How to move communities away from flooding risks with minimal harm

A Stanford analysis of planned relocations around the world reveals a blueprint for positive outcomes

Peer-Reviewed Publication

STANFORD UNIVERSITY

 

IMAGE: HOMES IN VUNIDOGOLOA, FIJI, A COMMUNITY RELOCATED TO AVOID THE EFFECTS OF A RISING SEA. view more 

CREDIT: NANSEN INITIATIVE

Watch video: https://www.youtube.com/watch?v=RrgCnkplfi0

As sea levels rise and flooding becomes more frequent, many countries are considering a controversial strategy: relocation of communities. A Stanford analysis of planned relocations around the world, published July 27 in Nature Climate Change, reveals a blueprint for positive outcomes from an approach often considered a measure of last resort. The authors find that community engagement matters: the more community members drive decisions about whether, where, and how to relocate, the more successful the outcomes.

“Planned relocation is complex and generally considered a ‘measure of last resort,’ but countries like Fiji are developing national policies to guide approaches,” said study lead author Erica Bower, a PhD student in the Emmett Interdisciplinary Program in Environment and Resources in the Stanford Doerr School of Sustainability. “The findings of this paper offer insights for policy- and decision-makers to help ensure relocated communities are not left in worse circumstances.”

Rising seas, rising risks

Every year, flooding drives millions of people from their homes. This nightmare scenario is likely to become more common as rising seas and heavier rainfall increase risks. Moving communities away from these danger zones in a planned and anticipatory way can prevent future forced displacement, but has been considered an option of last resort because of its potential to lead to unemployment, food insecurity, heritage loss, and other damages.

To understand options for getting the best from a challenging situation, the researchers examined six types of outcomes for completed relocations around the world. Across 14 planned relocations, from Allenville, Arizona, to Vunidogoloa, Fiji, cultural dimensions, such as access to ancestral burial sites and places of worship, fared the worst. Conversely, physical, human, and natural dimensions were more often positive, while financial and social outcomes were mixed.

No single aspect of the planning or execution was consistent across all relocations deemed successful or unsuccessful in terms of the six outcomes required for sustainable livelihoods. However, most successful relocations were initiated and driven by community members rather than governments. This finding confirms insights from previous studies about the importance of meaningful community engagement at all stages of the relocation process.

The analysis also showed evidence for the first time that the pace of the relocation influences the prospects for positive outcomes, but in contrasting ways for small and large communities. Small, tightknit communities with a shared identity achieve the best results with slow, careful efforts. The slow pace can help keep the community together, not only philosophically but also physically through, for example, shared temporary housing. On the other hand, large communities benefit from processes that are speedy and efficient often as an urgent response to a disaster.

Perhaps most surprisingly, the analysis found the distance a community moved made little difference for livelihood outcomes. One possible explanation is that the success of a relocation is controlled more by elevation change than horizontal distance. Another is that cultural and jurisdictional factors may matter more than distance, especially for indigenous and other communities with strong attachment to place. In Fiji, for example, distance mattered less than whether the move took place within land already owned by the community, ensuring the move did not challenge territorial sovereignty, protected connection to place, had historical precedent, and enabled continuity of everyday practices and livelihoods, including small‐scale farming and fishing.

“It would be great if people never had to move,” said study co-author Chris Field, the Perry L. McCarty Director of the Stanford Woods Institute for the Environment within the Stanford Doerr School of Sustainability. “But relocations will be necessary, and we should be doing everything we can to ensure that, when people need to move, it is to locations that are safer and lives that are better.”

Study co-authors also include Anvesh Badamikar, a graduate student in Civil and Environmental Engineering; and Gabrielle Wong-Parodi, an assistant professor of Earth system science and a center fellow at the Stanford Woods Institute for the Environment.

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JOURNAL

Nature Climate Change

ARTICLE PUBLICATION DATE

27-Jul-2023

 

Sri Lanka floods easier to predict with India weather tool

Peer-Reviewed Publication

UNIVERSITY OF READING

Floods and landslides in Sri Lanka could be better predicted by applying weather forecasting techniques currently used in India, a study has found.

The new research has the potential to help communities better prepare for extreme rainfall in Sri Lanka. The South Asian island is extremely vulnerable to floods and landslides caused by heavy rainfall. In May 2017, more than 150 people were killed after Sri Lanka experienced flooding triggered by monsoon rains.

