Juri Toomre

JILA Graduate Student Connor Bice awarded the Richard Nelson Thomas Award

Steven Burrows — Tue, 16 May 2023 17:22:37 +0000

JILA Graduate Student Connor Bice awarded the Richard Nelson Thomas Award Steven Burrows Tue, 05/16/2023 - 11:22

Kenna Hughes-Castleberry / JILA Science Communicator

Connor Bice, a recently graduated JILA graduate student, has been awarded the 2023 Richard Nelson Thomas award.

Before graduating, JILA graduate student Connor Bice recently received the 2023 Richard Nelson Thomas Award. This annual award is given to the most outstanding graduate student in astrophysics at the University of Colorado Boulder in honor of Dr. Richard Nelson Thomas. Dr. Thomas was one of the founding members of JILA and an influential scientist in astrophysics.

The JILA astrophysical faculty nominate outstanding students each year and vote to determine the award recipient. JILA Fellow Juri Toomre, Bice's former advisor, presented the award, along with Dr. Nelson's widow, Nora Thomas. Both spoke of Bice's research rigor and dedication to the field of astrophysics. Bice's research focused on activities around m-dwarf stars. Congratulations!

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Tackling the Sun’s Tachocline

Steven Burrows — Mon, 05 Dec 2022 17:20:47 +0000

Tackling the Sun’s Tachocline Steven Burrows Mon, 12/05/2022 - 10:20

Kenna Hughes-Castleberry / JILA Science Communicator

A rendering of how a solar tachocline moves.

Image Credit: Steven Burrows / JILA

Sitting 150 million kilometers away from the Earth, the Sun produces puzzling phenomena, like solar flares, that physicists are working to understand. One of these puzzles involves the Sun's tachocline, a belt of heat transition. “A tachocline is when the radiative interior of a star rotates like a solid ball, but the convection zone [an unstable outer heat layer in a star] rotates differently,” explained former JILA graduate student Loren Matilsky. “For geometric reference in the Sun, the outer 30% by radius is the convection zone, and the inner 70% by radius is the radiative interior.” Before leaving JILA to become a postdoctoral researcher at the University of California Santa Cruz, Matilsky collaborated with JILA Fellow Juri Toomre and his group at JILA to study the Sun's tachocline using computer simulations. In a new paper published in The Astrophysical Journal Letters, Matilsky, Toomre, and the team developed a new type of simulation, one where the tachocline is self-consistent and not artificially enforced, meaning that it arises on its own. According to Matilsky: “As far as we know, it's the first time this type of self-consistent tachocline behavior has been published for a fully nonlinear fluid dynamical global simulation.”

Sun Simulations
There are many types of simulations astrophysicists use to learn more about the Sun's tachocline. When the tachocline was discovered in 1992, physicists needed to come up with different reasons for why this transitional belt of heat existed on the Sun’s surface, including whether a magnetic field affected the flow. “Immediately after the tachocline's discovery, there were two [main] competing theoretical explanations for why it would exist,” said Matilsky. “The ‘fast’ scenario means there is no magnetic field, so the fluid outer layers are essentially experiencing shear instabilities, which arise when the spatial rate of change of the rotation rate is strong compared to the viscosity.” When the outer convective zone tries to burrow inward toward the solid core, the fluid dynamics of the scenario flattens out the zone. Which, according to the “fast” theory creates the transition layer of the tachocline. “It's called fast,” Matilsky continued, “ because these instabilities operate on a timescale of a convective overturning time or rotation period, so about every few days or maybe a month.” Compared to the millions of years it takes for a star to mature, a few days to a month seems rather rapid.

In contrast, the second “slow” scenario, includes a magnetic field as a source of the tachocline. As Matilsky explained: “The scenario is 'slow' because it wipes out differential rotation [the convective zone] on the long timescale (around billions of years) of deep global circulation.” The magnetic field helps to create the tachocline in this scenario, as opposed to fluid dynamics in the first scenario. Matilsky compared the magnetic fields to rubber bands that would prefer not to be bent. Similarly, if the field lines are stretched enough, there is a backlash and the burrowing process for the tachocline stops. There is, however, a third scenario involving a cyclical dynamo, a process that creates the Sun's magnetic field. “The dynamo field was assumed to follow the Sun's observed 22-year cycle for solar hotspots. The 22-year cycle refers to the fact that sunspots emerge in greater numbers, and with greater intensity, every 11 years. From one of these ‘sunspot cycles’ to the next, the field polarity reverses, making a 22-year cycle.” explained Matilsky. In the third scenario, the dynamo helps drive the tachocline, and this scenario is what Matilsky and Toomre explored in the simulation.

