Simulating the Interior Rotation and Dynamics of Stars and Giant Planets

Overview

In order to understand the structure and evolution of stars and planets, and accurately interpret our observations, we must find ways to learn more about the dynamic processes that occur within them. Modeling and simulation on high-performance computers are very effective tools for researching these phenomena. We have developed a 3D, spherical magnetohydrodynamic (MHD) code that can simulate the interior of giant planets, the Sun, and other stars. These studies can help us interpret various astronomical observations and make predictions that can help guide future observations.

Project Details

Using our MHD code on the Pleiades supercomputer at the NASA Advanced Supercomputer (NAS) facility, we have recently carried out two groundbreaking simulation studies. We performed the first MHD simulations of a "hot Jupiter" planet—a Jupiter-sized planet close to its host star—that included variable magnetic diffusivity and self-consistently modeled Ohmic heating.

We also carried out 2D and 3D self-consistent simulations that coupled the convective and radiative regions in a massive star in order to simulate the star's convectively driven internal gravity waves (IGWs). Our results showed that the waves could alter the surface rotation of these stars and can induce large-scale mean flows that vary in time and are potentially observable.

Results and Impact

Our hot Jupiter simulations show dynamics that are related solely to the presence of a magnetic field. The magnetic effects could influence winds on these exoplanets, which may be observable through temperature maps. Our results may lead to a method of inferring the planets' magnetic fields from observations made using NASA's Spitzer and Hubble space telescopes-and other currently available observatories. In addition, our results show that Ohmic heating had been overestimated in earlier, simplified calculations—and therefore likely cannot explain the observed inflated radii of these planets, as previously thought.

The flows predicted by our simulations of massive stars—in effect, momentum transport by IGWs—may explain a host of outstanding mysteries, including chemical anomalies that have been observed in such stars, the mechanism that drives the intermittent decretion disks of Be-class stars, and the origin of macroturbulence in intermediate and massive stars. These flows may also explain the observed dichotomy of obliquity between hot-Jupiter planets orbiting hot and cool stars—in other words, why the orbits of these planets around cool stars tend to align with the star's spin direction, while those around hot stars tend to have misaligned orbits.

Why HPC Matters

Our simulations required substantial computing resources. The relatively low-resolution hot Jupiter calculations required hundreds of thousands of processor hours on Pleiades; each single 2D massive star calculation also required hundreds of thousands of processor hours. Our 3D massive star calculations each required approximately one million processor hours, depending on complexity. In order to analyze the simulation results we need to store vast amounts of data—approximately 60 terabytes—on the Lou mass storage system at NAS. Without NASA's high-performance computing resources, our simulations and data analysis would not be possible.