Another intermediate-mass black hole discovered at the center of our galaxy

While researching a cluster of stars in the immediate vicinity of the supermassive black hole SgrA* (Sagittarius A*) at the centre of our galaxy, an international team of researchers led by PD Dr Florian Peißker has found signs of another, intermediate-mass black hole. Despite enormous research efforts, only about ten of these intermediate-mass black holes have been found in our entire universe so far. Scientists believe that they formed shortly after the Big Bang. By merging, they act as 'seeds' for supermassive black holes. The study 'The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. II. Kinematic structure' was published in The Astrophysical Journal.

The analysed star cluster IRS 13 is located 0.1 light years from the centre of our galaxy.

This is very close in astronomical terms, but would still require travelling from one end of our solar system to the other twenty times to cover the distance.

The researchers noticed that the stars in IRS 13 move in an unexpectedly orderly pattern.

They had actually expected the stars to be arranged randomly.

Two conclusions can be drawn from this regular pattern: On the one hand, IRS 13 appears to interact with SgrA*, which leads to the orderly motion of the stars.

On the other hand, there must be something inside the cluster for it to be able to maintain its observed compact shape.

Multi-wavelength observations with the Very Large Telescope as well as the ALMA and Chandra telescopes now suggest that the reason for the compact shape of IRS 13 could be an intermediate-mass black hole located at the centre of the star cluster.

This would be supported by the fact that the researchers were able to observe characteristic X-rays and ionized gas rotating at a speed of several 100 km/s in a ring around the suspected location of the intermediate-mass black hole.

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Study hints at the existence of the closest black holes to Earth in the Hyades star cluster

A paper published in the journal Monthly Notices of the Royal Astronomical Society hints at the existence of several black holes in the Hyades cluster -- the closest open cluster to our solar system -- which would make them the closest black holes to Earth ever detected. The study results from a collaboration between a group of scientists led by Stefano Torniamenti, from the University of Padua (Italy), with the significant participation of with Mark Gieles, ICREA professor at the Faculty of Physics, the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC), and Friedrich Anders (ICCUB-IEEC).

Specifically, the finding took place during a research stay of the expert Stefano Torniamenti at the ICCUB, one of the research units that make up the IEEC.

Black holes in the Hyades star cluster?

Since their discovery, black holes have been one of the most mysterious and fascinating phenomena in the Universe and have become the object of study for researchers all over the world. This is particularly true for small black holes because they have been observed during the detection of gravitational waves. Since the detection of the first gravitational waves in 2015, experts have observed many events that correspond to mergers of low-mass black hole pairs.

For the published study, the team of astrophysicists used simulations that track the motion and evolution of all the stars in the Hyades -- located at a distance from the Sun of about 45 parsecs or 150 light-years -- to reproduce their current state.

Open clusters are loosely bound groups of hundreds of stars that share certain properties such as age and chemical characteristics. The simulation results were compared with the actual positions and velocities of the stars in the Hyades, which are now known precisely from observations made by the European Space Agency's (ESA) Gaia satellite.

"Our simulations can only simultaneously match the mass and size of the Hyades if some black holes are present at the centre of the cluster today (or until recently)," says Stefano Torniamenti, postdoctoral researcher at the University of Padua and first author of the paper.

The observed properties of the Hyades are best reproduced by simulations with two or three black holes at present, although simulations where all the black holes have been ejected (less than 150 million years ago, roughly the last quarter of the cluster's age) can still give a good match, because the evolution of the cluster could not erase the traces of its previous black hole population.

The new results indicate that the Hyades-born black holes are still inside the cluster, or very close to the cluster. This makes them the closest black holes to the Sun, much closer than the previous candidate (namely the black hole Gaia BH1, which is 480 parsecs from the Sun).

In recent years, the breakthrough of the Gaia space telescope has made it possible for the first time to study the position and velocity of open cluster stars in detail and to identify individual stars with confidence.

"This observation helps us understand how the presence of black holes affects the evolution of star clusters and how star clusters in turn contribute to gravitational wave sources," says Mark Gieles, a member of the UB Department of Quantum Physics and Astrophysics and host of the first author in Barcelona. "These results also give us insight into how these mysterious objects are distributed across the galaxy."

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Celestial monsters at the origin of globular clusters

Globular clusters are the most massive and oldest star clusters in the Universe. They can contain up to 1 million of them. The chemical composition of these stars, born at the same time, shows anomalies that are not found in any other population of stars. Explaining this specificity is one of the great challenges of astronomy. After having imagined that supermassive stars could be at the origin, a team from the Universities of Geneva and Barcelona, and the Institut d'Astrophysique de Paris (CNRS and Sorbonne University) believes it has discovered the first chemical trace attesting to their presence in globular proto-clusters, born about 440 million years after the Big Bang. These results, obtained thanks to observations by the James-Webb space telescope, are to be found in Astronomy and Astrophysics.

