Particle Physics in Space: In Search of the Elusive Axion

Composite image of the Sun including high-energy X-ray data from NuSTAR (blue); low-energy X-ray data from Hinode (green); and ultraviolet data from Solar Dynamics Observatory (red). The centers of stars like or even bigger than our Sun are unimaginably hot environments that may hold the key to detecting dark matter. Download Image

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NuSTAR is best known for observing some of the hottest, brightest, and most extreme phenomena in the Universe, such as supernovae explosions and the immediate surroundings of black holes. But did you know that it can also be used to search for some of the smallest and most elusive particles in existence?

For decades we've known that a large portion of the mass in the Universe—even more than all the stars and galaxies that we can see—consists of dark matter. We know it's there because we can see its gravitational effects on the matter we can observe—just like we can't see air but can see its effect on the world around us when trees sway in the wind. But dark matter itself, like its name suggests, is invisible to us, as it does not give off or interact with light. This makes trying to find out what it's actually made of incredibly difficult.

Axions

Many different kinds of particles and objects have been put forward as possible candidates to be dark matter, one of which is the axion. If you think that sounds a bit like the name of a cleaning product, you'd be right—it was named after a laundry detergent brand because it 'cleans up' a messy problem in particle physics called charge-parity violation. Under the Standard Model of particle physics, the laws of physics apply in the same way to particles and their corresponding anti-particles. But if this were the case, why is there so much more matter in the Universe than anti-matter? Axions present a possible solution to this problem and would emerge naturally from the breaking of this matter/anti-matter symmetry.

If axions were to exist, they would be extremely light and interact so weakly with normal matter that trillions could pass through you every second and you wouldn't even notice. This also makes them a compelling candidate for dark matter. However, if we are to demonstrate that this theorized particle exists, first we need to detect it. And that's no small feat, since there is a wide range of possible masses and degrees to which they interact with light or matter that could apply to an axion—in other words, they might be out there but we're not exactly sure what they look like. Axions are so elusive that all ground-based efforts to date have failed to detect them. So, scientists have turned to space to continue the search.

The Sun as a Particle Physics Laboratory

Inside our own Sun, high-energy X-ray photons released in the thermonuclear core face a long, slow journey to the surface, repeatedly absorbed and re-emitted by the densely packed matter inside, losing energy along the way until they finally emerge as visible light thousands to millions of years later. However, the interaction of a high-energy photon with the electric fields of an atomic nucleus or electron could also generate an axion, which then streams directly out of the Sun. Once outside, the axion interacts again with the Sun's magnetic field and can turn back into a photon. Depending on the properties of the axion, this could be an X-ray photon.

This is where NuSTAR comes in. Unlike most sensitive X-ray astrophysics observatories, NuSTAR is able to safely look at the Sun, making it useful for studying flares and hotspots on the Sun's surface. Scientists can also use this Solar data to look for the distinct predicted signatures of axions.

So far, NuSTAR has not detected an axion signal from the Sun. In science, a non-detection is not bad news! Since we know a lot about the Sun's properties, we know that if axions were larger or more interactive than certain values, we would have detected them. The fact that we haven't allows us to rule out certain possibilities for those properties. If axions exist, they must be sufficiently light and non-interactive that the signal from the Sun is too weak to detect.

The next step is to find somewhere that might produce a stronger axion signal than the Sun. Axion emission is directly related to the temperature inside a star. In other words, we're going to need a bigger, hotter star.

Let's go bigger!

On the left shoulder of Orion is an enormous red supergiant star called Betelgeuse, the tenth-brightest star in our night sky. Hundreds of times larger than the Sun, its radius would engulf the orbit of Mars if it were in our Solar system. The temperature of its core is also far higher than the Sun's, implying that it could be a far more efficient source of axions. In 2019, NuSTAR observed Betelgeuse in search of signs of a specific theorized axion production mechanism that Betelgeuse is hot enough to achieve—the de-excitation of iron nuclei. This kind of atomic transition happens at a very specific energy, meaning that the axion-to-photon transformations in the Sun’s outer magnetic field would result in a distinctive X-ray emission line at 14.4 keV.

