in differentially rotating stellar radiative zones may result in large-scale fluid motion. We agree with prior published work both on the existence of the instability and its nearly horizontal geometry for perturbations. However, the approximations in the original Acheson dispersion relation are valid only for small length scales, and we disagree that the dispersion relation can be extrapolated to horizontal length scales of order the radius of the star. We contend that dynamical effects, in particular, angular momentum conservation, limit the maximum horizontal length scale. We therefore present transport coefficients for chemical mixing and angular momentum redistribution by magnetic torques that are significantly different from previous published values. The new magnetic viscosity is reduced by 2-3 orders of magnitude compared to the old one, and we find that magnetic angular momentum transport by this mechanism is very sensitive to gradients in the mean molecular weight. The revised coefficients are more compatible with empirical constraints on the timescale of core-envelope coupling in young stars than the previous ones. However, solar models including only this mechanism possess a rapidly rotating core, in contradiction with helioseismic data. Previous studies had found strong core-envelope coupling, both for solar models and for the cores of massive evolved stars. We conclude that the Tayler-Spruit mechanism may be important for envelope angular momentum transport but that some other process must be responsible for efficient spin-down of stellar cores.
Recently Gies & Lambert (1992) have reported on results of a composition analysis of a representa... more Recently Gies & Lambert (1992) have reported on results of a composition analysis of a representative sample of early B-type stars. Their primary purpose was to verify a conjecture made earlier by Lyubimkov (1991) that almost every early B-type main sequence star displayed in its atmosphere CN-cycled material, i.e. an enhanced N abundance accompanied by a deficit of C. Though Gies & Lambert have revealed a source of systematic error in Lyubimkov's approach, nevertheless, they did confirm the presence of products of the CN-cycle in some of the B-stars. In the present study, the evolution of a 10 solar mass star has been computed from the initial main sequence up to the first dredge-up phase. Hydrogen burning nuclear reactions have been taken into account in detail. To interpret the available observational data on the N overabundance in B-stars, rotationally induced turbulent diffusive mixing has been assumed to operate in the radiative envelope of the stellar models. We have used...
Intermediate-mass AGBs and low-mass stars having just passed the helium-flash are both potential ... more Intermediate-mass AGBs and low-mass stars having just passed the helium-flash are both potential contributors to chemical variations in GC stars. Both mechanisms face the difficulty of the short time available between the sweeping of the generated gas at each crossing of the galactic plane by the GCs
These notes provide a tutorial for those who want to use the state-of-the-art stellar evolution c... more These notes provide a tutorial for those who want to use the state-of-the-art stellar evolution code MESA and post-processing nucleosyntheis tools of NuGrid. As an example, an application of MESA and NuGrid tools for simulations of a nova outburst and associated nucleosynthesis occurring on a 1.3 Msun ONe white dwarf is presented.
... located well below the bump luminosity, where extra mixing is thought to start, and there ...... more ... located well below the bump luminosity, where extra mixing is thought to start, and there ... or these brightest red giants are experiencing something like enhanced extra mixing themselves, which ... Indeed, M13 is known to possess the most extreme star-to-star abundance variations ...
The model of magnetic braking of solar rotation considered by Charbonneau & MacGregor (1993) has ... more The model of magnetic braking of solar rotation considered by Charbonneau & MacGregor (1993) has been modified so that it is able to reproduce for the first time the rotational evolution of both the fastest and slowest rotators among solar-type stars in open clusters of different ages, without coming into conflict with other observational constraints, such as the time evolution of the atmospheric Li abundance in solar twins and the thinness of the solar tachocline. This new model assumes that rotation-driven turbulent diffusion, which is thought to amplify the viscosity and magnetic diffusivity in stellar radiative zones, is strongly anisotropic with the horizontal components of the transport coefficients strongly dominating over those in the vertical direction. Also taken into account is the poloidal field decay that helps to confine the width of the tachocline at the solar age. The model's properties are investigated by numerically solving the azimuthal components of the coupled momentum and magnetic induction equations in two dimensions using a finite element method
Angular momentum transport by internal magnetic fields is an important ingredient for stellar int... more Angular momentum transport by internal magnetic fields is an important ingredient for stellar interior models. In this paper we critically examine the basic heuristic assumptions in the model of the Tayler-Spruit dynamo, which describes how a pinch-type instability of a toroidal magnetic field in differentially rotating stellar radiative zones may result in large-scale fluid motion. We agree with prior published work both on the existence of the instability and its nearly horizontal geometry for perturbations. However, the approximations in the original Acheson dispersion relation are valid only for small length scales, and we disagree that the dispersion relation can be extrapolated to horizontal length scales of order the radius of the star. We contend that dynamical effects, in particular, angular momentum conservation, limit the maximum horizontal length scale. We therefore present transport coefficients for chemical mixing and angular momentum redistribution by magnetic torques that are significantly different from previous published values. The new magnetic viscosity is reduced by 2-3 orders of magnitude compared to the old one, and we find that magnetic angular momentum transport by this mechanism is very sensitive to gradients in the mean molecular weight. The revised coefficients are more compatible with empirical constraints on the timescale of core-envelope coupling in young stars than the previous ones. However, solar models including only this mechanism possess a rapidly rotating core, in contradiction with helioseismic data. Previous studies had found strong core-envelope coupling, both for solar models and for the cores of massive evolved stars. We conclude that the Tayler-Spruit mechanism may be important for envelope angular momentum transport but that some other process must be responsible for efficient spin-down of stellar cores.
Recently Gies & Lambert (1992) have reported on results of a composition analysis of a representa... more Recently Gies & Lambert (1992) have reported on results of a composition analysis of a representative sample of early B-type stars. Their primary purpose was to verify a conjecture made earlier by Lyubimkov (1991) that almost every early B-type main sequence star displayed in its atmosphere CN-cycled material, i.e. an enhanced N abundance accompanied by a deficit of C. Though Gies & Lambert have revealed a source of systematic error in Lyubimkov's approach, nevertheless, they did confirm the presence of products of the CN-cycle in some of the B-stars. In the present study, the evolution of a 10 solar mass star has been computed from the initial main sequence up to the first dredge-up phase. Hydrogen burning nuclear reactions have been taken into account in detail. To interpret the available observational data on the N overabundance in B-stars, rotationally induced turbulent diffusive mixing has been assumed to operate in the radiative envelope of the stellar models. We have used...
Intermediate-mass AGBs and low-mass stars having just passed the helium-flash are both potential ... more Intermediate-mass AGBs and low-mass stars having just passed the helium-flash are both potential contributors to chemical variations in GC stars. Both mechanisms face the difficulty of the short time available between the sweeping of the generated gas at each crossing of the galactic plane by the GCs
These notes provide a tutorial for those who want to use the state-of-the-art stellar evolution c... more These notes provide a tutorial for those who want to use the state-of-the-art stellar evolution code MESA and post-processing nucleosyntheis tools of NuGrid. As an example, an application of MESA and NuGrid tools for simulations of a nova outburst and associated nucleosynthesis occurring on a 1.3 Msun ONe white dwarf is presented.
... located well below the bump luminosity, where extra mixing is thought to start, and there ...... more ... located well below the bump luminosity, where extra mixing is thought to start, and there ... or these brightest red giants are experiencing something like enhanced extra mixing themselves, which ... Indeed, M13 is known to possess the most extreme star-to-star abundance variations ...
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Papers by Pavel Denissenkov