Led by Dr Akshay Deoras and published today (Thursday, 27 July) in Geophysical Research Letters, the study expands the list of countries the University of Reading is supporting in its research and highlights the importance of studying extreme weather in Sri Lanka.

Dr Deoras said: “Sri Lanka has largely been ignored by researchers despite being extremely vulnerable to catastrophic floods and extreme weather events, which are likely to intensify in the future due to climate change. Our study is the first to show that techniques already being used to improve weather forecasting in India can be used effectively in Sri Lanka as well.

“People who forecast the weather are actively monitoring big patterns of wind which can help them to predict what will come next. We have identified specific wind patterns that are most responsible for triggering extreme rainfall in Sri Lanka. Understanding the link between weather patterns and extreme rainfall is really important for helping Sri Lankan communities prepare and respond to deadly natural disasters and could ultimately save lives.”

Prediciting extreme rainfall

A monsoon climate is characterised by a dramatic seasonal change in direction of the prevailing winds over a region. The study found extreme rainfall in Sri Lanka occurred most frequently during the northeast monsoon (December-February) and second intermonsoon seasons (October-November). In contrast, the amount of rainfall was very small in weather patterns associated with the southwest monsoon (May-September) and first intermonsoon (March-April) seasons.  

Using multiple datasets, the research team also investigated the link between extreme rainfall in Sri Lanka, weather patterns and the Madden-Julian Oscillation (MJO). The MJO is an eastward moving band of clouds and rainfall over the equatorial Indian Ocean and the western Pacific Ocean, which influences the weather in the tropics. The researchers found that the location of the MJO is important in determining whether a weather pattern will trigger extreme rainfall in Sri Lanka. The likelihood of extreme rainfall in some weather patterns was considerably larger when the MJO was located over the Indian Ocean. It decreased by over 90% in the same weather patterns when the MJO was located over the western Pacific Ocean.

Leading weather prediction models can accurately predict the location of the MJO a week or so in advance. It is hoped the findings from this study will enhance the predictability of extreme rainfall in Sri Lanka given its link with the MJO. 

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JOURNAL

Geophysical Research Letters

DOI

10.1029/2023GL103727 

METHOD OF RESEARCH

Observational study

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

The influence of weather patterns and the Madden-Julian Oscillation on extreme precipitation over Sri Lanka.

ARTICLE PUBLICATION DATE

27-Jul-2023

 

Hubble sees evaporating planet getting the hiccups

Peer-Reviewed Publication

NASA/GODDARD SPACE FLIGHT CENTER

 

IMAGE: THIS ARTIST'S ILLUSTRATION SHOWS A PLANET (DARK SILHOUETTE) PASSING IN FRONT OF THE RED DWARF STAR AU MICROSCOPII. THE PLANET IS SO CLOSE TO THE ERUPTIVE STAR A FEROCIOUS BLAST OF STELLAR WIND AND BLISTERING ULTRAVIOLET RADIATION IS HEATING THE PLANET'S HYDROGEN ATMOSPHERE, CAUSING IT TO ESCAPE INTO SPACE. FOUR TIMES EARTH'S DIAMETER, THE PLANET IS SLOWLY EVAPORATING ITS ATMOSPHERE, WHICH STRETCHES OUT LINEARLY ALONG ITS ORBITAL PATH. THIS PROCESS MAY EVENTUALLY LEAVE BEHIND A ROCKY CORE. THE ILLUSTRATION IS BASED ON MEASUREMENTS MADE BY THE HUBBLE SPACE TELESCOPE. view more 

CREDIT: NASA, ESA, JOSEPH OLMSTED (STSCI)

A young planet whirling around a petulant red dwarf star is changing in unpredictable ways orbit-by-orbit. It is so close to its parent star that it experiences a consistent, torrential blast of energy, which evaporates its hydrogen atmosphere – causing it to puff off the planet.

But during one orbit observed with NASA's Hubble Space Telescope, the planet looked like it wasn't losing any material at all, while an orbit observed with Hubble a year and a half later showed clear signs of atmospheric loss.

