The Effect of Combined Magnetic Geometries on Thermally Driven Winds II: Dipolar, Quadrupolar and Octupolar Topologies

This is the final version of the article. Available from American Astronomical Society via the DOI in this record.The erratum to this article is appended at the end of the article and is separately in ORE at http://hdl.handle.net/10871/36354During the lifetime of sun-like or low mass stars a significant amount of angular momentum is removed through magnetised stellar winds. This process is often assumed to be governed by the dipolar component of the magnetic field. However, observed magnetic fields can host strong quadrupolar and/or octupolar components, which may influence the resulting spin-down torque on the star. In Paper I, we used the MHD code PLUTO to compute steady state solutions for stellar winds containing a mixture of dipole and quadrupole geometries. We showed the combined winds to be more complex than a simple sum of winds with these individual components. This work follows the same method as Paper I, including the octupole geometry which increases the field complexity but also, more fundamentally, looks for the first time at combining the same symmetry family of fields, with the field polarity of the dipole and octupole geometries reversing over the equator (unlike the symmetric quadrupole). We show, as in Paper I, that the lowest order component typically dominates the spin down torque. Specifically, the dipole component is the most significant in governing the spin down torque for mixed geometries and under most conditions for real stars. We present a general torque formulation that includes the effects of complex, mixed fields, which predicts the torque for all the simulations to within 20% precision, and the majority to within ~5%. This can be used as an input for rotational evolution calculations in cases where the individual magnetic components are known

Erratum "The Effect of Combined Magnetic Geometries on Thermally Driven Winds. II. Dipolar, Quadrupolar, and Octupolar Topologies" (2018, ApJ, 854, 78)

This is the final version. Available from American Astronomical Society via the DOI in this recordThe article to which this is the erratum is in ORE at http://hdl.handle.net/10871/31867This is an erratum for the article 2018 ApJ 854 78. DOI: 10.3847/1538-4357/aaaab

Magnetic braking of Sun-like and low-mass stars: Dependence on coronal temperature

This is the final version of the article. Available from American Astronomical Society via the DOI in this record.Sun-like and low-mass stars possess high temperature coronae and lose mass in the form of stellar winds, driven by thermal pressure and complex magnetohydrodynamic processes. These magnetized outflows probably do not significantly affect the star's structural evolution on the Main Sequence, but they brake the stellar rotation by removing angular momentum, a mechanism known as magnetic braking. Previous studies have shown how the braking torque depends on magnetic field strength and geometry, stellar mass and radius, mass-loss rate, and the rotation rate of the star, assuming a fixed coronal temperature. For this study we explore how different coronal temperatures can influence the stellar torque. We employ 2.5D, axisymmetric, magnetohydrodynamic simulations, computed with the PLUTO code, to obtain steady-state wind solutions from rotating stars with dipolar magnetic fields. Our parameter study includes 30 simulations with variations in coronal temperature and surface-magnetic-field strength. We consider a Parker-like (i.e. thermal-pressure-driven) wind, and therefore coronal temperature is the key parameter determining the velocity and acceleration profile of the flow. Since the mass loss rates for these types of stars are not well constrained, we determine how torque scales for a vast range of stellar mass loss rates. Hotter winds lead to a faster acceleration, and we show that (for a given magnetic field strength and mass-loss rate) a hotter outflow leads to a weaker torque on the star. We derive new predictive torque formulae for each temperature, which quantifies this effect over a range of possible wind acceleration profiles

The effect of magnetic variability on stellar angular momentum loss. I. the solar wind torque during sunspot cycles 23 and 24

