Outer Convection Zone Research Articles

SUPERGRANULES AS PROBES OF SOLAR CONVECTION ZONE DYNAMICS

Supergranules are convection cells seen at the Sun's surface as a space filling pattern of horizontal flows. While typical supergranules have diameters of about 35 Mm, they exhibit a broad spectrum of sizes from ~10 Mm to ~100 Mm. Here we show that supergranules of different sizes can be used to probe the rotation rate in the Sun's outer convection zone. We find that the equatorial rotation rate as a function of depth as measured by global helioseismology matches the equatorial rotation as a function of wavelength for the supergranules. This suggests that supergranules are advected by flows at depths equal to their wavelengths and thus can be used to probe flows at those depths. The supergranule rotation profiles show that the surface shear layer, through which the rotation rate increases inward, extends to depths of ~50 Mm and to latitudes of at least 70 degrees. Typical supergranules are well observed at high latitudes and have a range of sizes that extend to greater depths than those typically available for measuring subsurface flows with local helioseismology. These characteristics indicate that probing the solar convection zone dynamics with supergranules can complement the results of helioseismology.

Differential rotation and meridional flow on the lower zero‐age main sequence: Reynolds stress versus barocinic flow

AbstractWe study the variation of surface differential rotation and meridional flow along the lower part of the zero age main sequence (ZAMS). We first compute a sequence of stellar models with masses from 0.3 to 1.5 solar masses. We then construct mean field models of their outer convection zones and compute differential rotation and meridional flows by solving the Reynolds equation with transport coefficients from the second order correlation approximation. For a fixed rotation period of 2.5 d we find a strong dependence of the surface differential rotation on the effective temperature with weak surface shear for M dwarfs and very large values for F stars. The increase with effective temperature is modest below 6000 K but very steep above 6000 K. The meridional flow shows a similar variation with temperature but the increase with temperature is not quite so steep. Both the surface rotation and the meridional circulation are solar‐type over the entire temperature range. We also study the dependence of differential rotation and meridional flow on the rotation period for masses. from 0.3 to 1.1 solar masses. The variation of the differential rotation with period is weak except for very rapid rotation. The meridional flow shows a systematic increase of the flow speed with the rotation rate. Numerical experiments in which either the Λ effect is dropped in the Reynolds stress or the baroclinic term in the equation of motion is canceled show that for effective temperatures below 6000 K the Reynolds stress is the dominant driver of differential rotation (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars

Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediate-mass stars, focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram. A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase. The properties of the blue loop are changed by modification of the element profiles above the H-burning shell, which results from the incomplete mixing in the bottom overshooting region when the stellar model evolves up along the RGB. Such modification of the element profiles will lead to an increase of opacity in the region just above the H-burning shell and a decrease of opacity in the outer homogeneous convection zone, which will result in a quick decrease of the H-shell nuclear luminosity LH when the stellar model evolves from the RGB tip to its bottom and, finally, a much weaker and smaller convection zone will be obtained in the stellar envelope. This helps to form a longer blue loop. The extension of the blue loop is very sensitive to the parameters (CX and αTCM) of the diffusive mixing model and of the TCM. The results mainly show that: 1) comparing the results of the classical model with the mixing-length theory, the lengths of the obtained blue loops with different combinations of the values of CX and αTCM are all increased and the length of the blue loop increases with the values of parameters CX and αTCM; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram, even though the length of the blue loop for the 7M⊙ star has a less prominent difference between the classical and diffusive mixing model; 3) both the observations referring to the location of the Cepheid instability strip and the number ratio NB/NR of blue to red evolved stars in the Galactic open clusters can confine the two parameters in a range of 0.5 ≤ αTCM ≤ 0.9 and 10−5 ≤ CX ≤ 10−4 for the model of 5M⊙. However, for the case of the 7M⊙ star, there seems to be no such definite range to even only account for the observed number ratio NB/NR. In any case, our results based on the diffusive mixing model are on the whole in accordance with not only other theoretical ones but also the observations.