While the 14.4 keV line was not detected from Betelgeuse, this doesn’t imply that axions cannot exist. Once more, the lack of a signal instead rules out certain possible properties of axions, providing orders of magnitude better constraints on their mass and the strength of their interactions with normal matter than we could achieve with the Sun.

Since axions weren't detected from Betelgeuse, can we find an even bigger, hotter laboratory than that? What if we didn't just look at one star, but a whole galaxy of hot massive stars?

M82, also known as the Cigar Galaxy for its narrow, edge-on shape, is a nearby galaxy undergoing intense star formation, meaning that it is full of newly formed, very massive and very hot stars. If each of these stars could potentially be giving off a very faint axion signal, then by observing the galaxy NuSTAR could pick up their combined signal. This would appear as a high-energy X-ray glow around the galaxy.

"By analyzing over a million seconds of NuSTAR X-ray observations of M82, we found no excess X-ray signal attributable to decaying axions," said Francisco Rodríguez Candón, PhD student at the University of Zaragoza in Spain and the first author of a paper on this new approach. "This null result enabled us to set some of the strictest limits to date on axion properties."

Once more, no signal was detected—which means that we can rule out further swaths of possible combinations of axion mass and photon coupling from the potential axion parameter space. Little by little, we are narrowing down the possibilities and, if axions are truly what makes up dark matter, closing in on their nature.

In the meantime, the search for axion signals continues. These studies demonstrate the importance of using astronomical observations with X-ray telescopes to probe particle physics in environments and on scales that would be impossible to replicate on Earth. With the help of telescopes like NuSTAR, the Universe itself is our particle physics laboratory.

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/News Release

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Pre-Supernova Burps and Red Supergiant Reflux

SN 1987A is an example of a supernova that collided with circumstellar material as it expanded.
Credit: ESA/Hubble, NASA

Title: 3D Hydrodynamics of Pre-supernova Outbursts in Convective Red Supergiant Envelopes
Authors: Benny T.-H. Tsang, Daniel Kasen, and Lars Bildsten
First Author’s Institution: University of California, Berkeley
Status: Published in ApJ

All right, I know what you’re thinking: “What do my digestive problems (for the hopefully very few of you) have to do with a star’s last, quite spectacular, goodbye?” Unlike most humans, stars don’t possess working intestines, and their outbursts — supernovae — are far more impressive than anything we can manage. There are many different kinds of supernovae, but they are broadly split into two categories: Type I and Type II. We typically see no emission lines of hydrogen in the spectra of Type I supernovae (here’s an example of why), while the spectra of Type II supernovae do contain hydrogen lines. We will talk about this latter type in today’s bite.

Red Supergiant Burps and Interacting Supernovae

Before red supergiants go supernova, they are prone to breathtaking belches called pre-supernova outbursts. These outbursts push huge amounts of gas from the star out into the so-called circumstellar medium — the material in the star’s direct neighborhood. If this neighborhood is filled with enough gas when a star dies, the expanding supernova will push against and interact with this material. This interplay between the material around the star and the supernova can actually be observed from Earth, giving rise to what is known as a Type IIn or interacting supernova.

How visible this interaction is depends mostly on how much material is in the stellar neighborhood. This depends again on how much gas the star decides to throw out, and also on how these pre-supernova outbursts (or burps) are actually formed. The authors of today’s article show that this has a lot to do with convection in the red supergiant.

Red Supergiant Boiling Pot

Simulating these red supergiant outbursts shortly before they go supernova is not new, so we already know the causes of these pre-supernova outbursts:

• Increasingly unstable nuclear fusion in the core of the star causes powerful gravity waves (not to be confused with gravitational waves)
• Large-scale convection in the red supergiant carries around material in the star, which can destabilize the nuclear fusion in the core, giving a very variable energy output
• Pair instability can cause the core’s energy output to go through cycles of drops and spikes
• A binary companion star can disturb the red supergiant enough to cause the star to temporarily become unstable

The bottom line is that some process releases a large amount of extra energy inside the star, which, depending on how the star reacts to this energy release, can lead to different outbursts of gas. Until now, the simulations of these outbursts have usually been spherically symmetric, meaning that the simulation of the outburst looks exactly the same from any direction. You can also see this as a simulation along a single line of sight from the outside of the star inwards (i.e., one-dimensional).