From their simulation, the researchers found that the tachocline became self-contained, arising on its own. “It was a self-consistently enforced tachocline,” Matilsky said. “I think there is a definite possibility that if you don't try to confine the tachocline artificially, it might just be there if you add a magnetic field.”

This self-contained tachocline may help astrophysicists to learn more about another of the Sun's phenomena, its magnetic field. “The main goal, I would say, of all solar physics is to understand the Sun's dynamo, because we have these flares—coronal mass ejections—which makes the news because they are firing high-energy particles that hit satellites in space, they're disrupting power grids, and that actually affects life on Earth. That comes from the solar magnetic field.” Matilsky and other physicists reason that the Sun's dynamo must originate at the tachocline, as opposed to previous thinking which suggested something else caused the tachocline. “Because the tachocline is the interface between differential rotation and solid body rotation, there's a lot of shear happening, and many believe that that [shear] is what is creating this powerful magnetic field," Matilsky added.

More on Exoplanets and Stellar Evolution
While the new simulation suggests a strong link between the tachocline and magnetic field, it also hints at more explanations about exoplanets. “Our paper may also provide significant insights into stellar dynamos,” said Matilsky. “We discover exoplanets all the time. Their space weather is really violent compared to the stuff we experience on Earth. So, if other stars have tachoclines, that may tell us if they have dynamos, which could affect exoplanets’ space weather.”

Insight into these tachoclines may also uncover more history about our Sun's origins. “During the early stages of stellar evolution, there's a whole bunch of theories relying on whether the magnetic field homogenizes the rotation rate or not,” Matilsky added. “Throughout all this, the presence or not of a tachocline (the boundary between homogenous [solid body] rotation rate and differential rotation) would definitely affect stellar evolution.” While there is still more research to be done, this paper is one more step forward to learning more about how stars originated.

Sitting 150 million kilometers away from the Earth, the Sun produces puzzling phenomena, like solar flares, that physicists are working to understand. One of these puzzles involves the Sun's tachocline, a belt of heat transition. Before leaving JILA to become a postdoctoral researcher at the University of California Santa Cruz, Matilsky collaborated with JILA Fellow Juri Toomre and his group at JILA to study the Sun's tachocline using computer simulations.

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New Insights into Magnetic Fields of Red Dwarfs

Steven Burrows — Tue, 17 May 2022 16:56:31 +0000

New Insights into Magnetic Fields of Red Dwarfs Steven Burrows Tue, 05/17/2022 - 10:56

Kenna Hughes-Castleberry / JILA Science Communicator

Bice and Toomre studied the magnetic fields and the convective flows on three simulated red dwarfs. Image credit: Kenna Castleberry, Conner Bice and Juri Toomre

Of the many different objects in the galaxy, M-dwarf stars, also known as red dwarf stars, are of particular interest to astrophysicists. These small objects are the most common type of star in the universe and have unique properties. “If you lay out all of the different types of stars [in a plot of stellar color versus brightness] we can see, based on what color they are and how bright they are, [that] most stars fall on a line we call the ‘main sequence’,” explained graduate student Connor Bice. “That's where they are born, and they stay in that same spot for most of their lives. Down at the tail end of that [line] are red dwarfs. They're the least massive, the coldest, and the smallest type of main-sequence stars.” Bice is a researcher in JILA Fellow and astrophysicist Juri Toomre's group, and both he and Toomre have been looking at some of a red dwarf's unique properties, mainly their magnetic fields and convective flows. In a new paper published in the Astrophysical Journal, Bice and Toomre have found a link between the star’s convective cycles, or the heat cycles in a star’s atmosphere, and its magnetic fields, using fluid dynamics simulations.

The Smallest and Coldest

Red-dwarf stars may be the least massive and coldest, but according to Bice, “When we actually sit down and watch some of them, we find that a huge proportion are strikingly violent magnetically, and it really sets them apart. They put off flares constantly, and many of them are brighter than any flares we've seen on the Sun,” Bice continued. “Since they have to power that magnetic activity somehow, as small and dim as they are, it raises some very interesting questions about how they're able to do it.”

Both Toomre and Bice were intrigued by this magnetic violence and wanted to research the cause of this behavior. In order to study this magnetic activity, Bice and Toomre used an open-sourced software program called Rayleigh to run a series of massively parallel 3D simulations on NASA’s Pleiades supercomputer. The simulations studied the dynamics in the interiors of three virtual red dwarfs. The researchers wanted to better understand how the star’s dynamo action, the fluid processes by which it generates magnetic fields, might differ from those of more massive stars. When analyzing the influence of the convective flows, the researchers focused on a phenomenon called a convective nest. According to Bice, the convective nest is a concentration of the star’s vertical heat transport, from the core of the star to its atmosphere, into a large structure at the star's equator that continually travels around the star. Since the convective nest is an area of particularly vigorous flows, the researchers were hopeful the nest could provide insight into the cause of the violently magnetic behavior observed in many red dwarf stars.

