'Triple star' discovery could revolutionize understanding of stellar evolution

A ground-breaking new discovery by University of Leeds scientists could transform the way astronomers understand some of the biggest and most common stars in the Universe.

Research by PhD student Jonathan Dodd and Professor René Oudmaijer, from the University's School of Physics and Astronomy, points to intriguing new evidence that massive Be stars -- until now mainly thought to exist in double stars -- could in fact be "triples."

The remarkable discovery could revolutionise our understanding of the objects -- a subset of B stars -- which are considered an important "test bed" for developing theories on how stars evolve more generally.

These Be stars are surrounded by a characteristic disc made of gas -- similar to the rings of Saturn in our own Solar System.

And although Be stars have been known for about 150 years -- having first been identified by renowned Italian astronomer Angelo Secchi in 1866 -- until now, no one has known how they were formed.

Consensus among astronomers so far has said the discs are formed by the rapid rotation of the Be stars, and that itself can be caused by the stars interacting with another star in a binary system.

Triple systems

Mr Dodd, corresponding author of the research, said: "The best point of reference for that is if you've watched Star Wars, there are planets where they have two Suns."

But now, by analysing data from the European Space Agency's Gaia satellite, the scientists say they have found evidence these stars actually exist in triple systems -- with three bodies interacting instead of just two.

Mr Dodd added: "We observed the way the stars move across the night sky, over longer periods like 10 years, and shorter periods of around six months. If a star moves in a straight line, we know there's just one star, but if there is more than one, we will see a slight wobble or, in the best case, a spiral.

"We applied this across the two groups of stars that we are looking at -- the B stars and the Be stars -- and what we found, confusingly, is that at first it looks like the Be stars have a lower rate of companions than the B stars. This is interesting because we'd expect them to have a higher rate."

However, Principal Investigator Prof Oudmaijer said: "The fact that we do not see them might be because they are now too faint to be detected."

Mass transfer

The researchers then looked at a different set of data, looking for companion stars that are further away, and found that at these larger separations the rate of companion stars is very similar between the B and Be stars.

From this, they were able to infer that in many cases a third star is coming into play, forcing the companion closer to the Be star -- close enough that mass can be transferred from one to the other and form the characteristic Be star disc.

This could also explain why we do not see these companions anymore; they have become too small and faint to be detected after the "vampire" Be star has sucked in so much of their mass.

The discovery could have huge impacts on other areas of astronomy -- including our understanding of black holes, neutron stars and gravitational wave sources.

Prof Oudmaijer said: "There's a revolution going on in physics at the moment around gravitational waves. We have only been observing these gravitational waves for a few years now, and these have been found to be due to merging black holes.

"We know that these enigmatic objects -- black holes and neutron stars -- exist, but we don't know much about the stars that would become them. Our findings provide a clue to understanding these gravitational wave sources."

He added: "Over the last decade or so, astronomers have found that binarity is an incredibly important element in stellar evolution. We are now moving more towards the idea it is even more complex than that and that triple stars need to be considered."

"Indeed," Oudmaijer said, "triples have become the new binaries."

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Astronomers discover striking evidence of 'unusual' stellar evolution

Astronomers have found evidence that some stars boast unexpectedly strong surface magnetic fields, a discovery that challenges current models of how they evolve.

In stars like our sun, surface magnetism is linked to stellar spin, a process similar to the inner workings of a hand-cranked flashlight. Strong magnetic fields are seen in the hearts of magnetic sunspot regions, and cause a variety of space weather phenomena. Until now, low-mass stars -- celestial bodies of lower mass than our sun that can rotate either very rapidly or relatively slowly -- were thought to exhibit very low levels of magnetic activity, an assumption which has primed them as ideal host stars for potentially habitable planets.

In a new study, published today in The Astrophysical Journal Letters, researchers from The Ohio State University argue that a new internal mechanism called core-envelope decoupling -- when the surface and core of the star start out spinning at the same rate, then drift apart -- might be responsible for enhancing magnetic fields on cool stars, a process which could intensify their radiation for billions of years and impact the habitability of their nearby exoplanets.