The problem with this approach is that you cannot simulate convection this way. To deal with convection, the authors of today’s article took the brute-force approach and did a fully 3D simulation. They simulated the region of the star outside the nuclear core (called the envelope) and started with a large energy release at the innermost part of their simulation. The authors considered different styles of energy release in the envelope. These included:

• A large, sudden energy release, comparable to the energy needed to keep the star together by gravity. This can cause a mass ejection, quite like the Sun but on much larger scales.
• A slow release of energy, which causes a much steadier stream of mass flowing away from the star instead of an explosive loss of mass.
• Varying direction of energy release, which influences how (and where to) the pre-supernova outburst will occur.

A snapshot of the authors’ simulation is shown in Figure 1. Here, we see both the envelope density on the left and the velocity of the envelope gas in the radial direction on the right. In the velocity graph, we can see zones both moving away from the star and falling back towards the core. These are the same as convection cells we can find in daily life — like in a pot of boiling water.

Figure 1: Left: Density slice of the star’s outer layers, with radius (R) vs. the distance from the core to the pole (z). Right: Velocity in the radial direction (away from the core) slice with the same axes as on the left. Credit: Tsang et al. 2022

The convection cells leave “holes” or channels of lower density in the envelope from the outside to inner parts of the star. Through these channels, much more gas can escape than would be possible without convection.

We can also see this in Figure 2: the simulation in the left panel, which included the convection, resulted in much more mass loss than the simulation in the right panel, which did not. These channels of low density appear where most of the mass escapes in the convection simulation.

Figure 2: Two images of the star’s surface in Mollweide projection, showing how much mass has escaped. On the left is a model with convection, where the colors indicate the amount of mass lost per direction (or, specifically, solid angle). On the right is a simulation without convection. Credit: Tsang et al. 2022

This article shows the necessity of taking convection in 3D into account, where the loss of mass from the pre-supernova outbursts has mostly been underestimated. This increases the amount of gas in the neighborhood of the red supergiant, ultimately affecting how the interacting supernova will look to us on Earth.

Source: American Astronomical Society - AAS Nova 

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By Astrobites

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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About the author, Roel Lefever:

Roel is a first-year PhD student at Heidelberg University, studying astrophysics. He works on massive stars and simulates their atmospheres/outflows. In his spare time, he likes to hike/bike in nature, play (a whole lot of) video games, play/listen to music (movie soundtracks!), and to read (currently The Wheel of Time, but any fantasy really).

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Hubble Sees Red Supergiant Star Betelgeuse Slowly Recovering After Blowing Its Top

This illustration plots changes in the brightness of the red supergiant star Betelgeuse, following the titanic mass ejection of a large piece of its visible surface. The escaping material cooled to form a cloud of dust that temporarily made the star look dimmer, as seen from Earth. This unprecedented stellar convulsion disrupted the monster star’s 400-day-long oscillation period that astronomers had measured for more than 200 years. The interior may now be jiggling like a plate of gelatin dessert.Release Images

Analyzing data from NASA's Hubble Space Telescope and several other observatories, astronomers have concluded that the bright red supergiant star Betelgeuse quite literally blew its top in 2019, losing a substantial part of its visible surface and producing a gigantic Surface Mass Ejection (SME). This is something never before seen in a normal star's behavior.

Our Sun routinely blows off parts of its tenuous outer atmosphere, the corona, in an event known as a Coronal Mass Ejection (CME). But the Betelgeuse SME blasted off 400 billion times as much mass as a typical CME!

The monster star is still slowly recovering from this catastrophic upheaval. "Betelgeuse continues doing some very unusual things right now; the interior is sort of bouncing," said Andrea Dupree of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts.

These new observations yield clues as to how red stars lose mass late in their lives as their nuclear fusion furnaces burn out, before exploding as supernovae. The amount of mass loss significantly affects their fate. However, Betelgeuse's surprisingly petulant behavior is not evidence the star is about to blow up anytime soon. So the mass-loss event is not necessarily the signal of an imminent explosion.

Dupree is now pulling together all the puzzle pieces of the star's petulant behavior before, after, and during the eruption into a coherent story of a never-before-seen titanic convulsion in an aging star.

