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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - AWESoMeStars (Accretion, Winds, and Evolution of Spins and Magnetism of Stars)

Teaser

Summary

This project focuses on Sun-like stars, which possess convective envelopes and universally exhibit magnetic activity (in the mass range ~0.1 to ~1.3 MSun). The rotation of these stars influences their internal structure, energy and chemical transport, and magnetic field generation, as well as their external magnetic activity and environmental interactions. Due to the huge range of timescales, spatial scales, and physics involved, understanding how each of these processes relate to each other and to the long-term evolution remains an enormous challenge in astrophysics. To face this challenge, the AWESoMeStars project will develop a comprehensive, physical picture of the evolution of stellar rotation, magnetic activity, mass loss, and accretion.
In doing so, we will
(1) Discover how stars lose the vast majority of their angular momentum, which happens in the accretion phase.
(2) Explain the observed rotation-activity relationship and saturation in terms of the evolution of magnetic properties & coronal physics.
(3) Characterize coronal heating and mass loss across the full range of mass & age.
(4) Explain the Skumanich (1972) relationship and distributions of spin rates observed in young clusters & old field stars.
(5) Develop physics-based gyrochronology as a tool for using rotation rates to constrain stellar ages.
We will accomplish these goals using a fundamentally new and multi-faceted approach, which combines the power of multi-dimensional MHD simulations with long-timescale rotational-evolution models. Specifically, we will develop a next generation of MHD simulations of both star-disk interactions and stellar winds, to model stars over the full range of mass & age, and to characterize how magnetically active stars impact their environments. Simultaneously, we will create a new class of rotational-evolution models that include external torques derived from our simulations, compute the evolution of spin rates of entire star clusters, and compare with observations.

Work performed

During the first 30 months of the project, we have developed and exploited the first systematic magnetohydrodynamic simulation parameter studies for determining the effect of wind speed and complex magnetic geometries on the angular momentum loss. These were achieved with three main studies. The first demonstrated that, for a given mass loss rate, stellar winds that accelerate to faster speeds are less efficient at carrying away angular momentum. We derived a semi-analytic formulation for estimating the effects of stellar wind speed on the angular momentum loss, when simulations are not available. The next two studies determined how the angular momentum loss varies for all possible combinations of the axisymmetric dipole, quadrupole, and octupole components of magnetic field. From those studies, we derived a semi-analytic formulation to calculate the angular momentum loss, for any combination of those fields.

We have exploited our wind-torque formulations by applying them to the solar wind, and also to stars for which we have measurements of magnetic fields and mass-loss rates. For the sun, we computed the variation in the solar wind angular momentum loss rate over the past few magnetic cycles. This showed how the solar wind torque is quasi-periodic, like the sunspot cycle, and it varies by a factor of several throughout the magnetic cycle. In applying to other stars, we found that they undergo very similar variations in their angular momentum loss, with differences being explained in terms of differences in their magnetic variability. Within the context of these variability studies, we provided the best evidence for and first detailed characterization of a discrepancy between long-term evolution torques and dynamical models of stellar winds. The discrepancy is a factor of a few or more, and we have determined that something is missing in either the observations of stellar wind properties, the models of stellar winds, or the long-term rotational-evolution models. We have identified several possible sources of the discrepancy, which will be directly explored by this project but also by relevant scientific communities. These and some of our other studies also show whether and when the more complex (non-dipolar) magnetic fields are important for determining the stellar wind torques.

We have carried out a few studies on the rotational-evolution of stars. One study determined how the amount of magnetic flux that is open in a stellar wind might vary during stellar evolution. Another has provided a rotational-evolution torque that provides the best fit so far to all of the rotation period measurements of young clusters. This empirically-tuned torque predicts a super-saturation of the magnetic field properties and/or mass loss rates of rapid rotators, for which we do not yet have observations.

The project has made several other contributions to our understanding of stellar rotation and magnetism, including: studying the observed surface magnetic fields and chromospheric activity in young solar-type stars; examining rotation-activity relationships for M dwarf stars using K2 data with archival X-ray and UV data; hydrodynamics simulations of stellar interiors to characterize the effect of stellar structure and rotation on internal differential rotation and convection; and modeling the formation and ejection of prominences, as additional sources for stellar winds.

Final results

\"Our magnetohydrodynamic simulation developments continue to push the state-of-the-art, particularly in the development of our parameter studies, which are designed to systematically study the most important properties of angular momentum loss that have never before been explored. Our semi-analytic formulations for the angular momentum loss, which are derived from the simulations, are the first to quantify the effects of wind speed and magnetic geometry. These developments have enabled several studies described above and will continue to enable many future explorations by us and other groups.

We have also acheived many recent developments, which will produce results in the second half of the project. First of all, we have successfully adapted a method for fitting color-magnitude diagrams to the comparison of our rotational-evolution models to observational data in period-mass space. This gives us, for the first time, a robuts statistical test of goodness-of-fit, which uses all available data simultaneously, going far beond the usual \"\"by-eye\"\" fitting methods used in the past. We will exploit this method by next developing fitting tools, which will enable us to find the models that best fit all available observational data, as well as to easily encorporate new data as they become available. Second, we have nearly finished the development of our three-dimensional magnetohydrodynamical simulations, which we will use for systematic studies of the effects of non-axisymmetric magnetic fields on angular momentum loss. Third, we have begun the first studies of the effects of stellar metallicity on rotational-evolution, which may turn out to significantly change our interpretation of several recently-discovered, and otherwise unexplained, phenomena. Fourth, we have made significant developments for studies of the pre-main-sequence, accreting phase of stellar evolution. During the reporting period, we have not published any work on studies of this phase, but these developments guarantee many results in the second half of the project. In particular, we have already implemented star-disk interaction torques into our rotational-evolution models; we have been progressing in developing our numerical simulations of the magnetic star-disk interactions; we have begun work on using the TORUS radiative transfer code to simulation observational signatures of the star-disk interaction; and we have recently begun observational and data-archive studies to determine the effects of the environment on the rotational evolution of forming stars, which will also further inform our models and understanding of this phase of evolution.

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