This extreme variability between orbits shocked astronomers. "We've never seen atmospheric escape go from completely not detectable to very detectable over such a short period when a planet passes in front of its star," said Keighley Rockcliffe of Dartmouth College in Hanover, New Hampshire. "We were really expecting something very predictable, repeatable. But it turned out to be weird. When I first saw this, I thought 'That can't be right.'"

Rockcliffe was equally puzzled to see, when it was detectable, the planet's atmosphere puffing out in front of the planet, like a headlight on a fast-bound train. "This frankly strange observation is kind of a stress-test case for the modeling and the physics about planetary evolution. This observation is so cool because we're getting to probe this interplay between the star and the planet that is really at the most extreme," she said.

Located 32 light-years from Earth, the parent star AU Microscopii (AU Mic) hosts one of the youngest planetary systems ever observed. The star is less than 100 million years old (a tiny fraction of the age of our Sun, which is 4.6 billion years old). The innermost planet, AU Mic b, has an orbital period of 8.46 days and is just 6 million miles from the star (about 1/10th the planet Mercury's distance from our Sun). The bloated, gaseous world is about four times Earth's diameter.

AU Mic b was discovered by NASA’s Spitzer and TESS (Transiting Exoplanet Survey Satellite) space telescopes in 2020. It was spotted with the transit method, meaning telescopes can observe a slight dip in the star's brightness when the planet crosses in front of it.

Red dwarfs like AU Microscopii are the most abundant stars in our Milky Way galaxy. They therefore should host the majority of planets in our galaxy. But can planets orbiting red dwarf stars like AU Mic b be hospitable to life? A key challenge is that young red dwarfs have ferocious stellar flares blasting out withering radiation. This period of high activity lasts a lot longer than that of stars like our Sun.

The flares are powered by intense magnetic fields that get tangled by the roiling motions of the stellar atmosphere. When the tangling gets too intense, the fields break and reconnect, unleashing tremendous amounts of energy that are 100 to 1,000 times more energetic than our Sun unleashes in its outbursts. It's a blistering fireworks show of torrential winds, flares, and X-rays blasting any planets orbiting close to the star. "This creates a really unconstrained and frankly, scary, stellar wind environment that's impacting the planet's atmosphere," said Rockcliffe.

Under these torrid conditions, planets forming within the first 100 million years of the star's birth should experience the most amount of atmospheric escape. This might end up completely stripping a planet of its atmosphere.

"We want to find out what kinds of planets can survive these environments. What will they finally look like when the star settles down? And would there be any chance of habitability eventually, or will they wind up just being scorched planets?" said Rockcliffe. "Do they eventually lose most of their atmospheres and their surviving cores become super-Earths? We don't really know what those final compositions look like because we don't have anything like that in our solar system."

While the star's glare prevents Hubble from directly seeing the planet, the telescope can measure changes in the star's apparent brightness caused by hydrogen bleeding off the planet and dimming the starlight when the planet transits the star. That atmospheric hydrogen has been heated to the point where it escapes the planet's gravity.

The never-before-seen changes in atmospheric outflow from AU Mic b may indicate swift and extreme variability in the host red dwarf's outbursts. There is so much variability because the star has a lot of roiling magnetic field lines. One possible explanation for the missing hydrogen during one of the planet's transits is that a powerful stellar flare, seen seven hours prior, may have photoionized the escaping hydrogen to the point where it became transparent to light, and so was not detectable.

Another explanation is that the stellar wind itself is shaping the planetary outflow, making it observable at some times and not observable at other times, even causing some of the outflow to "hiccup" ahead of the planet itself. This is predicted in some models, like those of John McCann and Ruth Murray-Clay from the University of California at Santa Cruz, but this is the first kind of observational evidence of it happening and to such an extreme degree, say researchers.

Hubble follow-up observations of more AU Mic b transits should offer additional clues to the star and planet's odd variability, further testing scientific models of exoplanetary atmospheric escape and evolution.