Globular clusters are very dense groupings of stars distributed in a sphere, with a radius varying from a dozen to a hundred light years. They can contain up to 1 million stars and are found in all types of galaxies. Ours is home to about 180 of them. One of their great mysteries is the composition of their stars: why is it so varied? For instance, the proportion of oxygen, nitrogen, sodium and aluminium varies from one star to another. However, they were all born at the same time, within the same cloud of gas. Astrophysicists speak of ''abundance anomalies''.

Monsters with very short lives

A team from the universities of Geneva (UNIGE) and Barcelona, and the Institut d'Astrophysique de Paris (CNRS and Sorbonne University) has made a new advance in the explanation of this phenomenon. In 2018, it had developed a theoretical model according to which supermassive stars would have "polluted" the original gas cloud during the formation of these clusters, enriching their stars with chemical elements in a heterogeneous manner. ''Today, thanks to the data collected by the James-Webb Space Telescope, we believe we have found a first clue of the presence of these extraordinary stars,'' explains Corinne Charbonnel, a full professor in the Department of Astronomy at the UNIGE Faculty of Science, and first author of the study.

These celestial monsters are 5,000 to 10,000 times more massive and five times hotter at their centre (75 million °C) than the Sun. But proving their existence is complex. ''Globular clusters are between 10 and 13 billion years old, whereas the maximum lifespan of superstars is two million years. They therefore disappeared very early from the clusters that are currently observable. Only indirect traces remain,'' explains Mark Gieles, ICREA professor at the University of Barcelona and co-author of the study.

Revealed by light

Thanks to the very powerful infrared vision of the James-Webb telescope, the co-authors were able to support their hypothesis. The satellite captured the light emitted by one of the most distant and youngest galaxies known to date in our Universe. Located at about 13.3 billion light-years, GN-z11 is only a few tens of millions of years old. In astronomy, the analysis of the light spectrum of cosmic objects is a key element in determining their characteristics. Here, the light emitted by this galaxy has provided two valuable pieces of information.

''It has been established that it contains very high proportions of nitrogen and a very high density of stars,'' says Daniel Schaerer, associate professor in the Department of Astronomy at the UNIGE Faculty of Science, and co-author of the study. This suggests that several globular clusters are forming in this galaxy and that they still harbour an active supermassive star. ''The strong presence of nitrogen can only be explained by the combustion of hydrogen at extremely high temperatures, which only the core of supermassive stars can reach, as shown by the models of Laura Ramirez-Galeano, a Master's student in our team,'' explains Corinne Charbonnel.

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NASA's Webb uncovers star formation in cluster's dusty ribbons

NGC 346, one of the most dynamic star-forming regions in nearby galaxies, is full of mystery. Now, it is less mysterious with new findings from NASA's James Webb Space Telescope.

NCG 346 is located in the Small Magellanic Cloud (SMC), a dwarf galaxy close to our Milky Way. The SMC contains lower concentrations of elements heavier than hydrogen or helium, which astronomers call metals, compared to the Milky Way. Since dust grains in space are composed mostly of metals, scientists expected there would be low amounts of dust, and that it would be hard to detect. New data from Webb reveals the opposite.

Astronomers probed this region because the conditions and amount of metals within the SMC resemble those seen in galaxies billions of years ago, during an era in the universe known as "cosmic noon," when star formation was at its peak. Some 2 to 3 billion years after the big bang, galaxies were forming stars at a furious rate. The fireworks of star formation happening then still shape the galaxies we see around us today.

"A galaxy during cosmic noon wouldn't have one NGC 346 like the Small Magellanic Cloud does; it would have thousands" of star-forming regions like this one, said Margaret Meixner, an astronomer at the Universities Space Research Association and principal investigator of the research team. "But even if NGC 346 is now the one and only massive cluster furiously forming stars in its galaxy, it offers us a great opportunity to probe conditions that were in place at cosmic noon."

By observing protostars still in the process of forming, researchers can learn if the star formation process in the SMC is different from what we observe in our own Milky Way. Previous infrared studies of NGC 346 have focused on protostars heavier than about 5 to 8 times the mass of our Sun. "With Webb, we can probe down to lighter-weight protostars, as small as one tenth of our Sun, to see if their formation process is affected by the lower metal content," said Olivia Jones of the United Kingdom Astronomy Technology Centre, Royal Observatory Edinburgh, a co-investigator on the program.