The research was made possible due to a technique that Lyra Cao, lead author of the study and a graduate student in astronomy at Ohio State, and co-author Marc Pinsonneault, a professor of astronomy at Ohio State, developed earlier this year to make and characterize starspot and magnetic field measurements.

Although low-mass stars are the most common stars in the Milky Way and are often hosts to exoplanets, scientists know comparatively little about them, said Cao.

For decades, it was assumed that the physical processes of lower mass stars followed those of solar-type stars. Because stars gradually lose their angular momentum as they spin down, astronomers can use stellar spins as a device to understand the nature of a star's physical processes, and how they interact with their companions and their surroundings. However, there are times where the stellar rotation clock appears to stop in place, Cao said.

Using public data from the Sloan Digital Sky Survey to study a sample of 136 stars in M44, a star crib also known as Praesepe, or the Beehive cluster, the team found that the magnetic fields of the low-mass stars in the region appeared much stronger than current models could explain.

While previous research revealed that the Beehive cluster is home to many stars that defy current theories of rotational evolution, one of Cao's team's most exciting discoveries was determining that these stars' magnetic fields may be just as unusual -- far stronger than predicted by current models.

"To see a link between the magnetic enhancement and rotational anomalies was incredibly exciting," said Cao. "It indicates that there might be some interesting physics at play here." The team also hypothesized that the process of syncing up a star's core and the envelope might induce a magnetism found in these stars that would have a starkly different origin from the kind seen on the sun.

"We're finding evidence that there's a different kind of dynamo mechanism driving the magnetism of these stars," said Cao. "This work shows that stellar physics can have surprising implications for other fields."

According to the study, these findings have important implications for our understanding of astrophysics, particularly on the hunt for life on other planets. "Stars experiencing this enhanced magnetism are likely going to be battering their planets with high-energy radiation," Cao said. "This effect is predicted to last for billions of years on some stars, so it's important to understand what it might do to our ideas of habitability."

But these findings shouldn't put a damper on the search for extraplanetary existence. With further research, the team's discovery could help provide more insight into where to look for planetary systems capable of hosting life. But here on Earth, Cao believes her team's discoveries might lead to better simulations and theoretical models of stellar evolution.

"The next thing to do is verify that enhanced magnetism happens on a much larger scale," said Cao. "If we can understand what's going on in the interiors of these stars as they experience shear-enhanced magnetism, it's going to lead the science in a new direction."

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Astro­physics: Star-child­hood shapes stel­lar evo­lu­tion

In classical models of stellar evolution, so far little importance has been attached to the early evolution of stars. Scientists now show that the biography of stars is indeed shaped by their early stage.

From babies to teenagers: stars in their "young years" are a major challenge for science. The process of star formation is particularly complex and difficult to map in theoretical models. One of the few ways to learn more about the formation, structure or age of stars is to observe their oscillations. "Comparable to the exploration of the Earth's interior with the help of seismology, we can also make statements about their internal structure and thus also about the age of stars based on their oscillations" says Konstanze Zwintz. The astronomer is regarded as a pioneer in the young field of asteroseismology and heads the research group "Stellar Evolution and Asteroseismology" at the Institute for Astro- and Particle Physics at the University of Innsbruck. The study of stellar oscillations has evolved significantly in recent years because the possibilities for precise observation through telescopes in space such as TESS, Kepler, and James Webb have improved on many levels. These advances are now also shedding new light on decades-old theories of stellar evolution.

With a new model to zero hour of adult stars

Stars are called "children" as long as they are not yet burning hydrogen to helium in their cores. At this stage, they are on the pre-main sequence; after ignition, they become adults and move onto the main sequence. "Research on stars has so far focused mainly on adult stars -- such as our Sun" says Thomas Steindl, a member of Konstanze Zwintz's research group and lead author of the study. "Even if it sounds counterintuitive at first glance, so far little attention has been paid to the evolution of the pre-main sequence because the phase is very turbulent and difficult to model. It's only the technological advances of recent years that allow us a closer look at the infancy of stars -- and thus at that moment when the star begins to fuse hydrogen into helium." In their current study, the two Innsbruck researchers now present a model that can be used to realistically depict the earliest phases of a star's life long before they become adults. The model is based on the open-source stellar evolution program MESA (Modules for Experiments in Stellar Astrophysics).

