CAPTION These images compare a view of Beta Pictoris in scattered light as seen by the Hubble Space Telescope (top) with a similar view constructed from data in the SMACK simulation (red overlay, bottom). The X pattern in the Hubble image forms as a result of a faint secondary dust disk inclined to the main debris disk. Previous simulations were unable to reproduce this feature, but the SMACK model replicates the overall pattern because it captures the three-dimensional distribution of the collisions responsible for making the dust. CREDIT Courtesy of: Top, NASA/ESA and D. Golimowski (Johns Hopkins Univ.); bottom, NASA Goddard/E. Nesvold and M. Kuchner

A new NASA supercomputer simulation of the planet and debris disk around the nearby star Beta Pictoris reveals that the planet's motion drives spiral waves throughout the disk, a phenomenon that causes collisions among the orbiting debris. Patterns in the collisions and the resulting dust appear to account for many observed features that previous research has been unable to fully explain.

"We essentially created a virtual Beta Pictoris in the computer and watched it evolve over millions of years," said Erika Nesvold, an astrophysicist at the University of Maryland, Baltimore County, who co-developed the simulation. "This is the first full 3-D model of a debris disk where we can watch the development of asymmetric features formed by planets, like warps and eccentric rings, and also track collisions among the particles at the same time."

In 1984, Beta Pictoris became the second star known to be surrounded by a bright disk of dust and debris. Located only 63 light-years away, Beta Pictoris is an estimated 21 million years old, or less than 1 percent the age of our solar system. It offers astronomers a front-row seat to the evolution of a young planetary system and it remains one of the closest, youngest and best-studied examples today. The disk, which we see edge on, contains rock and ice fragments ranging in size from objects larger than houses to grains as small as smoke particles. It's a younger version of the Kuiper belt at the fringes of our own planetary system. 

Nesvold and her colleague Marc Kuchner, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md., presented the findings Thursday during the "In the Spirit of Lyot 2015" conference in Montreal, which focuses on the direct detection of planets and disks around distant stars. A paper describing the research has been submitted to The Astrophysical Journal.

In 2009, astronomers confirmed the existence of Beta Pictoris b, a planet with an estimated mass of about nine times Jupiter's, in the debris disk around Beta Pictoris. Traveling along a tilted and slightly elongated 20-year orbit, the planet stays about as far away from its star as Saturn does from our sun.

Astronomers have struggled to explain various features seen in the disk, including a warp apparent at submillimeter wavelengths, an X-shaped pattern visible in scattered light, and vast clumps of carbon monoxide gas. A common ingredient in comets, carbon monoxide molecules are destroyed by ultraviolet starlight in a few hundred years. To explain why the gas is clumped, previous researchers suggested the clumps could be evidence of icy debris being corralled by a second as-yet-unseen planet, resulting in an unusually high number of collisions that produce carbon monoxide. Or perhaps the gas was the aftermath of an extraordinary crash of icy worlds as large as Mars.

"Our simulation suggests many of these features can be readily explained by a pair of colliding spiral waves excited in the disk by the motion and gravity of Beta Pictoris b," Kuchner said. "Much like someone doing a cannonball in a swimming pool, the planet drove huge changes in the debris disk once it reached its present orbit."

Keeping tabs on thousands of fragmenting particles over millions of years is a computationally difficult task. Existing models either weren't stable over a sufficiently long time or contained approximations that could mask some of the structure Nesvold and Kuchner were looking for.

Working with Margaret Pan and Hanno Rein, both now at the University of Toronto, they developed a method where each particle in the simulation represents a cluster of bodies with a range of sizes and similar motions. By tracking how these "superparticles" interact, they could see how collisions among trillions of fragments produce dust and, combined with other forces in the disk, shape it into the kinds of patterns seen by telescopes. The technique, called the Superparticle-Method Algorithm for Collisions in Kuiper belts (SMACK), also greatly reduces the time required to run such a complex computation.

Using the Discover supercomputer operated by the NASA Center for Climate Simulation at Goddard, the SMACK-driven Beta Pictoris model ran for 11 days and tracked the evolution of 100,000 superparticles over the lifetime of the disk.

