A Cool Stars 20 Splinter Session
Highlighting computational advances at the frontiers in the heliospheric/asterospheric hierarchy, extending from the generation of solar-stellar activity to its impact on the ambient conditions of planetary systems.
We have entered a new era in astrophysics characterized by the advent of transformative capabilities in space and on the ground. In parallel, we are entering the era of multi-petascale computing. Powerful new simulation software—coupled to the new generation of massively parallel computers—is transforming how modeling and simulations interact with observations. Going from the data to interpretative frameworks, combined with a feedback loop from the observations back to the models, has become an imperative. Concurrent with these new computational approaches, open source software, high-performance hardware, and cloud computing capabilities are much more widely available than in the past. A full scientific exploration of the themes of the 20th Cool Stars Workshop is dependent on this new computational universe.
This splinter session will highlight computational advances at the frontiers in the heliospheric/asterospheric hierarchy, extending from the generation of solar-stellar activity to its impact on the ambient conditions of planetary systems. Efforts in petascale computing and associated visualizations, as applied to the solar-stellar-planet connection, will enable advances in both modeling and simulation and data analysis/reduction software, which promise to continue the transformation of observation and modeling interactions. Models with visualizations and simulations, together with observations, empower us to originate and explore new theories and to develop physical intuition about complex systems in ways never before attained.
The speakers will highlight recent research and future directions in computational astrophysics that seek to advance our understanding of the nature and origin of solar-stellar activity, including impacts on asterospheres and planetary systems, as an integrated system. An equally important goal is to provide an impetus and a context for building a hierarchy of community-based modeling codes that interface seamlessly, in which data can be assimilated smoothly from space-based observatories and ground-based facilities. Both observers and modelers could provide synergistic feedback to produce model-driven code applications and new, model-driven observations along with data-driven models.
Panel discussion participants will engage the audience in a variety of topics that, among other salient issues, address:
In this presentation I will give an overview of the latest progress made in simulating in 3-D the complex non linear interplay between convection, rotation, turbulence and magnetic field. Such simulations and intricate interactions lead to fascinating and time variable solar and stellar activity and help understanding how it comes about. We will further discuss how various stellar dynamo models lead to different dynamics and rotation-cycle period trends. We will end the talk by proposing future computational development to tackle even better these key questions on solar and stellar magnetism.
Our Sun is a quintessential example of a spotted star, for which we have detailed and long-term observations of individual active regions. Over the past few decades, much effort has been placed on studying the solar flux emergence process through computational simulations. Such simulations make contact with many facets of observed active regions and the solar storms they spawn. Now there is a growing landscape of starspot observations, though with far less fidelity than we can achieve for the Sun. Surveying the current status of both starspot observations and flux emergence simulations, perhaps the most accessible point of contact between the two is developing an intuition for the location of flux emergence on stellar surfaces.
Motivated by flux emergence simulations in the solar interior, we describe multiple sets of simulations that model how fibril magnetic fields traverse stellar convection zones. These assume the magnetism generated by underlying dynamo action can be modeled as thin flux tubes represented by a reduced set of 1D ideal MHD equations. The flux tubes are then embedded in hydrodynamic convection simulations, thereby capturing the global effects of turbulent fluid flows on buoyantly rising bundles of magnetism assumed to be starspot progenitors. Performing our simulations in this way allows us to capture much of the underlying physical processes at reduced computational costs while exploring a large parameter space. Our results in the solar regime reproduce multiple active region observables. Extension to other stars indicate that convection, rotation, and stellar structure are all important contributing factors to the general trend of flux emergence. In particular, we highlight outcomes of our work relevant to fully convective M dwarfs, which indicate preferred starspot emergence much different from the solar trend in some regimes. We also comment on the state of recent dynamo simulations that now show some aspects of self-consistent magnetic flux emergence, mention the possibility of performing flux emergence and convection simulations in truly full 3D spheres, and hope for the day when we may have magnetic polarity observations of individual active regions on stars other than the Sun.