As stars form, they gather gas and dust, which can look like ribbons in Webb imagery, from the surrounding molecular cloud. The material collects into an accretion disk that feeds the central protostar. Astronomers have detected gas around protostars within NGC 346, but Webb's near-infrared observations mark the first time they have also detected dust in these disks.

"We're seeing the building blocks, not only of stars, but also potentially of planets," said Guido De Marchi of the European Space Agency, a co-investigator on the research team. "And since the Small Magellanic Cloud has a similar environment to galaxies during cosmic noon, it's possible that rocky planets could have formed earlier in the universe than we might have thought."

The team also has spectroscopic observations from Webb's NIRSpec instrument that they are continuing to analyze. These data are expected to provide new insights into the material accreting onto individual protostars, as well as the environment immediately surrounding the protostar.

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Astronomy: Observation puzzles researchers

An international team of astrophysicists has made a puzzling discovery while analyzing certain star clusters. The University of Bonn played a major role in the study. The finding challenges Newton's laws of gravity, the researchers write in their publication. Instead, the observations are consistent with the predictions of an alternative theory of gravity. However, this is controversial among experts. The results have now been published in the Monthly Notices of the Royal Astronomical Society.

In their work, the researchers investigated the so-called open star clusters. These are formed when thousands of stars are born within a short time in a huge gas cloud. As they "ignite," the galactic newcomers blow away the remnants of the gas cloud. In the process, the cluster expands considerably. This creates a loose formation of several dozen to several thousand stars. The weak gravitational forces acting between them hold the cluster together.

"In most cases, open star clusters survive only a few hundred million years before they dissolve," explains Prof. Dr. Pavel Kroupa of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. In the process, they regularly lose stars, which accumulate in two so-called "tidal tails." One of these tails is pulled behind the cluster as it travels through space. The other, in contrast, takes the lead like a spearhead.

"According to Newton's laws of gravity, it's a matter of chance in which of the tails a lost star ends up," explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute of Radiation and Nuclear Physics. "So both tails should contain about the same number of stars. However, in our work we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars nearby to the cluster than the rear tail."

New method developed for counting stars

Until now, it has been almost impossible to determine from among the millions of stars close to a cluster those that belong to its tails. "To do this, you have to look at the velocity, direction of motion and age of each of these objects," explains Dr. Tereza Jerabkova. The co-author of the paper, who did her doctorate in Kroupa's group, recently moved from the European Space Agency (ESA) to the European Southern Observatory in Garching. She developed a method that allowed her to accurately count the stars in the tails for the first time. "So far, five open clusters have been investigated near us, including four by us," she says. "When we analyzed all the data, we encountered the contradiction with the current theory. The very precise survey data from ESA's Gaia space mission were indispensable for this."

The observational data, in contrast, fit much better with a theory that goes by the acronym MOND ("MOdified Newtonian Dynamics") among experts. "Put simply, according to MOND, stars can leave a cluster through two different doors," Kroupa explains. "One leads to the rear tidal tail, the other to the front. However, the first is much narrower than the second -- so it's less likely that a star will leave the cluster through it. Newton's theory of gravity, on the other hand, predicts that both doors should be the same width."

Star clusters are shorter-lived than Newton's laws predict

The team calculated the stellar distribution expected according to MOND. "The results correspond surprisingly well with the observations," highlights Dr. Ingo Thies, who played a key role in the corresponding simulations. "However, we had to resort to relatively simple computational methods to do this. We currently lack the mathematical tools for more detailed analyses of modified Newtonian dynamics." Nevertheless, the simulations also coincided with the observations in another respect: They predicted how long open star clusters should typically survive. And this time span is significantly shorter than would be expected according to Newton's laws. "This explains a mystery that has been known for a long time," Kroupa points out. "Namely, star clusters in nearby galaxies seem to be disappearing faster than they should."

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Webb reveals a galaxy sparkling with the universe's oldest star clusters

Using the James Webb Space Telescope (JWST), researchers from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS) team have identified the most distant globular clusters ever discovered. These dense groups of millions of stars may be relics that contain the first and oldest stars in the universe.

The early analysis of Webb's First Deep Field image, which depicts some of the universe's earliest galaxies, is published today in The Astrophysical Journal Letters.

"JWST was built to find the first stars and the first galaxies and to help us understand the origins of complexity in the universe, such as the chemical elements and the building blocks of life," says Lamiya Mowla, Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto and co-lead author of the study. "This discovery in Webb's First Deep Field is already providing a detailed look at the earliest phase of star formation, confirming the incredible power of JWST."