As the planet moves along its tilted path, it passes vertically through the disk twice each orbit. Its gravity excites a vertical spiral wave in the disk. Debris concentrates in the crests and troughs of the waves and collides most often there, which explains the X-shaped pattern seen in the dust and may help explain the carbon monoxide clumps.

The planet's orbit also is slightly eccentric, which means its distance from the star varies a little every orbit. This motion stirs up the debris and drives a second spiral wave across the face of the disk. This wave increases collisions in the inner regions of the disk, which removes larger fragments by grinding them away. In the real disk, astronomers report a similar clearing out of large debris close to the star.

"One of the nagging questions about Beta Pictoris is how the planet ended up in such an odd orbit," Nesvold explained. "Our simulation suggests it arrived there about 10 million years ago, possibly after interacting with other planets orbiting the star that we haven't detected yet."

CAPTION This image shows the gamma-ray signal produced in the computer simulation by annihilations of dark matter particles. Lighter colors indicate higher energies. The highest-energy gamma rays originate from the center of the crescent-shaped region at left, closest to the black hole's equator and event horizon. The gamma rays with the greatest chances of escape are produced on the side of the black hole that spins toward us. Such lopsided emission is typical for a rotating black hole. CREDIT Credit: NASA Goddard/Jeremy Schnittman

A new NASA supercomputer simulation shows that dark matter particles colliding in the extreme gravity of a black hole can produce strong, potentially observable gamma-ray light. Detecting this emission would provide astronomers with a new tool for understanding both black holes and the nature of dark matter, an elusive substance accounting for most of the mass of the universe that neither reflects, absorbs nor emits light.

"While we don't yet know what dark matter is, we do know it interacts with the rest of the universe through gravity, which means it must accumulate around supermassive black holes," said Jeremy Schnittman, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "A black hole not only naturally concentrates dark matter particles, its gravitational force amplifies the energy and number of collisions that may produce gamma rays." 

In a study published in The Astrophysical Journal on June 23, Schnittman describes the results of a supercomputer simulation he developed to follow the orbits of hundreds of millions of dark matter particles, as well as the gamma rays produced when they collide, in the vicinity of a black hole. He found that some gamma rays escaped with energies far exceeding what had been previously regarded as theoretical limits.

In the simulation, dark matter takes the form of Weakly Interacting Massive Particles, or WIMPS, now widely regarded as the leading candidate of what dark matter could be. In this model, WIMPs that crash into other WIMPs mutually annihilate and convert into gamma rays, the most energetic form of light. But these collisions are extremely rare under normal circumstances.

Over the past few years, theorists have turned to black holes as dark matter concentrators, where WIMPs can be forced together in a way that increases both the rate and energies of collisions. The concept is a variant of the Penrose process, first identified in 1969 by British astrophysicist Sir Roger Penrose as a mechanism for extracting energy from a spinning black hole. The faster it spins, the greater the potential energy gain.

In this process, all of the action takes place outside the black hole's event horizon, the boundary beyond which nothing can escape, in a flattened region called the ergosphere. Within the ergosphere, the black hole's rotation drags space-time along with it and everything is forced to move in the same direction at nearly speed of light. This creates a natural laboratory more extreme than any possible on Earth.

The faster the black hole spins, the larger its ergosphere becomes, which allows high-energy collisions further from the event horizon. This improves the chances that any gamma rays produced will escape the black hole. 

"Previous work indicated that the maximum output energy from the collisional version of the Penrose process was only about 30 percent higher than what you start with," Schnittman said. In addition, only a small portion of high-energy gamma rays managed to escape the ergosphere. These results suggested that clear evidence of the Penrose process might never be seen from a supermassive black hole.

But the earlier studies included simplifying assumptions about where the highest-energy collisions were most likely to occur. Moving beyond this initial work meant developing a more complete computational model, one that tracked large numbers of particles as they gathered near a spinning black hole and interacted among themselves.

Schnittman's supercomputer simulation does just that. By tracking the positions and properties of hundreds of millions of randomly distributed particles as they collide and annihilate each other near a black hole, the new model reveals processes that produce gamma rays with much higher energies, as well as a better likelihood of escape and detection, than ever thought possible. He identified previously unrecognized paths where collisions produce gamma rays with a peak energy 14 times higher than that of the original particles.