Rockcliffe is lead author on the science paper accepted for publication in The Astronomical Journal.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

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JOURNAL

The Astronomical Journal

DOI

10.3847/1538-3881/ace536 

METHOD OF RESEARCH

Observational study

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b

ARTICLE PUBLICATION DATE

27-Jul-2023

Violent atmosphere allows rare look at the early life of a planet

Neptune-sized planet weirdly and sporadically burps hydrogen as it circles its star

Peer-Reviewed Publication

DARTMOUTH COLLEGE

 

IMAGE: AN ILLUSTRATION OF THE PLANET AU MIC B SHEDDING ITS ATMOSPHERE AS IT ORBITS ITS SUN, AU MICROSCOPII. view more 

CREDIT: IMAGE BY NASA, ESA, JOSEPH OLMSTED, STSCI.

Trillions of miles from Earth, the violent and erratic shedding of a young planet's atmosphere could provide a rare glimpse into the tumultuous early life that besets most of the planets in our galaxy.

A new study led by Dartmouth researchers found that a Neptune-sized gas planet known as AU Mic b exhibited some bizarre behavior—it showed no atmospheric shedding during one orbit around its sun then spewed its hydrogen-rich atmosphere into the cosmos on its next go-round.

All planets with an atmosphere lose some gas as they orbit their suns—a process known as atmospheric escape—either subtly like Earth or in dramatic plumes like AU Mic b. But scientists have never before seen atmospheric escape stop and start between orbits, the researchers report in The Astronomical Journal.

"This is the first time we've seen a planet’s atmospheric escape go from unobservable to very, very observable," said first author Keighley Rockcliffe, a Ph.D. candidate in physics and astronomy at Dartmouth. "In addition, the hydrogen cloud was not a tail behind the planet like we normally see, but like a puff in front of the planet as it orbited. We don't usually think of planets as burping hydrogen as they go around a star."

"We are directly probing an essential evolutionary mechanism that the most common planets in our galaxy go through," Rockcliffe said. "We think our work captures the early stages of this extremely typical process, and we want to use our observations of this system to understand the most common experiences of planets beyond our solar system."

The planet—which is more than four times the diameter of Earth, orbits a star called AU Microscopii that is 32 light years (roughly 192 trillion miles) from Earth. In star terms, AU Microscopii is a youthful 23 million years old; our sun is roughly 4.6 billion years old. The planet AU Mic b is only 6 million miles from its sun—or one-tenth of the distance Mercury is from our sun.

Despite its size, AU Mic b completes a full orbit in less than nine Earth days. It's discovery by NASA's Spitzer and TESS space telescopes was published in the journal Nature in 2020. The latest study is based on data from the Hubble Space Telescope.

The young age and atmospheric behavior of AU Mic b and its sun suggest that the researchers have captured the early stages of planetary evolution, said co-author Elisabeth Newton, an assistant professor of physics and astronomy at Dartmouth. Most studies on planets outside—and even within—Earth's solar system pertain to very old worlds.

Older planets have already experienced a wide range of evolutionary processes that make it difficult to extrapolate to planetary evolution at large, Newton said. They're like trying to study developmental psychology by only observing adults, she said.

"This planet is like observing a totally generic toddler," Newton said. "Systems like AU Mic are our insight into the broader planetary-evolution process. Keighley is making very challenging observations, and there are limited opportunities to even attempt them."

The planet's sun is a common type of small, low-intensity star known as a red dwarf. Seventy percent of all stars are red dwarfs, Newton said, including Proxima Centauri, the closest star to our sun.

AU Mic b itself is a type of planet known as a "hot Neptune," a world similar in size to Neptune that orbits close to its parent star. The evolution of hot Neptunes is thought to be broadly applicable to other gas planets in the galaxy. Scientists think the planets quickly burn off their large gaseous layer and evolve into smaller planets, Rockcliffe said. Only one other young hot Neptune has been observed undergoing atmospheric escape.

"I'm trying to observe that loss of mass as it's happening to get a snapshot of how planets develop before they get to that smaller, potentially rockier endpoint," she said.

Young planets such as AU Mic b also provide scientists with opportunities to examine the tempestuous early years of their young stars. These observations can be used to fine-tune computer models of how planets evolve and interact with their stellar environment, Rockcliffe said.

"There's an enormous difference between a 23-million-year-old star and a 5-billion-year-old star. The very young stars are going to be throwing out lots of flares and very high-energy radiation. Because we're looking at young planets, we can see this very extreme but typical interaction happen and use our observations to see if we're understanding the physics correctly," Rockcliffe said.