In the finely detailed Webb's First Deep Field image, the researchers zeroed in on what they've dubbed "the Sparkler galaxy," which is nine billion light years away. This galaxy got its name for the compact objects appearing as small yellow-red dots surrounding it, referred to by the researchers as "sparkles." The team posited that these sparkles could either be young clusters actively forming stars -- born three billion years after the Big Bang at the peak of star formation -- or old globular clusters. Globular clusters are ancient collections of stars from a galaxy's infancy and contain clues about its earliest phases of formation and growth.

From their initial analysis of 12 of these compact objects, the researchers determined that five of them are not only globular clusters but among the oldest ones known.

"Looking at the first images from JWST and discovering old globular clusters around distant galaxies was an incredible moment, one that wasn't possible with previous Hubble Space Telescope imaging," says Kartheik G. Iyer, Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto and co-lead author of the study. "Since we could observe the sparkles across a range of wavelengths, we could model them and better understand their physical properties, like how old they are and how many stars they contain. We hope the knowledge that globular clusters can be observed at from such great distances with JWST will spur further science and searches for similar objects."

The Milky Way galaxy has about 150 globular clusters, and how and when exactly these dense clumps of stars formed is not well understood. Astronomers know that globular clusters can be extremely old, but it is incredibly challenging to measure their ages. Using very distant globular clusters to age-date the first stars in distant galaxies has not been done before and is only possible with JWST.

"These newly identified clusters were formed close to the first time it was even possible to form stars," says Mowla. "Because the Sparkler galaxy is much farther away than our own Milky Way, it is easier to determine the ages of its globular clusters. We are observing the Sparkler as it was nine billion years ago, when the universe was only four-and-a-half billion years old, looking at something that happened a long time ago. Think of it as guessing a person's age based on their appearance -- it's easy to tell the difference between a 5- and 10-year-old, but hard to tell the difference between a 50- and 55-year-old."

Until now, astronomers could not see the surrounding compact objects of the Sparkler galaxy with the Hubble Space Telescope (HST). This changed with JWST's increased resolution and sensitivity, unveiling the tiny dots surrounding the galaxy for the first time in Webb's First Deep Field image. The Sparkler galaxy is special because it is magnified by a factor of 100 due to an effect called gravitational lensing -- where the SMACS 0723 galaxy cluster in the foreground distorts what is behind it, much like a giant magnifying glass. Moreover, gravitational lensing produces three separate images of the Sparkler, allowing astronomers to study the galaxy in greater detail.

"Our study of the Sparkler highlights the tremendous power in combining the unique capabilities of JWST with the natural magnification afforded by gravitational lensing," says CANUCS team lead Chris Willott from the National Research Council's Herzberg Astronomy and Astrophysics Research Centre. "The team is excited about more discoveries to come when JWST turns its eye on the CANUCS galaxy clusters next month."

The researchers combined new data from JWST's Near-Infrared Camera (NIRCam) with HST archival data. NIRCam detects faint objects using longer and redder wavelengths to observe past what is visible to the human eye and even HST. Both magnifications due to the lensing by the galaxy cluster and the high resolution of JWST are what made observing compact objects possible.

The Canadian-made Near-Infrared Imager and Slitless Spectrograph (NIRISS) instrument on the JWST provided independent confirmationthat the objects are old globular clusters because the researchers did not observe oxygen emission lines -- emissions with measurable spectra given off by young clusters that are actively forming stars. NIRISS also helped unravel the geometry of the triply lensed images of the Sparkler.

"JWST's made-in-Canada NIRISS instrument was vital in helping us understand how the three images of the Sparkler and its globular clusters are connected," says Marcin Sawicki, Canada Research Chair in Astronomy, professor at Saint Mary's University and study co-author. "Seeing several of the Sparkler's globular clusters imaged three times made it clear that they are orbiting around the Sparkler galaxy rather than being simply in front of it by chance."

JWST will observe the CANUCS fields starting in October 2022, leveraging JWST data to examine five massive clusters of galaxies, around which the researchers expect to find more such systems. Future studies will also model the galaxy cluster to understand the lensing effect and execute more robust analyses to explain the star formation histories.

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Hydrogen-burning white dwarfs enjoy slow aging

The prevalent view of white dwarfs as inert, slowly cooling stars has been challenged by observations from the NASA/ESA Hubble Space Telescope. An international group of astronomers have discovered the first evidence that white dwarfs can slow down their rate of ageing by burning hydrogen on their surface.