Above the photospheres of the sun and cool stars, the interaction between surface convection and magnetic flux drives stellar winds and produces a wide range of observable phenomena, broadly referred to as “magnetic activity.” As a consequence, the environments and planets around cool stars are bathed in both high-energy radiation and high-velocity, magnetized plasmas. We have little information about the winds and associated mass loss rates because they are so tenuous that they are extremely difficult to observe. Furthermore, the physics of stellar coronae are complex and elusive, which means that even first-principles physical models still have large uncertainties. Fortunately, since winds remove angular momentum, we can get complementary information by studying stellar rotation. We now have large samples of observed rotation rates, revealing how stellar rotation varies systematically with mass and age. I will describe current efforts to use the observed rotation rates, together with physical models of stellar spin-down and numerical simulations of stellar wind torques, to constrain the wind properties of cool stars. These efforts give us valuable new information about stellar winds and magnetism, which helps to characterize the environments of these stars and helps to develop a comprehensive understanding of how stars build magnetic fields and drive magnetic activity.
We investigate the feasibility of representing a structured multi-dimensional stellar atmosphere with a single one-dimensional average stratification for the purpose of spectral diagnosis of the atmosphere's average spectrum. In particular, we construct four different one-dimensional stratifications from a single snapshot of a magnetohydrodynamic simulation of solar convection: one by averaging its properties over surfaces of constant height and three by averaging over surfaces of constant optical depth at 500 nm. Using these models, we calculate continuum and atomic and molecular line intensities and their center-to-limb variations. From an analysis of the emerging spectra, we identify three main reasons why these average representations are inadequate for accurate determination of stellar atmospheric properties through spectroscopic analysis. These reasons are nonlinearity in the Planck function with temperature, which raises the average emergent intensity of an inhomogeneous atmosphere above that of an average-property atmosphere, even if their temperature-optical depth stratification is identical; nonlinearities in molecular formation with temperature and density, which raise the abundance of molecules of an inhomogeneous atmosphere over that in a one-dimensional model with the same average properties; and the anisotropy of convective motions, which strongly affects the center-to-limb variation of line-core intensities. We argue therefore that a one-dimensional atmospheric model that reproduces the mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the average physical properties of that atmosphere and is therefore inherently unreliable.
Great advancements have been achieved in the last two decades in the field of computational heliophysics. The majority of the modeling work has been driven by the need to develop reliable modeling tools for space weather prediction. This effort has led to the development of extremely sophisticated numerical tools that include coupling of different models (system science approach) and the implementation of small-scale physics and nonlinear effects in large-scale models (multi-scale, multi-physics approach). In the talk, I will review these recent computational advances and will discuss potential applications of these numerical tools to stellar astrophysics and the study of protoplanetary discs.
Lessons learned from our closest star enable us to anticipate that stellar activity is a primary driver of space weather impacting the environments around accompanying planets. Thanks to detailed multi-wavelength high-resolution observations from space and ground-based observatories, we learn about manifestations and general properties of various phenomena, such as the global and local dynamo, emergence of magnetic flux, sunspots and eruptions. Three-dimensional numerical simulations based on first physical principles are enabling interpretation and understanding of the complex physical processes of magnetic energy accumulation and release. These simulations take into account all essential ingredients, such as realistic abundances and EOS, radiative energy transport, sub-grid scale turbulent and magnetic effects etc. The numerical modeling approach has been validated by available high-resolution observations of the Sun, and brought important insights into observed phenomena. We apply this approach to realistically model stellar dynamics from the convection zone to the corona. These simulations are critical not only for understanding the basic physics, but also for interpreting observational results and planning observational programs. One important example is identification of the stellar convective noise (‘stellar jitter’) and its impact on detection of Earth-size exoplanets using spectroscopic observations, or photometric data from the upcoming TESS mission. Numerous detection of Jupiter-mass planets with close orbits to their stars are motivating the development of models aimed at understanding star-exoplanet interactions and impacts of stellar activity in extreme conditions. This talk will give a brief review of current status and perspectives of realistically modeling stellar dynamics, and star-planet interactions.
The stellar winds produced by young, Sun-like stars are expected to be orders of magnitude more powerful than those of the present-day Sun. These strong winds affect the stars themselves through angular momentum loss, and they potentially reduce the habitability of orbiting planets by eroding their atmospheres.
The USQ Starwinds project aims to extend understanding of the Solar wind in time by presenting a collection of stellar wind maps for young, Sun-like stars, based on published surface magnetic measurements from the TOUPIES study. The first set of wind maps are for stars aged 25 to 250 Myr, an age in which the Sun's magnetic field is believed to have undergone important changes. From the wind maps we calculate stellar parameters such as mass loss, angular momentum loss, and high-energy radiation emission. We characterize the wind ram pressure, a key parameter determining atmospheric erosion, at a range of distances including 1 AU.