Inspired by a talk given by astronomer Eduard Vorobyov of the University of Vienna at a 2019 meeting, Thomas Steindl spent months refining the method for using this stellar evolution code to recreate the chaotic phase of early star formation and then predict their specific oscillations. "Our data show that stars on the pre-main sequence take a very chaotic course in their evolution. Despite its complexity, we can now use it in our new theoretical model." Steindl said. Thus, the astronomer shows that the way the star is formed has an impact on the oscillation behaviour even after ignition of nuclear fusion on the main sequence: "The infancy has an influence on the later pulsations of the star: This sounds very simple, but it was strongly in doubt. The classical theory assumes that the time before ignition is simply irrelevant. This is not true: Comparable to a musical instrument, even subtle differences in the composition lead to significant changes in the tone. Thus, our modern models better describe the oscillations in real stars."

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Newly-found planets on the edge of destruction

Three newly-discovered planets have been orbiting dangerously close to stars nearing the end of their lives.

Out of the thousands of extrasolar planets found so far, these three gas giant planets first detected by the NASA TESS (Transiting Exoplanet Survey Satellite) Mission, have some of the shortest-period orbits around subgiant or giant stars. One of the planets, TOI-2337b, will be consumed by its host star in less than 1 million years, sooner than any other currently known planet.

"These discoveries are crucial to understanding a new frontier in exoplanet studies: how planetary systems evolve over time," explained lead author Samuel Grunblatt, a postdoctoral fellow at the American Museum of Natural History and the Flatiron Institute in New York City. Grunblatt, who earned his PhD from the University of Hawai?i Institute for Astronomy (UH IfA), added that "these observations offer new windows into planets nearing the end of their lives, before their host stars swallow them up."

Grunblatt announced the discovery and confirmation of these planets -- TOI-2337b, TOI-4329b, and TOI-2669b -- at an American Astronomical Society press conference today; the study has been accepted for publication in the Astronomical Journal.

The researchers estimate that the planets have masses between 0.5 and 1.7 times Jupiter's mass, and sizes that range from slightly smaller to more than 1.6 times the size of Jupiter. They also span a wide range of densities, from styrofoam-like to three times denser than water, implying a wide variety of origins.

These three planets are believed to be just the tip of the iceberg. "We expect to find tens to hundreds of these evolved transiting planet systems with TESS, providing new details on how planets interact with each other, inflate, and migrate around stars, including those like our Sun," said Nick Saunders, a graduate student at UH IfA and co-author of the study.

The planets were first found in NASA TESS Mission full-frame image data taken in 2018 and 2019. Grunblatt and his collaborators identified the candidate planets in TESS data, and then used W. M. Keck Observatory's High-Resolution Echelle Spectrometer (HIRES) on Maunakea, Hawai?i to confirm the existence of the three planets.

"The Keck observations of these planetary systems are critical to understanding their origins, helping reveal the fate of solar systems like our own," said UH IfA Astronomer Daniel Huber, who co-authored the study.

Current models of planet dynamics suggest that planets should spiral in toward their host stars as the stars evolve over time, particularly in the last 10 percent of the star's lifetime. This process also heats the planets, potentially causing their atmospheres to inflate. However, this stellar evolution will also cause the orbits of planets around the host star to come closer to one another, increasing the likelihood that some of them will collide, or even destabilize the entire planetary system.

The wide variety of planet densities found in the study suggests that these planetary systems have been shaped through chaotic planet-to-planet interactions. This could also have resulted in unpredictable heating rates and timescales for these planets, giving them the wide range in densities we observe today.

Future observations of one of these systems, TOI-4329, with the recently-launched James Webb Space Telescope could reveal evidence for water or carbon dioxide in the planet's atmosphere. If these molecules are seen, the data would provide constraints on where these planets formed, and what sort of interactions had to occur to produce the planetary orbits we see today.

