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Magnetic massive stars

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Research shines new light on link between rotation speed, brightness

Two researchers at a laptop

​New discoveries about magnetic massive stars have emerged as astronomer Matt Shultz (left) and theoretical physicist Stan Owocki combined forces with other researchers. Shultz was among those who found that the star’s magnetic field strength and its rotation speed are closely related, and Owocki and his team showed how the energy for these stars’ radio emissions was likely generated.

Oh, those twinkling, twinkling little stars. They make the night sky seem so peaceful, so idyllic, so quiet. It’s hard to believe that they are actually powerful furnaces, where explosive power, extreme heat and swirling, energized gases make them something more like a hydrogen bomb than anything imagined in that child’s lullaby.

Some of us settle for the wistful bit of that lullaby: “How I wonder what you are.” Others press on with increasingly challenging questions in a lifelong quest to learn, discover and inform the rest of us about what those twinkling little stars really are.

Matt Shultz does that as the Annie Jump Cannon Postdoctoral Fellow at the University of Delaware. Stan Owocki does that as professor of physics and astronomy at UD. With collaborators at UD and around the world, they recently joined forces on three articles printed by the Royal Astronomical Society, pointing to important new discoveries that could change how measurements of stars are done and how their brightness can be predicted.

It started a few years ago. Shultz was minding his astronomy at UD and Paolo Leto, a researcher at the Catania Astrophysical Observatory, was doing the same in Italy. Both were studying magnetic massive stars — a rare variety of bright, hot stars with magnetic fields thousands of times stronger than the sun’s. These stars are emerging as promising laboratories for studying plasmas under extreme conditions, Shultz said.

“Like the sun, massive stars have stellar winds — constant outflows of charged particles,” he said. “Because the wind is a plasma — a gas of charged particles — it responds to a magnetic field.”

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That creates a magnetosphere, which all stars and planets with magnetic fields — including Earth — have. Some are complex, as our sun’s magnetosphere is. Others are more stable, as the magnetospheres around these rare stars are.

Working independently, Shultz and Leto came to the same surprising conclusion — that the rotation speed of these stars is closely related to how bright they are at radio wavelengths.

Rotation speed wasn’t part of the previous model. But as they collected data from all of the previous studies they could find, assembling a large sample of these magnetic stars — 131 of them — they confirmed that rotation speed is a critical factor.

With the help of Owocki, a theoretical astrophysicist, they were also able to identify a plausible source of energy driving the powerful radio emissions of these stars.

Owocki showed that the connection between rotation and radio observations is consistent with what happens in centrifugal breakout events. Breakout occurs in the magnetospheres of fast-rotating stars. The plasma, which can move at thousands of kilometers per second, is subject to extreme centrifugal forces. It is held in place by the magnetic field, but as the wind feeds more plasma into the magnetosphere it eventually overpowers that restraint, bursting free and erupting into interstellar space. During the breakout, the magnetic field reconnects, releasing an enormous amount of energy and accelerating electrons to the high velocities needed to generate radio waves.

Twinkle, twinkle, indeed.

“We now know there is a close relationship with magnetic field strength and the rotation and thanks to Stan’s theoretical work, the real breakthrough is that we were able to tie this to the centrifugal breakout mechanism,” Shultz said.

Centrifugal breakout had been predicted more than a decade earlier in simulations performed by Asif ud-Doula, who was a student of Owocki’s, earned his doctorate in 2002 and now is an associate professor at Penn State-Scranton.

These kinds of magnetic-field dynamics are similar to processes within Earth’s magnetosphere which can affect space weather. Space weather can have real consequences for humans, as Elon Musk’s team experienced in February, when dozens of SpaceX satellites were yanked out of orbit by a massive geomagnetic storm, re-entered the Earth's atmosphere and burned up.

With multiple companies — including SpaceX — planning to launch new satellites and expand internet access around the world, understanding these dynamics and learning how to cope with them is crucial.

The discovery may also enhance our ability to find planets around other stars that we cannot otherwise detect now. Leto showed that the relationship between rotation and radio brightness is consistent with the radio emission of Jupiter, suggesting that the same mechanisms might be at work within the magnetospheres of other planets as well as much cooler stars.