Magnetic activity of cool stars in the Hertzsprung-Russell diagram

AbstractI review the X-ray emission from cool stars with outer convection zones in comparison to the Sun with a focus on the properties of low-activity stars. I present the recent results of long-term X-ray monitoring which demonstrate the existence of X-ray cycles on stars with known calcium cycles. The evidence of a minimum stellar X-ray flux is presented and arguments are put forward for the view that the Sun in its extended minimum between 2008 - 2009 behaved very much like a Maunder-minimum Sun.

Variable turbulent convection as the cause of the Blazhko effect - testing the Stothers model

The amplitude and phase modulation observed in a significant fraction of the RR Lyrae variables - the Blazhko effect - represents a long-standing enigma in stellar pulsation theory. No satisfactory explanation for the Blazhko effect has been proposed so far. In this paper we focus on the Stothers (2006) idea, in which modulation is caused by changes in the structure of the outer convective zone, caused by a quasi-periodically changing magnetic field. However, up to this date no quantitative estimates were made to investigate whether such a mechanism can be operational and whether it is capable of reproducing the light variation we observe in Blazhko variables. We address the latter problem. We use a simplified model, in which the variation of turbulent convection is introduced into the non-linear hydrodynamic models in an ad hoc way, neglecting interaction with the magnetic field. We study the light curve variation through the modulation cycle and properties of the resulting frequency spectra. Our results are compared with Kepler observations of RR Lyr. We find that reproducing the light curve variation, as is observed in RR Lyr, requires a huge modulation of the mixing length, of the order of +/-50 per cent, on a relatively short time-scale of less than 40 days. Even then, we are not able to reproduce neither all the observed relations between modulation components present in the frequency spectrum, nor the relations between Fourier parameters describing the shape of the instantaneous light curves.

Magnetic flux generation and transport in cool stars

The Sun and other cool stars harbouring outer convection zones manifest magnetic activity in their atmospheres. The connection between this activity and the properties of a deep-seated dynamo generating the magnetic flux is not well understood. By employing physical models, we study the spatial and temporal characteristics of the observable surface field for various stellar parameters. We combine models for magnetic flux generation, buoyancy instability, and transport, which encompass the entire convection zone. The model components are: (1) a thin-layer alpha-Omega dynamo at the base of the convection zone; (2) buoyancy instabilities and the rise of flux tubes through the convection zone in 3D, which provides a physically consistent determination of emergence latitudes and tilt angles; and (3) horizontal flux transport at the surface. For solar-type stars and rotation periods longer than about 10 days, the latitudinal dynamo waves generated by the deep-seated alpha-Omega dynamo are faithfully reflected by the surface distribution of magnetic flux. For rotation periods of the order of two days, however, Coriolis acceleration of rising flux loops leads to surface flux emergence at much higher latitudes than the dynamo waves at the bottom of the convection zone reach. A similar result is found for a K0V star with a rotation period of two days. In the case of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy activity cycles and mixed-polarity fields at high latitudes.

Rotation and lithium abundance of solar-analog stars

Rotational velocity, lithium abundance, and the mass depth of the outer convective zone are key parameters in the study of the processes at work in the stellar interior, in particular when examining the poorly understood processes operating in the interior of solar-analog stars. We investigate whether the large dispersion in the observed lithium abundances of solar-analog stars can be explained by the depth behavior of the outer convective zone masses, within the framework of the standard convection model based on the local mixing-length theory. We also aims to analyze the link between rotation and lithium abundance in solar-analog stars. We computed a new extensive grid of stellar evolutionary models, applicable to solar-analog stars, for a finely discretized set of mass and metallicity. From these models, the stellar mass, age, and mass depth of the outer convective zone were estimated for 117 solar-analog stars, using Teff and [Fe/H] available in the literature, and the new HIPPARCOS trigonometric parallax measurements. We determine the age and mass of the outer convective zone for a bona fide sample of 117 solar-analog stars. No significant on-to-one correlation is found between the computed convection zone mass and published lithium abundance, indicating that the large A(Li) dispersion in solar analogs cannot be explained by the classical framework of envelope convective mixing coupled with lithium depletion at the bottom of the convection zone. These results illustrate the need for an extra-mixing process to explain lithium behavior in solar-analog stars, such as, shear mixing caused by differential rotation. To derive a more realistic definition of solar-analog stars, as well as solar-twin, it seems important to consider the inner physical properties of stars, such as convection, hence rotation and magnetic properties.