Continued monitoring of these systems with the NASA TESS telescope will constrain the rate at which these planets are spiraling into their host stars. So far, no clear signal of orbital decay has been observed in any of the systems, but a longer baseline of observations with the TESS Extended Missions will provide much tighter constraints on planet in-spiral than are currently possible, revealing how strongly planetary systems are affected by stellar evolution.

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In planet formation, it's location, location, location

Astronomers using NASA's Hubble Space Telescope are finding that planets have a tough time forming in the rough-and-tumble central region of the massive, crowded star cluster Westerlund 2. Located 20,000 light-years away, Westerlund 2 is a unique laboratory to study stellar evolutionary processes because it's relatively nearby, quite young, and contains a large stellar population.

A three-year Hubble study of stars in Westerlund 2 revealed that the precursors to planet-forming disks encircling stars near the cluster's center are mysteriously devoid of large, dense clouds of dust that in a few million years could become planets.

However, the observations show that stars on the cluster's periphery do have the immense planet-forming dust clouds embedded in their disks. Researchers think our solar system followed this recipe when it formed 4.6 billion years ago.

So why do some stars in Westerlund 2 have a difficult time forming planets while others do not? It seems that planet formation depends on location, location, location. The most massive and brightest stars in the cluster congregate in the core, which is verified by observations of other star-forming regions. The cluster's center contains at least 30 extremely massive stars, some weighing up to 80 times the mass of the Sun. Their blistering ultraviolet radiation and hurricane-like stellar winds of charged particles blowtorch disks around neighboring lower-mass stars, dispersing the giant dust clouds.

"Basically, if you have monster stars, their energy is going to alter the properties of the disks around nearby, less massive stars," explained Elena Sabbi, of the Space Telescope Science Institute in Baltimore and lead researcher of the Hubble study. "You may still have a disk, but the stars change the composition of the dust in the disks, so it's harder to create stable structures that will eventually lead to planets. We think the dust either evaporates away in 1 million years, or it changes in composition and size so dramatically that planets don't have the building blocks to form."

The Hubble observations represent the first time that astronomers analyzed an extremely dense star cluster to study which environments are favorable to planet formation. Scientists, however, are still debating whether bulky stars are born in the center or whether they migrate there. Westerlund 2 already has massive stars in its core, even though it is a comparatively young, 2-million-year-old system.

Using Hubble's Wide Field Camera 3, the researchers found that of the nearly 5,000 stars in Westerlund 2 with masses between 0.1 to 5 times the Sun's mass, 1,500 of them show fluctuations in their light as the stars accrete material from their disks. Orbiting material clumped within the disk would temporarily block some of the starlight, causing brightness fluctuations.

However, Hubble detected the signature of such orbiting material only around stars outside the cluster's packed central region. The telescope witnessed large drops in brightness for as much as 10 to 20 days around 5% of the stars before they returned to normal brightness. They did not detect these dips in brightness in stars residing within four light-years of the center. These fluctuations could be caused by large clumps of dust passing in front of the star. The clumps would be in a disk tilted nearly edge-on to the view from Earth. "We think they are planetesimals or structures in formation," Sabbi explained. "These could be the seeds that eventually lead to planets in more evolved systems. These are the systems we don't see close to very massive stars. We see them only in systems outside the center."

Thanks to Hubble, astronomers can now see how stars are accreting in environments that are like the early universe, where clusters were dominated by monster stars. So far, the best known nearby stellar environment that contains massive stars is the starbirth region in the Orion Nebula. However, Westerlund 2 is a richer target because of its larger stellar population.

"Hubble's observations of Westerlund 2 give us a much better sense of how stars of different masses change over time, and how powerful winds and radiation from very massive stars affect nearby lower-mass stars and their disks," Sabbi said. "We see, for example, that lower-mass stars, like our Sun, that are near extremely massive stars in the cluster still have disks and still can accrete material as they grow. But the structure of their disks (and thus their planet-forming capability) seems to be very different from that of disks around stars forming in a calmer environment farther away from the cluster core. This information is important for building models of planet formation and stellar evolution."