Using the results of this new calculation, Schnittman created a simulated image of the gamma-ray glow as seen by a distant observer looking along the black hole's equator. The highest-energy light arises from the center of a crescent-shaped region on the side of the black hole spinning toward us. This is the region where gamma rays have the greatest chance of exiting the ergosphere and being detected by a telescope.

The research is the beginning of a journey Schnittman hopes will one day culminate with the incontrovertible detection of an annihilation signal from dark matter around a supermassive black hole.

"The simulation tells us there is an astrophysically interesting signal we have the potential of detecting in the not too distant future, as gamma-ray telescopes improve," Schnittman said. "The next step is to create a framework where existing and future gamma-ray observations can be used to fine-tune both the particle physics and our models of black holes."

CAPTION Green boxes indicate the primary input: data derived from measured solar magnetic fields. Red boxes indicate coronal model output, including simulated emission and white light images. Adopted from Linker (2011).

Space Weather research programs began around the world in the 1990's. Presently, three larger research groups are modeling key portions of the solar-terrestrial environment (i.e. solar wind). These groups are (i) Center for Integrated Space Weather Modeling (CISM) at Boston University (ii) the Center for Space Environment Modeling (CSEM) at the University of Michigan and (iii) the Solar-Interplanetary-Geomagnetic (SIGMA) group of the State Key Laboratory at the Chinese Academy of Sciences. The models developed by the three group are summarized as follows:

A. Combination of kinematic and numerical model: Corona-Heliosphere (CORHEL) model

Workers at the CISM of Boston University have assembled a number of current models for space weather prediction formalism (Goodrich et al. 2004) based on the physics of each region. A flow chart of the key components of corona-heliosphere models (CORHEL) is given in Fig. 1.

B. Self-Consistent magnetohydrodynamic models

There are two groups using this framework, the CSEM at the University of Michigan and the SIGMA at Chinese Academy of Sciences (CAS). Methods used by these two groups are described in the following sections.

B.1. SWMF (Space Weather Modeling Framework) BATS-R-US code

The CSEM at the University of Michigan has developed the SWMF that provides a high-performance flexible frame-work for physics-based space weather simulation model. The SWMF integrates numerical models of the Eruptive Event Generator (EE), Lower Corona (LC), Solar Corona (SC), Inner Heliosphere (IH), Outer Heliosphere (OH), Solar Energetic Particles (SEP), Global Magnetosphere (GM), Inner Magnetosphere (IM), Radiation Belt (RB), Polar Wind (PW), Ionosphere Electrodynamics (IE) and Upper Atmosphere (UA) into a high-performance coupled model (Toth et al. 2012).

B.2. SWIM (Space Weather Integrated Model: SIP-CESE-MHD Code

The SIGMA weather group at the Key Laboratory for Space Weather of the CAS has developed the SWIM system. This system is different from the other two systems because it constitutes one uniform numerical system from the solar surface to Earth's environment (1AU) and beyond. In this sense, there is no interface between different regions. This methodology could improve the numerical efficiency and accuracy. The numerical code for SWIM is based on the space time conservation element and solution element (CESE) method. The SWIM system is a framework for integrating and running various space weather models. It provides researchers an uniform environment to integrate the models of different spatial regions into a workflow that prepare the observational data and then analyzes and visualizes the output of the models. SWIM is programmed by using a combination of languages: C, FORTRAN and Python. SWIM is modularized into 4 layers: application layer (AL), Execution layer (EL), Calculation Layer (CL), and Foundation layer (FL).

Night-side view of magnetic field lines in a simulation of a "hot Jupiter" exoplanet. Simulations like these help researchers better understand the interior dynamics of these planets and learn more about how they may have formed. Magenta indicates magnetic fields with positive polarity, and blue indicates fields with negative polarity. Tamara Rogers, Jess Vriesema, University of Arizona

In the two decades since the first exoplanets were found in the mid-1990s, astronomers have discovered nearly 2,000 planets outside our solar system. Many of these are known as "hot Jupiters," planets that are similar in size to Jupiter but are much closer to their host stars, and therefore have faster orbits and much hotter surface temperatures.

To learn more about the interior dynamics of hot Jupiter exoplanets and their stars, astrophysicist Tamara Rogers and her team at the University of Arizona's Lunar and Planetary Laboratory ran a series of groundbreaking simulations on the Pleiades supercomputer, located at the NASA Advanced Supercomputing (NAS) facility at Ames Research Center.

