Internal mixing of rotating stars inferred from dipole gravity modes (original) (raw)

Astronomy and Astrophysics, 2003

In this paper we study the effects of rotation in low-mass, low-metallicity RGB stars. We present the first evolutionary models taking into account self-consistently the latest prescriptions for the transport of angular momentum by meridional circulation and shear turbulence in stellar interiors as well as the associated mixing processes for chemicals computed from the ZAMS to the upper RGB. We discuss in details the uncertainties associated with the physical description of the rotational mixing and study carefully their effects on the rotation profile, diffusion coefficients, structural evolution, lifetimes and chemical signatures at the stellar surface. We focus in particular on the various assumptions concerning the rotation law in the convective envelope, the initial rotation velocity distribution, the presence of µ-gradients and the treatment of the horizontal and vertical turbulence. This exploration leads to two main conclusions : (1) After the completion of the first dredge-up, the degree of differential rotation (and hence mixing) is maximised in the case of a differentially rotating convective envelope (i.e., j CE (r) = cst), as anticipated in previous studies. (2) Even with this assumption, and contrary to some previous claims, the present treatment for the evolution of the rotation profile and associated meridional circulation and shear turbulence does not lead to enough mixing of chemicals to explain the abundance anomalies in low-metallicity field and globular cluster RGB stars observed around the bump luminosity. This study raises questions that need to be addressed in a near future. These include for example the interaction between rotation and convection and the trigger of additional hydrodynamical instabilities.

Semiconvective mixing in low-mass stars

Astrophysics and Space Science, 2010

Mixing processes such as convection, overshooting and rotational mixing have long been known to affect the evolutionary properties of low-mass stars. While modeling a 1.2 M ⊙ star, we encountered a semiconvective region outside the fully convective core, reminiscent of the well-known situation for massive stars. In this study we focus on low-mass stars presenting convective cores and, by applying different prescriptions for the determination of the convective boundaries and using different mixing descriptions for the dynamical processes in the core, we look for the effects of semiconvective mixing in the interior structure of the stars and its observable quantities. With this purpose, we have constructed different sets of evolutionary models using a stellar evolution code (GARSTEC), and analyzed the models looking for imprints of these processes.

Rotational mixing in low-mass stars: I Effect of the ?-gradients in main sequence and subgiant Pop I stars

Astron Astrophys, 2003

Rotational mixing in early-type stars: the main-sequence evolution of a 9 Mo star

1996

We describe the main-sequence evolution of a rotating 9 M star. Its interior rotation profile is determined by the redistribution of angular momentum through the meridian circulation and through the shear turbulence generated by the differential rotation; the possible effect of internal waves is neglected. We examine the mixing of chemicals produced by the same internal motions. Our modelization is based on the set of equations established by Zahn (1992) and completed in Matias, . Our calculations show that the amount of mixing associated with a typical rotation velocity of ∼ 100 km s −1 yields stellar models whose global parameters are very similar to those obtained with the moderate overshooting (d/H P 0.2) which has been invoked until now to fit the observations. Fast rotation (∼ 300 km s −1 ) leads to significant changes of the C/N and O/N surface ratios, but the abundance of He is barely increased. The modifications of the internal composition profile due to such rotational mixing will certainly affect the post-main-sequence evolution.

Effects of rotational mixing on the asteroseismic properties of solar-type stars

Astronomy and Astrophysics, 2010

Context. Observations of solar-like oscillations obtained either from the ground or from space stimulated the study of the effects of various physical processes on the modelling of solar-type stars. Aims. The influence of rotational mixing on the evolution and asteroseismic properties of solar-type stars is studied. Methods. Global and asteroseismic properties of models of solar-type stars computed with and without a comprehensive treatment of shellular rotation are compared. The effects of internal magnetic fields are also discussed in the framework of the Tayler-Spruit dynamo.

Evolution of the First Stars: the major role of rotation for mixing and mass loss

We show that rotation plays a major role for very low metallicity stars, even if the distribution of angular velocities Ω with respect to critical values is the same as at solar Z.T h e internal gradients of Ω are much larger at lower metallicity Z, which enhance internal mixing and give rise to N-enrichments. Low Z stars easily reach break-up during MS evolution and lose mass. They also lose mass as red giants or supergiants due to their surface enrichments in heavy elements. The winds of low Z stars make peculiar contributions to the chemical yields. We suggest that the helium rich blue Main Sequence (bMS) of ω Centauri bears the signature of such enrichments by the stellar winds of rotating stars in the first stellar generations.