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordThe rotational evolution of cool stars is governed by magnetized stellar winds that slow the stellar rotation during their main sequence lifetimes. Magnetic variability is commonly observed in Sun-like stars, and the changing strength and topology of the global field is expected to affect the torque exerted by the stellar wind. We present three different methods for computing the angular momentum loss in the solar wind. Two are based on MHD simulations from Finley & Matt (2018), with one using the open flux measured in the solar wind, and the other using remotely observed surface magnetograms. Both methods agree in the variation of the solar torque seen through the solar cycle and show a 30%-40% decrease from cycles 23 to 24. The two methods calculate different average values, 2.9 ×1030 erg (open flux) and 0.35 ×1030 erg (surface field). This discrepancy results from the already well-known difficulty of reconciling the magnetograms with the observed open flux, which is currently not understood, leading to an inability to discriminate between these two calculated torques. The third method is based on the observed spin rates of Sun-like stars, which decrease with age, directly probing the average angular momentum loss. This method gives 6.2 ×1030 erg for the solar torque, larger than the other methods. This may be indicative of further variability in the solar torque on timescales much longer than the magnetic cycle. We discuss the implications for applying the formula to other Sun-like stars, where only surface field measurements are available, and where the magnetic variations are ill-constrained

On the diversity of magnetic interactions in close-in star-planet systems

PublishedJournal Article© 2014. The American Astronomical Society. All rights reserved..Magnetic interactions between close-in planets and their host star can play an important role in the secular orbital evolution of the planets, as well as the rotational evolution of their host. As long as the planet orbits inside the Alfvén surface of the stellar wind, the magnetic interaction between the star and the planet can modify the wind properties and also lead to direct angular momentum transfers between the two. We model these star-planet interactions using compressible magnetohydrodynamic (MHD) simulations, and quantify the angular momentum transfers between the star, the planet, and the stellar wind. We study the cases of magnetized and non-magnetized planets and vary the orbital radius inside the Alfvén surface of the stellar wind. Based on a grid of numerical simulations, we propose general scaling laws for the modification of the stellar wind torque, for the torque between the star and the planet, and for the planet migration associated with the star-planet magnetic interactions. We show that when the coronal magnetic field is large enough and the star is rotating sufficiently slowly, the effect of the magnetic star-planet interaction is comparable to tidal effects and can lead to a rapid orbital decay.This work was supported by the ANR 2011 Blanc Toupies and the ERC project STARS2. A.S. acknowledges support from the Canada's Natural Sciences and Engineering Research Council. We acknowledge access to supercomputers through GENCI (project 1623), Prace, and ComputeCanada infrastructures

Numerical Aspects of 3D Stellar Winds

This paper explores and compares the pitfalls of modelling the three-dimensional wind of a spherical star with a cartesian grid. Several numerical methods are compared, using either uniform and stretched grid or adaptative mesh refinement (AMR). An additional numerical complication is added, when an orbiting planet is considered. In this case a rotating frame is added to the model such that the orbiting planet is at rest in the frame of work. The three-dimensional simulations are systematically compared to an equivalent two-dimensional, axisymmetric simulation. The comparative study presented here suggests to limit the rotation rate of the rotating frame below the rotating frame of the star and provides guidelines for further three-dimensional modelling of stellar winds in the context of close-in star-planet interactions.AS thanks T. Matsakos for discussions about the modelling of star-planet systems in 3D. This work was supported by the ANR 2011 Blanc Toupies and the ERC project STARS2 (207430). The authors acknowledge CNRS INSU/PNST and CNES/Solar Orbiter fundings. AS acknowledges support from the Canada’s Natural Sciences and Engineering Research Council and from the Canadian Institute of Theoretical Astrophysics (National fellow). We acknowledge access to supercomputers through GENCI (project 1623), Prace, and ComputeCanada infrastructures

The influence of the magnetic topology on the wind braking of sun-like stars.

Stellar winds are thought to be the main process responsible for the spin down of main-sequence stars. The extraction of angular momentum by a magnetized wind has been studied for decades, leading to several formulations for the resulting torque. However, previous studies generally consider simple dipole or split monopole stellar magnetic topologies. Here we consider in addition to a dipolar stellar magnetic field, both quadrupolar and octupolar configurations, while also varying the rotation rate and the magnetic field strength. 60 simulations made with a 2.5D, cylindrical and axisymmetric set-up and computed with the PLUTO code were used to find torque formulations for each topology. We further succeed to give a unique law that fits the data for every topology by formulating the torque in terms of the amount of open magnetic flux in the wind. We also show that our formulation can be applied to even more realistic magnetic topologies, with examples of the Sun in its minimum and maximum phase as observed at the Wilcox Solar Observatory, and of a young K-star (TYC-0486-4943-1) whose topology has been obtained by Zeeman-Doppler Imaging (ZDI).We would like to thank Colin Folsom, Pascal Petit for the magnetic field decomposition coefficients of TYC- 0486-4943-1, J´erome Bouvier and the ANR TOUPIES project which aim to understand the evolution of star?s spin rates, the ERC STARS2 (www.stars2.eu) and CNES support via our Solar Orbiter funding