This cluster will be an excellent laboratory for follow-up observations with NASA's upcoming James Webb Space Telescope, an infrared observatory. Hubble has helped astronomers identify the stars that have possible planetary structures. With Webb, researchers can study which disks around stars are not accreting material and which disks still have material that could build up into planets. This information on 1,500 stars will allow astronomers to map a path on how star systems grow and evolve. Webb also can study the chemistry of the disks in different evolutionary phases and watch how they change, and help astronomers determine what influence environment plays in their evolution.

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Turbulent convection at the heart of stellar activity

Different stars can exhibit different levels of activity. The Sun's signs of solar activity are rather feeble on an astronomical scale. Other stars are up to ten times more active. While researchers have identified the magnetic fields generated in the interior of stars in a dynamo process as drivers of activity, the exact workings of this dynamo are unclear. Scientists now find that a common, turbulence-dependent dynamo mechanism plays a crucial role for stellar activity in all stages of stellar evolution.

In their interiors, stars are structured in a layered, onion-like fashion. In those with solar-like temperatures, the core is followed by the radiation zone. There, the heat from within is led outwards by means of radiation. As the stellar plasma becomes cooler farther outside, heat transport is dominated by plasma flows: hot plasma from within rises to the surface, cools, and sinks down again. This process is called convection. At the same time, the star's rotation, which depends on stellar latitude, introduces shear movements. Together, both processes twist and twirl magnetic field lines and create a star's complex magnetic fields in a dynamo process that is not yet fully understood.

"Unfortunately, we cannot look directly into the Sun and other stars to see these processes in action, but have to resort to more indirect methods," says Dr. Jyri Lehtinen from the Max Planck Institute for Solar System Research (MPS) in Germany, first author of the new paper published today in Nature Astronomy. In their current study, the researchers compared different stars' activity levels on the one hand, and their rotational and convective properties on the other. The goal was to determine, which properties have a strong influence on activity. This can help to understand the specifics of the dynamo process within.

Several models of the stellar dynamo have been proposed in the past, but two main paradigms prevail. While one of them puts a greater emphasis on the rotation and assumes only subtle effects of convectional flows, the other depends crucially on turbulent convection. In this type of convection, the hot stellar plasma does not rise to the surface in large-scale, sedate motions. Rather, small-scale vigorous flows dominate.

In order to find evidence for one or the other of the two paradigms, Lehtinen and his colleagues for the first time took a look at 224 very different stars. Their sample contained both main sequence stars, which are so to say in the prime of their life, and older, more evolved giant stars. Typically, both convection and rotational properties of stars change as they age. Compared to main sequence stars, evolved stars exhibit a thicker convection zone often expanding over much of the star's diameter and sometimes superseding the radiation zone completely. This leads to longer turnover times for convective heat transport. At the same time, rotation usually slows down.

For their study, the researchers analyzed a data set obtained at Mount Wilson Observatory in California (USA), which over several years recorded the stars' emissions in wavelengths typical of calcium ions found in the stellar plasma. These emissions are not only correlated with the stars' activity level. Complex data processing also made it possible to infer the stars' rotation periods.

Like the Sun, stars are sometimes dappled with regions of extremely high magnetic field strength, so-called active regions, which are often associated with dark spots on the stars' visible surface. "As a star rotates, these regions come into view and pass out of it leading to a periodic rise and fall in emission brightness," Prof. Dr. Maarit Käpylä from Aalto University in Finland, who also heads the research group "Solar and Stellar Dynamos" at MPS, explains. However, since stellar emissions can also fluctuate due to other effects, identifying periodic variations -- especially over long periods -- is tricky.

"Some of the stars we studied show rotation periods of several hundreds of days, and surprisingly still a magnetic activity level similar to the other stars, and remarkably even magnetic cycles like the Sun," says Dr. Nigul Olspert from MPS, who analyzed the data. The Sun, in comparison, rotates rather briskly with a rotation period of only approximately 25 days at the solar equator. The convective turnover times were calculated by means of stellar structure modelling taking into account each star's mass, chemical composition, and evolutionary stage.