The Secrets of the Nest

When looking at the interactions between the convective nests and the magnetic fields in the simulations, the researchers found a couple of interesting things. “First, we're seeing that turbulent induction [a circular movement] by the nest tends to cause the magnetic fields within it to reverse their direction,” explained Bice. As the nest spun in a distinct pattern, it caused the magnetic fields entangled within it to reverse direction, similar to a snake eating its own tail. “This is comparable in some ways to the solar cycle, where the global magnetic fields of the Sun reverse direction every 11 years, though that's by a very different mechanism,” Bice added. In one of the simulations, the magnetic field reversal expanded to the rest of the star, showing a larger effect beyond the convective nest. From these results, the researchers concluded that the convective nest had a direct influence on the magnetic field of the red dwarf.

The other interesting result had to do with the outward movement of the magnetic field from the star’s convection zone. “Above this convective nest, at the surface of the star, we're seeing the outward magnetic fields get amplified,” Bice said. This amplification, where the magnetic field got stronger, not just on a large scale (covering an entire hemisphere of the star), but also on a smaller scale (a more local area), could lead to the formation of starspots, temporary dark areas on the surface of a star. Starspots come from a reduced surface temperature, caused by strong magnetic fields preventing cooled material from sinking back below the surface. In studying this amplification further, Bice and Toomre hoped to build on previous research showing that starspots can affect stellar rotation measurements, by examining the starspot origin process.

For Bice and Toomre, this study of convective nests is just a part of a much bigger picture. “These three simulations were actually drawn from a larger survey of models that we're looking to publish a paper on this summer,” Bice said. “It's shaping up as a broader take on several of the behaviors we've seen a lot since turning to red dwarfs, that you don't really see when modeling Sun-like stars.” Understanding more about how the convective nests shape magnetic fields not only allows the researchers to learn more about the properties of red dwarfs but can also help astrophysicists better calculate stellar rotation and other properties of these unique stars.

In a new paper published in the Astrophysical Journal, Bice and Toomre have found a link between a red dwarf's convective cycles, or the heat cycles in a star’s atmosphere, and its magnetic fields, using fluid dynamics simulations.

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Loren Matilsky and Heather Wernke win 2020 R. N. Thomas Award

Steven Burrows — Fri, 04 Dec 2020 22:01:03 +0000

Loren Matilsky and Heather Wernke win 2020 R. N. Thomas Award Steven Burrows Fri, 12/04/2020 - 15:01

Steven Burrows / JILA Science Communications Manager

Loran Matilsky and Heather Wernke

The Richard Nelson Thomas Award was established by the friends and family of R.N. "Dick" Thomas to provide an annual award to the year's most outstanding graduate student in astrophysics. Each year, the JILA astrophysical faculty nominates outstanding students and vote to determine the recipient of the award.

This year two graduate students have won the award:

Loren Matilsky

Loren is majoring in global-scale simulations studying highly nonlinear stellar and solar convection and dynamo processes, and has published three papers in The Astrophysical Journal this year and last. He published another paper on Rossby vortices in accretion disks in Monthly Notices of the Royal Astronomical Society (MNRAS), along with a number of conference proceedings. His work has revealed how strong wreaths of magnetism can be build within turbulent convection zones that go through complex cycling, and has tackled the subtleties of achieving differential rotations profiles that capture the spirit of helioseismic findings.

He works under Fellow Juri Toomre and Senior Research Associate Brad Hindman. In announcing the award to JILA, Toomre said, “Loren is highly motivated and very effective in such research in astrophysical fluid dynamics. He is also a decidedly graceful and friendly young colleague, and keenly involved in helping to consider diversity issues.

Loren will be defending his thesis in the spring or early summer.

Heather Wernke

Heather plans to defend her thesis in 2021 on the dynamics of stars in eccentric disks around massive black holes. Her research involves "observing" simulations of such systems with a view to comparing them directly to observations of local galactic nuclei. She has uncovered interesting results particularly with respect to the location of compact objects (i.e. stellar mass black holes, etc.) in these systems.

Fellow Ann-Marie Madigan said, "I am very happy to nominate Heather for this award. She is a wonderful teacher and is looking forward to continuing teaching physics and astronomy past graduation."

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