This includes new spectroscopic and imaging data from the STELLA robotic observatory , the Fred L. Whipple Observatory's Tillinghast Reflector Echelle Spectrograph (TRES) , NASA's Solar Terrestrial Relations Observatory spacecraft (STEREO-A) , NASA's Hubble Space Telescope , and the American Association of Variable Star Observers (AAVSO) . Dupree emphasizes that the Hubble data was pivotal to helping sort out the mystery.

"We've never before seen a huge mass ejection of the surface of a star. We are left with something going on that we don't completely understand. It's a totally new phenomenon that we can observe directly and resolve surface details with Hubble. We're watching stellar evolution in real time."

The titanic outburst in 2019 was possibly caused by a convective plume, more than a million miles across, bubbling up from deep inside the star. It produced shocks and pulsations that blasted off the chunk of the photosphere leaving the star with a large cool surface area under the dust cloud that was produced by the cooling piece of photosphere. Betelgeuse is now struggling to recover from this injury.

Weighing roughly several times as much as our Moon, the fractured piece of photosphere sped off into space and cooled to form a dust cloud that blocked light from the star as seen by Earth observers. The dimming, which began in late 2019 and lasted for a few months, was easily noticeable even by backyard observers watching the star change brightness. One of the brightest stars in the sky, Betelgeuse is easily found in the right shoulder of the constellation Orion.

Even more fantastic, the supergiant's 400-day pulsation rate is now gone, perhaps at least temporarily. For almost 200 years astronomers have measured this rhythm as evident in changes in Betelgeuse's brightness variations and surface motions. Its disruption attests to the ferocity of the blowout.

The star's interior convection cells, which drive the regular pulsation may be sloshing around like an imbalanced washing machine tub, Dupree suggests. TRES and Hubble spectra imply that the outer layers may be back to normal, but the surface is still bouncing like a plate of gelatin dessert as the photosphere rebuilds itself.

Though our Sun has coronal mass ejections that blow off small pieces of the outer atmosphere, astronomers have never witnessed such a large amount of a star's visible surface get blasted into space. Therefore, surface mass ejections and coronal mass ejections may be different events.

Betelgeuse is now so huge now that if it replaced the Sun at the center of our solar system, its outer surface would extend past the orbit of Jupiter. Dupree used Hubble to resolve hot spots on the star's surface in 1996. This was the first direct image of a star other than the Sun.

NASA's Webb Space Telescope may be able to detect the ejected material in infrared light as it continues moving away from the star.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, 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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Credits:

Release: NASA, ESA, STScI

Media Contact:

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Science Contact:

Andrea Dupree
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts

Permissions: Content Use Policy

Contact Us:

Direct inquiries to the News Team.

Related Links and Documents:

• Science Paper: The science paper by A. Dupree et al., PDF (1.08 MB)
• NASA's Hubble Portal
• Center for Astrophysics Harvard & Smithsonian's Release

Source:HubbleSite/News

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Supergiant Atmosphere of Antares Revealed by Radio Telescopes

Artist impression of the atmosphere of Antares. As seen with the naked eye (up until the photosphere), Antares is around 700 times larger than our sun, big enough to fill the solar system beyond the orbit of Mars (Solar System scale shown for comparison). But ALMA and VLA showed that its atmosphere, including the lower and upper chromosphere and wind zones, reaches out 12 times farther than that. Credit: NRAO/AUI/NSF, S. Dagnello. Hi-Res File

Radio images of Antares with ALMA and the VLA. ALMA observed Antares close to its surface in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. In the VLA image a huge wind is visible on the right, ejected from Antares and lit up by its smaller but hotter companion star Antares B. Credit: ALMA (ESO/NAOJ/NRAO), E. O’Gorman; NRAO/AUI/NSF, S. Dagnello. Hi-Res File

Artist impression of red supergiant star Antares

Credit: NRAO/AUI/NSF, S. Dagnell. Hi-Res File

Antares Star Chart

Star chart showing the location of the bright red star Antares (encircled in red). Antares is the closest red supergiant to Earth (555 light years away) and is located in the constellation of Scorpius (The Scorpion). Credit: ESO, IAU, Sky & Telescope. Hi-Res File

Press Release Video

Brief video (1:33) explaining this research result.