The impact of metallicity on the evolution of the rotation and magnetic activity of Sun-like stars

This is the author accepted articleThe rotation rates and magnetic activity of Sun-like and low-mass (.1.4M) main-sequence stars are knownto decline with time, and there now exist several models for the evolution of rotation and activity. However,the role that chemical composition plays during stellar spin-down has not yet been explored. In this work,we use a structural evolution code to compute the rotational evolution of stars with three different masses (0.7, 1.0, and 1.3M) and six different metallicities, ranging from [Fe/H]=−1.0 to [Fe/H]= +0.5. We also implement three different wind-braking formulations from the literature (two modern and one classical) and compare their predictions for rotational evolution. The effect that metallicity has on stellar structural properties,and in particular the convective turnover timescale, leads the two modern wind-braking formulations to predict a strong dependence of the torque on metallicity. Consequently, they predict that metal rich stars spin-down more effectively at late ages (>1 Gyr) than metal poor stars, and the effect is large enough to be detectable with current observing facilities. For example, the formulations predict that a Sun-like (solar-mass and solar-aged) star with [Fe/H]=−0.3 will have a rotation period of less than 20 days. Even though old, metal poor stars are predicted to rotate more rapidly at a given age, they have larger Rossby numbers and are thus expected to have lower magnetic activity levels. Finally, the different wind-braking formulations predict quantitative differences in the metallicity-dependence of stellar rotation, which may be used to test themER

Developing Preservice Primary Teachers’ Confidence and Competence in Arts Education using Principles of Authentic Learning.

Arts education research over the years has highlighted the situation of non-specialist preservice primary arts teachers as having little confidence in their own artistic ability and their ability to teach the arts to children. Added to this, problems such a lack of resources, confidence, priority, time, knowledge and experience appear to inhibit the regular teaching of the arts by generalist classroom teachers while at the same time, face-to-face hours for preservice primary arts education have decreased significantly over the recent years. This paper describes how one subject within a Primary Teacher Education course responded to these challenges. This subject was based on Herrington, Oliver and Reeves’ (2003) framework for creating authentic learning environments then triangulates this authentic learning framework with what students wanted to learn in the subject and how they perceived they had developed their confidence and competence in creative arts educatio

Statistical Fitting of Evolutionary Models to Rotation Rates of Sun-like Stars

This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this record.We apply for the first time a two-dimensional fitting statistic, τ2, to rotational-evolution models (REMs) of stars (0.1–1.3 M⊙) on the period–mass plane. The τ2 statistic simultaneously considers all cluster rotation data to return a goodness of fit, allowing for data-driven improvement of REMs. We construct data sets for Upper Sco, the Pleiades, and Praesepe, to which we tune our REMs. We use consistently determined stellar masses (calculated by matching Ks magnitudes to isochrones) and literature rotation periods. As a first demonstration of the τ2 statistic, we find the best-fitting gyrochronology age for Praesepe, which is in good agreement with the literature. We then systematically vary three parameters that determine the dependence of our stellar wind torque law on the Rossby number in the saturated and unsaturated regimes, and the location of the transition between the two. By minimizing τ2, we find best-fit values for each parameter. These values vary slightly between clusters, mass determinations, and initial conditions, highlighting the precision of τ2 and its potential for constraining REMs, gyrochronology, and our understanding of stellar physics. Our resulting REMs, which implement the best-possible fitting form of a broken-power-law torque, are statistically improved on previous REMs using similar formulations, but still do not simultaneously describe the observed rotation distributions of the lowest masses, which have both slow and fast rotators by the Praesepe age, and the shape of the converged sequence for higher masses. Further complexity in the REMs is thus required to accurately describe the data.European Commissio