The wind is modeled as a fully 3-dimensional magnetohydrodynamic fluid using the Space Weather Modeling Framework, developed at the University of Michigan. We use the latest version of Alfvén wave solar model (AWSoM) with counter-propagating Alfvén waves and physically realistic heating terms.
The Starwinds project should produce the largest three-dimensional wind atlas of young, Sun-like stars to date. We expect that the results should improve understanding of the young Sun's behavior and have relevance to models of planetary evolution in our Solar system and beyond.
One of the most important problems in the context of cataclysmic variables (CVs) is the lack of observations of systems with periods between 1 and 3.12 hours, known as the period gap. The orbital evolution of CVs with periods shorter than those in the gap is dominated by gravitational radiation while for periods exceeding those of the gap it is dominated by magnetic braking of the secondary star. Spruit & Ritter (1983) have shown that a sharp decline in magnetic dynamo and braking efficiency as periods approach 3 hours and secondary stars become fully convective would result in such a gap. Recent X-ray observations showing coronal magnetic energy dissipation is similar in fully convective and partly radiative M dwarfs cast this theory into doubt.
In this work, we use Zeeman-Doppler observations culled from the literature to show that the complexity of the surface magnetic fields of rapidly rotating M dwarfs increases with decreasing rotation period. Recently, Garraffo et al. (2018) presented a new spin-down model calibrated using MHD modeling. They show that the efficiency of angular momentum loss of cool stars declines strongly with increasing complexity of their surface magnetic field. By generating synthetic CV populations, we show that the CV period gap can naturally arise as a consequence of a rise in secondary star magnetic complexity near the long period edge of the gap that renders a sharp decline in their angular momentum loss rate.
Regular spaceborne measurements revealed that solar brightness varies on all timescales at which it has ever been measured. The state-of-the-art models attribute solar brightness variations to a joint action of surface magnetic field and constantly evolving granulation pattern on the solar surface, and reproduce available measurements of solar brightness variability in great detail.
Studies of solar brightness variability have always been of high importance for stellar astronomers, who have been comparing it with the variability of other lower main sequence stars. The interest in solar-stellar comparison has been recently brought to a new level by the unprecedented precision of broadband stellar photometry achieved with the launch of the Kepler and CoRoT space missions. In general, Sun-like stars show brightness variations similar to those of the Sun but with much wider variety of patterns.
The enormous progress in MHD simulations of solar and stellar atmospheres as well as the development of new computationally efficient radiative transfer schemes makes it finally possible to extend physics-based solar irradiance models to other stars and connect related solar and stellar studies. We present our recent achievements in modeling stellar brightness variations and show how our modeling can help to answer a very fundamental question of how typical is the Sun as an active star.
| Time | Presenter | Type | Title |
|---|---|---|---|
| 1400 – 1405 | Mark Giampapa | Introductory Remarks | |
| 1405 – 1420 | Sacha Brun | Invited | The Solar-stellar Connection: New Simulations and Results |
| 1420 – 1435 | Maria Weber | Invited | Applying Solar Insight to Study Flux Emergence in Late-type Stars |
| 1435 – 1445 | Sasha Shapiro | Contributed | Linking Solar and Stellar Variabilities |
| 1445 – 1500 | Sean Matt | Invited | Stellar Winds, Angular Momentum Evolution and Activity |
| 1500 – 1510 | Dag Evensberget | Contributed | Characterising the Solar Wind in Time Using Young Stellar Proxies: The USQ Starwinds Project |
| 1510 – 1535 | Han Uitenbroek | Invited | Multi-dimensional RT Models in Inhomogeneous Atmospheres and the Interpretation of Stellar Spectra |
| 1530 – 1600 | POSTER SESSION AND COFFEE BREAK | ||
| 1600 – 1615 | Nagi Mansour | Invited | Heliophysics integrated modeling: From Sun-Earth to Star-Exoplanet |
| 1615 – 1630 | Ofer Cohen | Invited | Computational Heliophysics as Applied to Stellar Astrophysics |
| 1630 – 1640 | Cecilia Garraffo | Contributed | On the Magnetic Nature of the CV PeriodGap |
| 1640 – 1725 | PANEL SESSION | Ben Brown, Jane Pratt, Bob Rosner, Dan Spicer. Facilitator: Peter Foukal | |
| 1725 – 1730 | Peter Foukal | Closing Remarks | |
If you have questions about or are interested in participating in this splinter session, send email to:
Madhulika Guhathakurta (Lika) Guhathakurta
Madhulika.Guhathakurta@nasa.gov
Mark Giampapa
giampapa@nso.edu