"I'm becoming more convinced that AU Mic b is this nice example of a planet undergoing all these violent but typical processes at once," she said. "It can hit the corners of many different models and ensure we're making the most accurate models possible when we're talking about planet evolution."

With every new planet discovered, the question arises: Could it be another Earth? Some rocky planets experience an early stage similar to what AU Mic b is going through now, Newton said. But even if they don't, red dwarf systems are currently the best places to find habitable, Earth-like planets.

"Stars like AU Mic are potential hunting grounds for an Earth 2.0," Newton said. "By understanding this system, we can answer questions about what an Earth-like planet orbiting a red dwarf star would have to contend with early in its evolution."

"The systems we're looking at are extremely different from our solar system. We can't really extrapolate from our own experience of atmospheric mass loss," Rockcliffe added. But "knowing how atmospheres evolve and which planets will have stable atmospheres is important for finding life on other planets. Atmospheres are essential in understanding how life can form and persist."

The paper "The variable detection of atmospheric escape around the young, hot Neptune AU Mic b" was published in The Astronomical Journal on July 27, 2023.

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JOURNAL

The Astronomical Journal

DOI

10.3847/1538-3881/ace536 

METHOD OF RESEARCH

Observational study

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b

ARTICLE PUBLICATION DATE

27-Jul-2023

Listen to a star ‘twinkle’

New study is first to determine how much stars should innately twinkle

Peer-Reviewed Publication

NORTHWESTERN UNIVERSITY

 

VIDEO: VISUALIZATION OF "JUPITER" BY GUSTAV HOLST PLAYED THROUGH THREE SIZES OF MASSIVE STARS. view more 

CREDIT: NORTHWESTERN UNIVERSITY

Many people know that stars appear to twinkle because our atmosphere bends starlight as it travels to Earth. But stars also have an innate “twinkle” — caused by rippling waves of gas on their surfaces — that is imperceptible to current Earth-bound telescopes.

In a new study, a Northwestern University-led team of researchers developed the first 3D simulations of energy rippling from a massive star’s core to its outer surface. Using these new models, the researchers determined, for the first time, how much stars should innately twinkle. 

And, in yet another first, the team also converted these rippling waves of gas into sound waves, enabling listeners to hear both what the insides of stars and the “twinkling” should sound like. And it is eerily fascinating.

The study will be published on July 27, in the journal Nature Astronomy.

“Motions in the cores of stars launch waves like those on the ocean,” said Northwestern’s Evan Anders, who led the study. “When the waves arrive at the star’s surface, they make it twinkle in a way that astronomers may be able to observe. For the first time, we have developed computer models which allow us to determine how much a star should twinkle as a result of these waves. This work allows future space telescopes to probe the central regions where stars forge the elements we depend upon to live and breathe.”

Anders is a postdoctoral fellow in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He is advised by study coauthor Daniel Lecoanet, an assistant professor of engineering sciences and applied mathematics in Northwestern’s McCormick School of Engineering and member of CIERA.

Chaotic convection

All stars have a convection zone, a wild and disorderly place where gases churn to push heat outward. For massive stars (stars at least about 1.2 times the mass of our sun), this convection zone resides at their cores.

“Convection within stars is similar to the process that fuels thunderstorms,” Anders said. “Cooled air drops, warms and rises again. It’s a turbulent process that transports heat.”

It also makes waves — small rivulets that cause starlight to dim and brighten, producing a subtle twinkle. Because the cores of massive stars are shrouded from view, Anders and his team sought to model their hidden convection. Building upon studies that examined properties of turbulent core convection, characteristics of waves and possible observational features of those waves, the team’s new simulations include all relevant physics to accurately predict how a star’s brightness changes depending upon convection-generated waves.

‘Soundproofing’ stars

After convection generates waves, those waves bounce around inside of the simulated star. While some waves eventually emerge to the star’s surface to produce a twinkling effect, other waves become trapped and continue to bounce around. To isolate the waves that launch to the surface and create twinkling, Anders and his team built a filter that describes how waves bounce around inside of the simulations.

“We first put a damping layer around the star — like the padded walls you would have in a recording studio — so we could measure exactly how the core convection makes waves,” Anders explained. 