"Modeling and simulation on high-performance computers are very effective tools for researching the dynamical processes that occur within stars and planets," said Rogers, now a lecturer at Newcastle University in the U.K. "Understanding these phenomena can help us learn how hot Jupiters formed and how they affect the evolution of planetary systems."

The team's simulations of hot Jupiters—which were the first to include magnetic fields—along with their massive star simulations, can help astronomers interpret data collected from space-based observatories like NASA's Kepler, Spitzer, and Hubble telescopes. For example, the team's findings may help explain some puzzling observations, such as why planets circling cool stars tend to have orbits that align with the star's spin direction while those around hot stars often have misaligned orbits; and why many hot Jupiters are bigger and less dense than expected given their mass, even accounting for their extreme temperatures. The simulation results also reveal how magnetic effects can influence winds on these planets, a finding that could provide a method for estimating the planets' magnetic fields based on observations of their atmospheres.

By studying hot Jupiters, so different from the gas giants that slowly circle our own Sun, astronomers are expanding their knowledge of planetary structure and evolution—research that is crucial to the search for rocky, Earth-like exoplanets that may support life.

Collision of two icy spheres with a diameter of about one kilometer. After a first impact the bodies separate and reimpact a day later.

Rosetta's target "Chury" and other comets observed by space missions show common evidence of layered structures and bi-lobed shapes. With 3D supercomputer simulations Martin Jutzi of PlanetS at the University of Bern was able to reconstruct the formation of these features as a result of gentle collisions and mergers. The study has now been published online in the journal Science Express.

In a video sequence based on a supercomputer simulation two icy spheres with a diameter of about one kilometer are moving towards each other. They collide at bicycle speed, start mutually rotating and separate again after the smaller body has left traces of material on the larger one. The time sequence shows that the smaller object is slowed down by mutual gravity. After about 14 hours it returns back to reimpact a day after the first collision. The two bodies finally merge to form one body that somehow looks familiar: The bi-lobed frame resembles the shape of comet 67P/Churyumov-Gerasimenko imaged by ESA's Rosetta mission.

The simulation is part of a study published in Science Express by Bernese astrophysicist Martin Jutzi and his US colleague Erik Asphaug (Arizona State University). With their three-dimensional supercomputer models the researchers reconstruct what happened in the early solar system. "Comets or their precursors formed in the outer planets region, possibly millions of years before planet formation," explains Martin Jutzi. "Reconstructing the formation process of comets can provide crucial information about the initial phase of planet formation, for instance, the initial sizes of the building blocks of planets, the so-called planetesimals or cometesimals in the outer solar system." About 100 simulations were performed, each one taking one to several weeks to complete, depending on the collision type. The work was supported from the Swiss National Science Foundation through the Ambizione program and in part carried out within the frame of the Swiss National Centre for Competence in Research "PlanetS".

67P/Churyumov-Gerasimenko isn't the only comet showing a bi-lobed shape and evidence for a layered structure. Crashing on 9P/Tempel 1 in 2005, NASA's Deep Impact showed similar layers, a feature that is also presumed on two other comets visited by NASA missions. Half of the comet nuclei that spacecraft have observed so far have bi-loped shapes among them comets 103P/Hartley 2 and 19P/Borelly. "How and when these features formed is much debated, with distinct implications for solar system formation, dynamics, and geology," says Martin Jutzi.

Primordial remnants of a quiet phase

In their study the researchers applied 3D collisional models, constrained by these shape and topographic data, to understand the basic accretion mechanism and its implications for internal structure. As their three-dimensional supercomputer simulations indicate, the major structural features observed on cometary nuclei can be explained by the pairwise low velocity accretion of weak cometesimals. The model is also compatible with the observed low bulk densities of comets as the collisions result in only minor compaction.

"These slow mergers might represent the quiet, early phase of planet formation, before large bodies excited the system to disruptive velocities, supporting the idea that cometary nuclei are primordial remnants of early agglomeration of small bodies," says Martin Jutzi. Alternatively, the same processes of coagulation might have occurred among debris clumps ejected from much larger parent bodies. Along with future space missions using radar to directly image internal structure, the 3D supercomputer simulations are an important step to clarify the question of how the cometary nuclei were assembled.

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