Large-scale magnetic fields of low-mass dwarfs: the many faces of dynamo

AbstractMagnetic field are ubiquitous to low-mass stars and can potentially impact their evolution and their internal structure; yet the physical processes (called dynamo) that succeed at generating them in the stellar convective zones of cool dwarfs are still enigmatic. Although theoretical modelling and numerical simulations of stellar dynamo action showed breathtaking progress in the last decade, they are not yet in the state of accurately predicting the various magnetic topologies that different low-mass stars can generate.Thanks to the advent of new-generation instruments, spectropolarimetric observations can now reveal the large-scale magnetic topologies of cool dwarfs, from the brown dwarf threshold (spectral type M8) up to the limit beyond which outer convective zones get vanishingly small (spectral type F5). In particular, they can reconstruct through tomographic methods the poloidal and toroidal components of the large-scale field, hence offering a fresh option for guiding dynamo theories to a more mature state.We review here the latest observational advances, showing in particular that magnetic topologies of low-mass dwarfs can drastically vary with mass and rotation rate, and discuss their implications for our understanding of dynamo processes.

Stochastic excitation of gravity modes in massive main-sequence stars

We investigate the possibility that gravity modes can be stochastically excited by turbulent convection in massive main-sequence (MS) stars. We build stellar models of MS stars with masses M=10 M⊙,15 M⊙, and 20 M⊙. For each model, we then compute the power supplied to the modes by turbulent eddies in the convective core (CC) and the outer convective zones (OCZ). We found that, for asymptotic gravity modes, the major part of the driving occurs within the outer iron convective zone, while the excitation of low n order modes mainly occurs within the CC. We compute the mode lifetimes and deduce the expected mode amplitudes. We finally discuss the possibility of detecting such stochastically-excited gravity modes with the CoRoT space-based mission.

Lithium destruction induced by planetary accretion in solar-type stars

AbstractAccretion of planetary (metal-rich) material onto a star in its early phases can produce episodes of thermohaline convection below the outer convective zone. These extra-mixing phases lead to rapid lithium destruction. The observed dispersion of lithium abundances in solar-type stars can be related to such events.

The main-sequence rotation���colour relation in the Coma Berenices open cluster

We present the results of a photometric survey of rotation rates in the Coma Berenices (Melotte 111) open cluster, using data obtained as part of the SuperWASP exoplanetary transit-search programme. The goal of the Coma survey was to measure precise rotation periods for main-sequence F, G and K dwarfs in this intermediate-age (~600 Myr) cluster, and to determine the extent to which magnetic braking has caused the stellar spin periods to converge. We find a tight, almost linear relationship between rotation period and J-K colour with a root-mean square scatter of only 2 percent. The relation is similar to that seen among F, G and K stars in the Hyades. Such strong convergence can only be explained if angular momentum is not at present being transferred from a reservoir in the deep stellar interiors to the surface layers. We conclude that the coupling timescale for angular momentum transport from a rapidly-spinning radiative core to the outer convective zone must be substantially shorter than the cluster age, and that from the age of Coma onward, stars rotate effectively as solid bodies. The existence of a tight relationship between stellar mass and rotation period at a given age supports the use of stellar rotation period as an age indicator in F, G and K stars of Hyades age and older. We demonstrate that individual stellar ages can be determined within the Coma population with an internal precision of order 9 percent (RMS), using a standard magnetic braking law in which rotation period increases with the square root of stellar age. We find that a slight modification to the magnetic-braking power law, P proportional to t^0.56, yields rotational and asteroseismological ages in good agreement for the Sun and other stars of solar age for which p-mode studies and photometric rotation periods have been published.