Anders compares it to a music studio, which leverages soundproof padded walls to minimize the acoustics of an environment so musicians can extract the “pure sound” of the music. Musicians then apply filters and engineer those recordings to produce the song how they want. 

Similarly, Anders and his collaborators applied their filter to the pure waves they measured coming out of the convective core. They then followed waves bouncing around in a model star, ultimately finding that their filter accurately described how the star changed the waves coming from the core. The researchers then developed a different filter for how waves should bounce around inside of a real star. With this filter applied, the resulting simulation shows how astronomers expect waves to appear if viewed through a powerful telescope.

“Stars get a little brighter or a little dimmer depending on various things happening dynamically inside the star,” Anders said. “The twinkling that these waves cause is extremely subtle, and our eyes are not sensitive enough to see it. But powerful future telescopes may be able to detect it.”

Music in the stars

Taking the recording studio analogy one step further, Anders and his collaborators next used their simulations to generate sound. Because these waves are outside the range of human hearing, the researchers uniformly increased the frequencies of the waves to make them audible.

Depending on how large or bright a massive star is, the convection produces waves corresponding to different sounds. Waves emerging from the core of a large star, for example, make sounds like a warped ray gun, blasting through an alien landscape. But the star alters these sounds as the waves reach the star’s surface. For a large star, the ray gun-like pulses shift into a low echo reverberating through an empty room. Waves at the surface of a medium-sized star, on the other hand, conjure images of a persistent hum through a windswept terrain. And surface waves on a small star sound like a plaintive alert from a weather siren.

Next, Anders and his team passed songs through different stars to listen to how the stars change the songs. They passed a short audio clip from “Jupiter”(a movement from “The Planets” orchestral suite by composer Gustav Holst) and from “Twinkle, Twinkle, Little Star” through three sizes (large, medium and small) of massive stars. When propagated through stars, all songs sound distant and haunting — like something from “Alice in Wonderland.”

“We were curious how a song would sound if heard as propagated through a star,” Anders said. “The stars change the music and, correspondingly, change how the waves would look if we saw them as twinkling on the star’s surface.”

The study, “The photometric variability of massive stars due to gravity waves excited by core convection,” was supported by CIERA, NASA and the National Science Foundation.

"Twinkle, Twinkle, Little Star [VIDEO] | 

JOURNAL

Nature Astronomy

DOI

10.1038/s41550-023-02040-7 

METHOD OF RESEARCH

Computational simulation/modeling

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

The photometric variability of massive stars due to gravity waves excited by core convection

ARTICLE PUBLICATION DATE

27-Jul-2023

 

Twinkling of giant stars reveals how their innards churn in first-ever simulations

Astrophysicists at the Flatiron Institute and their colleagues have created the first computer simulations showing how convection in the cores of massive stars generates waves that result in flickering starlight.

Peer-Reviewed Publicatio

SIMONS FOUNDATION

 

VIDEO: A 3D SIMULATION OF HOW TURBULENT CONVECTION IN THE CORE OF A LARGE STAR (CENTER) CAN GENERATE WAVES THAT RIPPLE OUTWARD AND POWER RESONANT VIBRATIONS NEAR THE STAR’S SURFACE. BY STUDYING CHANGES IN THE STAR’S BRIGHTNESS CAUSED BY THE VIBRATIONS, SCIENTISTS COULD ONE DAY BETTER UNDERSTAND THE PROCESSES DEEP IN THE HEARTS OF LARGE STARS. view more 

CREDIT: E.H. ANDERS ET AL./NATURE ASTRONOMY 2023

Secrets hide in the twinkling of stars.

A research team led by scientists at the Flatiron Institute and Northwestern University has created first-of-their-kind computer simulations showing how churning deep in a star’s depths can cause the star’s light to flicker. This effect is different from the visible twinkling of stars in the night sky caused by Earth’s atmosphere.

By closely observing the innate twinkling of stars, astronomers could one day use the simulations to learn more about what goes on inside stars larger than our sun, the researchers report on July 27 in Nature Astronomy.

The effects are too small for current telescopes to pick up, says study co-author Matteo Cantiello, a research scientist at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City. That could change with improved telescopes. “We’ll be able to see the signature of the core,” Cantiello says, “which will be quite interesting because it will be a way to probe the very inner regions of stars.”