The Royal Astronomical Society published complementary articles from both angles, with Shultz and Leto presenting the observations and experimental data and Owocki and team explaining how centrifugal breakout events would provide all the energy needed for such phenomena.

Including rotation speed in calculations makes it possible to more accurately predict the radio brightness of the star, Shultz said.

“If you know just three things — the size of the star, the strength of its magnetic field and how fast it’s rotating — you can predict how bright it will be,” Shultz said.

Finding the rotational velocity and the size of the star are fairly easy measurements (for an astronomer). Measuring the strength of the magnetic field is trickier, because the measurement relies on a subtle quantum mechanical phenomenon (known as the Zeeman effect), which is very difficult to detect.

“You need a big telescope and a lot of time on that telescope,” Shultz said.

Using a new kind of measurement tool that includes the rotational velocity can help. Plugging that into a calculation that includes the star’s radio luminosity, the distance to the star and its radius may help scientists determine the magnetic field strength indirectly.

And because of the stability of these stars, observations here can help scientists understand these dynamics in more turbulent environments.

“What is nice about this result, is that it gives you a laboratory where you know exactly what’s causing the magnetic fields to stretch and reconnect,” Owocki said. “It’s not an isolated result. There are implications of how you could use it in different areas.”

Another UD-affiliated researcher contributed to this work — Barnali Das, a postdoctoral researcher from India who works with UD Associate Professor Veronique Petit.

Das said that although these results were drawn from hot stars, they can apply in other contexts.

“This is also applicable for very cool objects, going down to very cool stars,” she said. “It demonstrates how we can use this. I think this is the first time it becomes evident that research in the field of magnetic massive stars has application over a much broader field, such as searching for the magnetic fields of exoplanets.”

The previous model used to explain the energy behind radio emissions from hot stars was generated by “current sheets” — the place at the equator of a magnetic field where the polarity changes from north to south or from south to north. The magnetic field becomes weaker as it moves away from the star while the wind grows stronger, eventually pushing through the magnetic field.

“The problem with that paradigm is that rotation plays no role in it,” Shultz said. “It turns out not to be the case at all.”

Even when radio emission from magnetic stars was first discovered in 1987, researchers suspected that rotation should play a role in the phenomenon, Shultz said, and while they looked for it they did not have rotation data on enough stars at that time.

Many more observations are needed to further confirm these findings and how they apply in other contexts.

“As new telescopes come online, this work will give ideas of targets to look for,” Owocki said.

And our understanding will expand and evolve, as the scientific method is applied.

“None of our truths are absolute,” Owocki said. “This theory could fall apart. If an idea you have doesn’t work, you don’t want to make a square peg fit into a round hole. You abandon that and move on to what does work.”

Owocki said he draws energy and insight from these collaborative projects.

“What I really love is how interactive it is with other people,” he said. “I learn from students and postdocs. Even if you’re an expert on one thing, not on all the other stuff, having collaborators who can tell you about this and that — a collaboration that goes on to push the ball ahead — that’s what I find most rewarding.”

Other collaborators also included researchers from the National Research Council of Canada, Howard University in Washington D.C., NASA, the National Centre for Radio Astrophysics at Pune, India; the University of Moncton in New Brunswick, Canada, Uppsala University in Sweden, the Armagh Observatory and Planetarium in the United Kingdom, the University of Western Ontario, Canada; Paris Observatory in France; the European Organization for Astronomical Research in Santiago, Chile; and the Royal Military College of Canada.

This work was supported by the Annie Jump Cannon Fellowship, NASA and India’s Department of Atomic Energy.

To understand more about how rotation speed and centrifugal breakout affect star brightness, see this explanatory video.

Article by Beth Miller; photo by Evan Krape; video by Jeffrey C. Chase

Published June 8, 2022

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UD researchers in astronomy and physics, working with collaborators, have made discoveries about a rare variety of bright, hot stars with magnetic fields thousands of times stronger than the sun’s.
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