Nitrogen isotopes in the recent solar wind from the analysis of Genesis targets: Evidence for large scale isotope heterogeneity in the early solar system

We have analyzed nitrogen, neon and argon abundances and isotopic ratios in target material exposed in space for 27 months to solar wind (SW) irradiation during the Genesis mission. SW ions were extracted by sequential UV (193 nm) laser ablation of gold-plated material, purified separately in a dedicated line, and analyzed by gas source static mass spectrometry. We analyzed gold-covered stainless steel pieces from the Concentrator, a device that concentrated SW ions by a factor of up to 50. Despite extensive terrestrial N contamination, we could identify a non-terrestrial, 15N-depleted nitrogen end-member that points to a 40% depletion of 15N in solar-wind N relative to inner planets and meteorites, and define a composition for the present-day Sun ( 15N/ 14N = [2.26 ± 0.67] × 10 −3, 2σ), which is indistinguishable from that of Jupiter’s atmosphere. These results indicate that the isotopic composition of nitrogen in the outer convective zone of the Sun has not changed through time, and is representative of the protosolar nebula. Large 15N enrichments due to e.g., irradiation, low temperature isotopic exchange, or contributions from 15N-rich presolar components, are therefore required to account for inner planet values.

The lithium history of NGC 6397

AbstractThe primordial lithium abundance inferred from WMAP and standard Big Bang nucleosysnthesis is approximately three times higher than the plateau value measured in old metal-poor Population II stars, suggesting that these stars have undergone atmospheric Li depletion. To constrain the physics responsible for such depletion, we conducted a homogeneous analysis of a large sample of stars in the metal-poor globular cluster NGC 6397, covering all evolutionary phases from below the main-sequence turnoff to high up the red-giant branch (RGB). The dwarf, turnoff, and early subgiant stars form a thin abundance plateau, with a sharpe edge in the middle of the subgiant branch, where Li dilution caused by the inward extension of the convective envelope starts (the beginning of the so-called first dredge up). A second steep abundance drop is seen at the RGB bump, again highlighting the need for the onset of nonstandard mixing in this evolutionary phase. Moreover, by also measuring the sodium abundances of the targets, we have gained insight into the degree of pollution by early cluster self-enrichement, and may separate highly polluted, Li-poor and Na-rich stars from stars formed from pristine material. Our observational findings strictly limit both the extent of lithium surface depletion, which in turn constrains the efficiency of mixing below the outer convection zone, and the resulting spread in lithium abundance in metal-poor turn-off stars.

Modelling solar and stellar differential rotation

We present a model of stellar differential rotation based on the mean-field theory of fluid dynamics. DR is driven by Reynolds stress and anisotropic heat transport caused by the Coriolis force. Our model reproduces the rotation pattern in the solar convection zone and allows predictions for other stars with outer convection zones. We present results for a range of spectral types, including the limiting case of very shallow convection zones, and discuss the dependence of DR on the rotation rate and spectral type for main sequence stars.

The Sun in time: age, rotation, and magnetic activity of the Sun and solar-type stars and effects on hosted planets

