Periodic Reporting for period 4 - AWESoMeStars (Accretion, Winds, and Evolution of Spins and Magnetism of Stars)
Reporting period: 2021-01-01 to 2022-06-30
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.
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.
During the project, we have developed and exploited systematic magnetohydrodynamic simulation parameter studies for determining the effect of several different complexities in stellar winds and star-disk interactions on the mass and angular momentum flows. We characterized the effects of varying wind speeds/acceleration, magnetic field strengths and geometries, accretion rate, stellar spin rate, surface differential rotation, and mass loss rate. We also developed semi-analytic formulae for predicting the torques due to all of these effects.
We have exploited our 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. We have also used them to carry out studies on the rotational-evolution of stars, computing the rotational evolution of stars in entire clusters. To facilitate comparison with observational data, we adapted statistical methods that enable models to be compared to all available data, simultaneously. In addition, the project has made several other contributions to our understanding of stellar rotation and magnetism, such as: 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; discovery that stellar composition has a measurable effect on the rotational evolution and magnetic activity signatures; and more.
We have exploited our 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. We have also used them to carry out studies on the rotational-evolution of stars, computing the rotational evolution of stars in entire clusters. To facilitate comparison with observational data, we adapted statistical methods that enable models to be compared to all available data, simultaneously. In addition, the project has made several other contributions to our understanding of stellar rotation and magnetism, such as: 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; discovery that stellar composition has a measurable effect on the rotational evolution and magnetic activity signatures; and more.
Our magnetohydrodynamic simulation developments pushed 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, were the first to quantify the effects of wind speed and magnetic geometry, and our star-disk-interaction studies were the most comprehensive attempt so far to systematically characterize the torques. These developments have enabled several studies described above and will continue to enable many future explorations by us and other groups.
We 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 goes far beyond the usual "by-eye" fitting methods used in the past. We exploited this method by developing fitting tools, which enabled us to find the models that best fit available observational data. We completed several studies of the effects of stellar metallicity on rotational-evolution and magnetic activity, which significantly changes 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, including publishing the most comprehensive simulation-based torque formulations, studying the effect of birth environment on the rotation-rate distributions in clusters, and using the TORUS radiative transfer code to elucidate the origin of hydrogen emission in accreting young stars. These and our various other results have been published in more than 50 refereed journal articles, during the duration of the project.
We 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 goes far beyond the usual "by-eye" fitting methods used in the past. We exploited this method by developing fitting tools, which enabled us to find the models that best fit available observational data. We completed several studies of the effects of stellar metallicity on rotational-evolution and magnetic activity, which significantly changes 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, including publishing the most comprehensive simulation-based torque formulations, studying the effect of birth environment on the rotation-rate distributions in clusters, and using the TORUS radiative transfer code to elucidate the origin of hydrogen emission in accreting young stars. These and our various other results have been published in more than 50 refereed journal articles, during the duration of the project.