A better understanding of stellar innards will help astronomers learn how stars form and evolve, how galaxies assemble, and how heavy elements such as the oxygen we breathe are created, says study lead author Evan Anders, a postdoctoral researcher at Northwestern University.

“Motions in the cores of stars launch waves like those on the ocean,” Anders says. “When the waves arrive at the star’s surface, they make it twinkle in a way that astronomers may be able to observe. For the first time, we have developed computer models which allow us to determine how much a star should twinkle as a result of these waves. This work allows future space telescopes to probe the central regions where stars forge the elements we depend upon to live and breathe.”

Intriguingly, the new simulations also widen a years-long stellar mystery. Astronomers have consistently observed an unexplained pulsing — or ‘red noise’ — causing fluctuations in the brightness of hot, massive stars. A popular proposal was that convection in the stars’ cores causes this flickering. The new simulations, however, show that the twinkling induced by core convection is far too faint to match the observed red noise. Something else must be responsible, the researchers report in their new paper.

A Deep Squeeze

A star’s convection is powered by the nuclear reactor at its core. In the heart of a star, intense pressure squeezes hydrogen atoms together to form helium atoms plus a bit of excess energy. That energy generates heat, which causes clumps of plasma to rise like the goo in a lava lamp. But unlike a lava lamp, the convection is turbulent like a pot of boiling water. This movement generates waves just like those found in Earth’s oceans. Those waves then ripple outward to the star’s surface, where they compress and decompress the star’s plasma, causing brightening and dimming of the star’s light. By studying a star’s brightness, scientists realized they might be able to glean what’s going in the star’s core.

Simulating the wave generation and propagation in a computer is absurdly difficult, though, Cantiello says. That’s because while a wave-generating flow in the star’s core lasts a few weeks, the waves generated can linger for hundreds of thousands of years. Connecting those drastically different timescales — weeks and hundreds of millennia — posed a serious challenge.

The researchers took inspiration from a different form of waves: the sound waves that make up music. They realized that the convection-induced wave generation in the core is like a group of musicians in a concert hall. The musicians strumming their instruments produce a sound that is altered as it bounces around the venue. The researchers found they could first calculate the unaltered “song” of the convection-induced waves and then apply a filter that replicated the star’s acoustic properties — a similar process to that of a professional sound engineer.

The researchers tested their method using sound waves from real music, including “Jupiter” from Gustav Holst’s orchestral suite “The Planets” and, rather appropriately, “Twinkle, Twinkle, Little Star.” They simulated how those sound waves would bounce around inside stars of different sizes, producing a haunting result.

After this validation of their approach, the researchers simulated the convection-induced waves and resulting starlight fluctuations of stars whose masses are three, 15 and 40 times that of our sun. For all three sizes, the core convection did indeed cause flickering light intensity near the surface, but not at the frequencies or intensities characteristic of the red noise astronomers had seen.

Convection may still be responsible for red noise, Cantiello says, but it would likely be far nearer to the star’s surface and therefore less telling of what’s going on in the star’s deep interior.

The researchers are now improving their simulations to consider additional effects, such as the rapid spinning of a star around its axis, a common feature of stars more massive than our sun. They’re curious if fast-spinning stars have a strong enough flickering induced by core convection to be picked up by current telescopes. “It’s an interesting question we’re hoping to get an answer to,” Cantiello says.

Surface Waves [AUDIO] | EurekAlert! 

Unaltered Audio [AUDIO] | EurekAlert! 

Audio (Three Solar Masses) [AUDIO] |

Audio (15 Solar Masses) [AUDIO] |

Audio (40 Solar Masses) [AUDIO] |

ABOUT THE FLATIRON INSTITUTE

The Flatiron Institute is the research division of the Simons Foundation. The institute's mission is to advance scientific research through computational methods, including data analysis, theory, modeling and simulation. The institute's Center for Computational Astrophysics creates new computational frameworks that allow scientists to analyze big astronomical datasets and to understand complex, multi-scale physics in a cosmological context.

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JOURNAL

Nature Astronomy

DOI

10.1038/s41550-023-02040-7 

METHOD OF RESEARCH

Computational simulation/modeling

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

The photometric variability of massive stars due to gravity waves excited by core convection

ARTICLE PUBLICATION DATE

27-Jul-2023