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A stripped helium star solves the massive black hole mystery

Stellar black holes form when massive stars end their life in a dramatic collapse. Observations have shown that stellar black holes typically have masses of about ten times that of the Sun, in accordance with the theory of stellar evolution. Recently, a Chinese team of astronomers claimed to have discovered a black hole as massive as 70 solar masses, which, if confirmed, would severely challenge the current view of stellar evolution. The publication immediately triggered theoretical investigations as well as additional observations by other astrophysicists. Among those to take a closer look at the object was a team of astronomers from the Universities of Erlangen-Nürnberg and Potsdam. They discovered that it may not necessarily be a black hole at all, but possibly a massive neutron star or even an 'ordinary' star.

The putative black hole was detected indirectly from the motion of a bright companion star, orbiting an invisible compact object over a period of about 80 days. From new observations, a Belgian team showed that the original measurements were misinterpreted and that the mass of the black hole is, in fact, very uncertain. The most important question, namely how the observed binary system was created, remains unanswered. A crucial aspect is the mass of the visible companion, the hot star LS V+22 25. The more massive this star is, the more massive the black hole has to be to induce the observed motion of the bright star. The latter was considered to be a normal star, eight times more massive than the Sun.

A team of astronomers from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and the University of Potsdam had a closer look at the archival spectrum of LS V+22 25, taken by the Keck telescope at Mauna Kea, Hawaii. In particular, they were interested in studying the abundances of the chemical elements on the stellar surface. Interestingly, they detected deviations in the abundances of helium, carbon, nitrogen, and oxygen compared to the standard composition of a young massive star. The observed pattern on the surface showed ashes resulting from the nuclear fusion of hydrogen, a process that only happens deep in the core of young stars and would not be expected to be detected at its surface.

'At first glance, the spectrum did indeed look like one from a young massive star. However, several properties appeared rather suspicious. This motivated us to have a fresh look at the archival data', said Andreas Irrgang, the leading scientist of this study and a member of the Dr. Karl Remeis-Observatory in Bamberg, the Astronomical Institute of FAU.

The authors concluded that LS V+22 25 must have interacted with its compact companion in the past. During this episode of mass-transfer, the outer layers of the star were removed and now the stripped helium core is visible, enriched with the ashes from the burning of hydrogen.

However, stripped helium stars are much lighter than their normal counterparts. Combining their results with recent distance measurements from the Gaia space telescope, the authors determined a most likely stellar mass of only 1.1 (with an uncertainty of +/-0.5) times that of the Sun. This yields a minimum mass of only 2-3 solar masses for the compact companion, suggesting that it may not necessarily be a black hole at all, but possibly a massive neutron star or even an 'ordinary' star.

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Veiled supernovae provide clue to stellar evolution

This is an artist's impression of a red supergiant surrounded with thick circumstellar matter.

At the end of its life, a red supergiant star explodes in a hydrogen-rich supernova. By comparing observation results to simulation models, an international research team found that in many cases this explosion takes place inside a thick cloud of circumstellar matter shrouding the star. This result completely changes our understanding of the last stage of stellar evolution.

The research team led by Francisco Förster at the University of Chile used the Blanco Telescope to find 26 supernovae coming from red supergiants. Their goal was to study the shock breakout, a brief flash of light preceding the main supernova explosion. But they could not find any signs of this phenomenon. On the other hand, 24 of the supernovae brightened faster than expected.

To solve this mystery, Takashi Moriya at the National Astronomical Observatory of Japan (NAOJ) simulated 518 models of supernovae brightness variations and compared them with the observational results. The team found that models with a layer of circumstellar matter about 10 percent the mass of the Sun surrounding the supernovae matched the observations well. This circumstellar matter hides the shock breakout, trapping its light. The subsequent collision between the supernova ejecta and the circumstellar matter creates a strong shock wave that produces extra light, causing it to brighten more quickly.

Moriya explains, "Near the end of its life, some mechanism in the star's interior must cause it to shed mass that then forms a layer around the star. We don't yet have a clear idea of the mechanism causing this mass loss. Further study is needed to get a better understanding of the mass loss mechanism. This will also be important in revealing the supernova explosion mechanism and the origin of the diversity in supernovae."

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