AbstractMulti-wavelength studies of solar analogs (G0–5 V stars) with ages from ~50 Myr to 9 Gyr have been carried out as part of the “Sun in Time” program for nearly 20 yrs. From these studies it is inferred that the young (ZAMS) Sun was rotating more than 10× faster than today. As a consequence, young solar-type stars and the early Sun have vigorous magnetohydrodynamic (MHD) dynamos and correspondingly strong coronal X-ray and transition region/chromospheric FUV–UV emissions (up to several hundred times stronger than the present Sun). Also, rotational modulated, low amplitude light variations of young solar analogs indicate the presence of large starspot regions covering ~5–30% of their surfaces. To ensure continuity and homogeneity for this program, we use a restricted sample of G0–5 V stars with masses, radii, Teff, and internal structure (i.e. outer convective zones) closely matching those of the Sun. From these analogs we have determined reliable rotation-age-activity relations and X-ray–UV (XUV) spectral irradiances for the Sun (or any solar-type star) over time. These XUV irradiance measures serve as input data for investigating the photo-ionization and photo-chemical effects of the young, active Sun on the paleo-planetary atmospheres and environments of solar system planets. These measures are also important to study the effects of these high energy emissions on the numerous exoplanets hosted by solar-type stars of different ages. Recently we have extended the study to include lower mass, main-sequence (dwarf) dK and dM stars to determine relationships among their rotation spin-down rates and coronal and chromospheric emissions as a function of mass and age. From rotation-age-activity relations we can determine reliable ages for main-sequence G, K, M field stars and, subsequently, their hosted planets. Also inferred are the present and the past XUV irradiance and plasma flux exposures that these planets have endured and the suitability of the hosted planets to develop and sustain life.

The evolution of ‘the moment of inertia’ of stars

AbstractObservations of the rotation periods of cool open cluster stars display a distinctive dichotomy when plotted against stellar mass/color. Other measures of stellar activity are also known to be dependent on stellar mass and structure, especially the onset and characteristics of convection zones. One proposal for understanding the observed rotation period dichotomy suggested dependencies on the moment of inertia of either the whole star or that of only the outer convection zone (Barnes 2003).The moment of inertia of stars with the mass between 0.1Msun and 3.0Msun have been calculated using a version of Yale Stellar evolution code (aka YREC). Each star has been evolved from stellar birthline to the onset of the core He burning. For easy comparison to observations, we have calculated the isochrones of these quantities as well as the convective turnover time, of interest to the activity community.

Surface convection in Population II stars

AbstractThe initial surface abundances of Population II stars have been altered by the interplay between convection, rotational mixing and diffusion. In particular the shallower the outer convection zone the stronger the diffusion impact. We present preliminary results on constraining the extension of the convection zones of Population II stars thanks to 3D hydrodynamical simulations.

Convective envelopes in rotating OB stars

Aims. We study the effects of rotation on the outer convective zones of massive stars. Methods. We examine the effects of rotation on the thermal gradient and on the Solberg-Hoiland term by analytical developments and by numerical models. Results. Writing the criterion for convection in rotating envelopes, we show that the effects of rotation on the thermal gradient are much larger and of opposite sign to the effect of the Solberg-Hoiland criterion. On the whole, rotation favors convection in stellar envelopes at the equator and to a smaller extent at the poles. In a rotating 20 $M_{\odot}$ star at 94% of the critical angular velocity, there are two convective envelopes, with the bigger one having a thickness of 13.2% of the equatorial radius. In the non-rotating model, the corresponding convective zone has a thickness of only 4.6% of the radius. The occurrence of outer convection in massive stars has many consequences.

Tidal dissipation in binary systems

To first approximation, a binary system conserves its angular momentum while it evolves to its state of minimum kinetic energy: circular orbit, all spins aligned, and components rotating in synchronism with the orbital motion. The pace at which this final state is achieved depends on the physical processes that are responsible for the dissipation of the tidal kinetic energy. For stars (or planets) with an outer convection zone, the dominant mechanism identified so far is the viscous dissipation acting on the equilibrium tide. For stars with an outer radiation zone, it is the radiative damping operating on the dynamical tide. After a brief presentation of the tides, I shall review these physical processes; I shall discuss the uncertainties of their present treatment, describe the latest developments, and compare the theoretical predictions with the observed properties concerning the orbital circularization of close binaries.

A coupled model of magnetic flux generation and transport in stars

AbstractWe present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin‐layer α Ω dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large‐scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar‐type stars. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)