Credit: NRAO/AUI/NSF, S. Dagnello. Download Video

An international team of astronomers has created the most detailed map yet of the atmosphere of the red supergiant star Antares. The unprecedented sensitivity and resolution of both the Atacama Large Millimeter/submillimeter Array (ALMA) and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) revealed the size and temperature of Antares’ atmosphere from just above the star’s surface, throughout its chromosphere, and all the way out to the wind region.

Red supergiant stars, like Antares and its more well-known cousin Betelgeuse, are huge, relatively cold stars at the end of their lifetime. They are on their way to run out of fuel, collapse, and become supernovae. Through their vast stellar winds, they launch heavy elements into space, thereby playing an important role in providing the essential building blocks for life in the universe. But it is a mystery how these enormous winds are launched. A detailed study of the atmosphere of Antares, the closest supergiant star to Earth, provides a crucial step towards an answer.

The ALMA and VLA map of Antares is the most detailed radio map yet of any star, other than the Sun. ALMA observed Antares close to its surface (its optical photosphere) in shorter wavelengths, and the longer wavelengths observed by the VLA revealed the star’s atmosphere further out. As seen in visible light, Antares’ diameter is approximately 700 times larger than the Sun. But when ALMA and the VLA revealed its atmosphere in radio light, the supergiant turned out to be even more gigantic.

“The size of a star can vary dramatically depending on what wavelength of light it is observed with,” explained Eamon O’Gorman of the Dublin Institute for Advanced Studies in Ireland and lead author of the study published in the June 16 edition of the journal Astronomy & Astrophysics. “The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”

The radio telescopes measured the temperature of most of the gas and plasma in Antares’ atmosphere. Most noticeable was the temperature in the chromosphere. This is the region above the star’s surface that is heated up by magnetic fields and shock waves created by the vigorous roiling convection at the stellar surface – much like the bubbling motion in a pot of boiling water. Not much is known about chromospheres, and this is the first time that this region has been detected in radio waves.

Thanks to ALMA and the VLA, the scientists discovered that the star’s chromosphere extends out to 2.5 times the star’s radius (our Sun’s chromosphere is only 1/200th of its radius). They also found that the temperature of the chromosphere is lower than previous optical and ultraviolet observations have suggested. The temperature peaks at 3,500 degrees Celsius (6,400 degrees Fahrenheit), after which it gradually decreases. As a comparison, the Sun’s chromosphere reaches temperatures of almost 20,000 degrees Celsius.

“We found that the chromosphere is ‘lukewarm’ rather than hot, in stellar temperatures,” said O’Gorman. “The difference can be explained because our radio measurements are a sensitive thermometer for most of the gas and plasma in the star’s atmosphere, whereas past optical and ultraviolet observations were only sensitive to very hot gas and plasma.”

“We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere,” said co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile who previously observed Antares’ atmosphere in infrared light. “Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individually, but we want to try that in future studies.”

In the ALMA and VLA data, astronomers for the first time saw a clear distinction between the chromosphere and the region where winds start to form. In the VLA image, a huge wind is visible, ejected from Antares and lit up by its smaller but hotter companion star Antares B.

“When I was a student, I dreamt of having data like this,” said co-author Graham Harper of the University of Colorado, Boulder. “Knowing the actual sizes and temperatures of the atmospheric zones gives us a clue of how these huge winds start to form and how much mass is being ejected.”

“Our innate understanding of the night sky is that stars are just points of light. The fact we can map the atmospheres of these supergiant stars in detail, is a true testament to technological advances in interferometry. These tour de force observations bring the universe close, right into our own backyard,” said Chris Carilli of the National Radio Astronomy Observatory, who was involved in the first observations of Betelgeuse at multiple radio wavelengths with the VLA in 1998.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Source: National Radio Astronomy Observatory (NRAO)/Newa

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Media contact:

Iris Nijman
NRAO News and Public Information Manager
inijman@nrao.edu

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This research was presented in a paper titled “ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse,” by E. O’Gorman et al., appearing in the journal Astronomy & Astrophysics. www.aanda.org/10.1051/0004-6361/202037756

Read our press release on SpaceScoop.org, the astronomy news service for children!

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

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