Chemical mixing in low mass stars

Astronomy & Astrophysics, 2019

Context. When modelling stars with masses higher than 1.2 M⊙ with no observed chemical peculiarity, atomic diffusion is often neglected because, on its own, it causes unrealistic surface abundances compared with those observed. The reality is that atomic diffusion is in competition with other transport processes. Rotation is one of the processes able to prevent excessively strong surface abundance variations. Aims. The purpose of this study is to quantify the opposite or conjugated effects of atomic diffusion (including radiative acceleration) and rotationally induced mixing in stellar models of low mass stars, and to assess whether rotational mixing is able to prevent the strong abundance variations induced by atomic diffusion in F-type stars. Our second goal is to estimate the impact of neglecting both rotational mixing and atomic diffusion in stellar parameter inferences for stars with masses higher than 1.3 M⊙. Methods. Using the Asteroseismic Inference on a Massive Scale (AIMS)...

Mixing by overshooting and rotation in intermediate-mass stars

Monthly Notices of the Royal Astronomical Society, 2019

Double-line eclipsing binaries (DLEBs) have been recently used to constrain the amount of central mixing as a function of stellar mass, with contrasting results. In this work, we reanalyse the DLEB sample by Claret & Torres, using a Bayesian method and new PARSEC tracks that account for both convective core overshooting and rotational mixing. Using overshooting alone, we obtain that, for masses larger than about 1.9 M , the distribution of the overshooting parameter, λ ov , has a wide dispersion between 0.3 and 0.8, with essentially no values below λ ov = 0.3 and 0.4. While the lower limit supports a mild convective overshooting efficiency, the large dispersion derived is difficult to explain in the framework of current models of that process, which leave little room for large randomness. We suggest that a simple interpretation of our results can be rotational mixing: Different initial rotational velocities, in addition to a fixed amount of overshooting, could reproduce the high dispersion derived for intermediatemass stars. After a reanalysis of the data, we find good agreement with models computed with a fixed overshooting parameter, λ ov = 0.4, and initial rotational rates, ω, uniformly distributed in a wide range between 0 and 0.8 times the break-up value, at varying initial mass. We also find that our best-fitting models for the components of α Aurigae and TZ Fornacis agree with their observed rotational velocities, thus providing independent support to our hypothesis. We conclude that a constant efficiency of overshooting in concurrence with a star-to-star variation in the rotational mixing might be crucial in the interpretation of such data.

Rotational mixing in massive binaries. Detached short-period systems

Astronomy & Astrophysics, 2009

Models of rotating single stars can successfully account for a wide variety of observed stellar phenomena, such as the surface enhancements of N and He observed in massive main-sequence stars. However, recent observations have questioned the idea that rotational mixing is the main process responsible for the surface enhancements, emphasizing the need for a strong and conclusive test for rotational mixing. We investigate the consequences of rotational mixing for massive main-sequence stars in short-period binaries. In these systems the tides are thought to spin up the stars to rapid rotation, synchronous with their orbital revolution. We use a state-of-the-art stellar evolution code including the effect of rotational mixing, tides, and magnetic fields. We adopt a rotational mixing efficiency that has been calibrated against observations of rotating stars under the assumption that rotational mixing is the main process responsible for the observed surface abundances. We find that the primaries of massive close binaries (M 1 ≈ 20 M ⊙ , P orb 3 days) are expected to show significant enhancements in nitrogen (up to 0.6 dex in the Small Magellanic Cloud) for a significant fraction of their core hydrogen-burning lifetime. We propose using such systems to test the concept of rotational mixing. As these short-period binaries often show eclipses, their parameters can be determined with high accuracy. For the primary stars of more massive and very close systems (M 1 ≈ 50 M ⊙ , P orb 2 days) we find that centrally produced helium is efficiently mixed throughout the envelope. The star remains blue and compact during the main sequence evolution and stays within its Roche lobe. It is the less massive star, in which the effects of rotational mixing are less pronounced, which fills its Roche lobe first, contrary to what standard binary evolution theory predicts. The primaries will appear as "Wolf-Rayet stars in disguise": core hydrogen-burning stars with strongly enhanced He and N at the surface. We propose that this evolution path provides an alternative channel for the formation of tight Wolf-Rayet binaries with a main-sequence companion and might explain massive black hole binaries such as the intriguing system M33 X-7.

Stellar mixing

Astronomy & Astrophysics, 2011

In this paper we use the Reynolds stress models (RSM) to derive algebraic expressions for the following variables: a) heat fluxes; b) μ fluxes; and c) momentum fluxes. These relations, which are fully 3D, include: 1) stable and unstable stratification, represented by the Brunt-Väisäla frequency, N 2 = −gH −1 p (∇ − ∇ ad)(1 − R μ); 2) double diffusion, salt-fingers, and semi-convection, represented by the density ratio R μ = ∇ μ (∇ − ∇ ad) −1 ; 3) shear (differential rotation), represented by the mean squared shear Σ 2 or by the Richardson number, Ri = N 2 Σ −2 ; 4) radiative losses represented by a Peclet number, Pe; 5) a complete analytical solution of the 1D version of the model. In general, the model requires the solution of two differential equations for the eddy kinetic energy K and its rate of dissipation, ε. In the local and stationary cases, when production equals dissipation, the model equations are all algebraic.

Internal mixing of rotating stars inferred from dipole gravity modes (original) (raw)

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