"We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity," explained Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research. "This was quite a surprise, as it is at odds with what is commonly believed."

White dwarfs are the slowly cooling stars which have cast off their outer layers during the last stages of their lives. They are common objects in the cosmos; roughly 98% of all the stars in the Universe will ultimately end up as white dwarfs, including our own Sun. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well.

To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity but the populations of stars which will eventually give rise to white dwarfs are different. In particular, the overall colour of stars at an evolutionary stage known as the Horizontal Branch are bluer in M13, indicating a population of hotter stars. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool.

"The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters," continued Chen. "This allowed us to really contrast how stars evolve in M3 and M13."

Using Hubble's Wide Field Camera 3 the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs in the two clusters. They found that M3 contains standard white dwarfs which are simply cooling stellar cores. M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those which have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly.

Comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that roughly 70% of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling.

This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way. The evolution of white dwarfs has previously been modelled as a predictable cooling process. This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters. However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as 1 billion years.

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Astronomers discover an oversized black hole population in the star cluster Palomar 5

Palomar 5 is a unique star cluster. This is firstly because it is one of the "fluffiest" clusters in the halo of our Galaxy, with the average distance between the stars being a few light-years, comparable to the distance from the Sun to the nearest star. Secondly, it has a specular stellar stream associated with it that spans more than 20 degrees across the sky. In a paper published today in Nature Astronomy, an international team of astronomers and astrophysicists led by the University of Barcelona show that both distinguishing features of Palomar 5 are likely the result of an oversized black hole population of more than 100 black holes in the center of the cluster.

"The number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20% of the total cluster mass is made up of black holes. They each have a mass of about 20 times the mass of the Sun, and they formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young" says Prof Mark Gieles, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and lead author of the paper.

Tidal streams are streams of stars that were ejected from disrupting star clusters or dwarf galaxies. In the last few years, nearly thirty thin streams have been discovered in the Milky Way halo. "We do not know how these streams form, but one idea is that they are disrupted star clusters. However, none of the recently discovered streams have a star cluster associated with them, hence we can not be sure. So, to understand how these streams formed, we need to study one with a stellar system associated with it. Palomar 5 is the only case, making it a Rosetta Stone for understanding stream formation and that is why we studied it in detail" explains Gieles.

The authors simulate the orbits and the evolution of each star from the formation of the cluster until the final dissolution. They varied the initial properties of the cluster until a good match with observations of the stream and the cluster was found. The team finds that Palomar 5 formed with a lower black hole fraction, but stars escaped more efficiently than black holes, such that the black hole fraction gradually increased. The black holes dynamically puffed up the cluster in gravitational slingshot interactions with stars, which led to even more escaping stars and the formation of the stream. Just before it completely dissolves -- roughly a billion years from now -- the cluster will consist entirely of black holes. "This work has helped us understand that even though the fluffy Palomar 5 cluster has the brightest and longest tails of any cluster in the Milky Way, it is not unique. Instead, we believe that many similarly puffed up, black hole-dominated clusters have already disintegrated in the Milky Way tides to form the recently discovered thin stellar streams" says co-author Dr. Denis Erkal at the University of Surrey.

Gieles points out that in this paper "we have shown that the presence of a large black hole population may have been common in all the clusters that formed the streams." This is important for our understanding of globular cluster formation, the initial masses of stars and the evolution of massive stars. This work also has important implications for gravitational waves. "It is believed that a large fraction of binary black hole mergers form in star clusters. A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we can not see black holes. Our method gives us a way to learn how many BHs there are in a star cluster by looking at the stars they eject.'', says Dr. Fabio Antonini from Cardiff University, a co-author of the paper.

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Surprise twist suggests stars grow competitively

A survey of star formation activity in the Orion Nebula Cluster found similar mass distributions for newborn stars and dense gas cores, which may evolve into stars. Counterintuitively, this means that the amount of gas a core accretes as it develops, and not the initial mass of the core, is the key factor in deciding the final mass of the produced star.

The Universe is populated with stars of various masses. Dense cores in clouds of interstellar gas collapse under their own gravity to form stars, but what determines the final mass of the star remains an open question. There are two competing theories. In the core-collapse model, larger stars form from larger cores. In the competitive accretion model, all cores start out about the same mass but accrete different amounts of gas from the surroundings as they grow.

To distinguish between these two scenarios, a research team led by Hideaki Takemura at the National Astronomical Observatory of Japan created a map of the Orion Nebula Cluster where new stars are forming, based on data from the American CARMA interferometer and NAOJ's own Nobeyama 45-m Radio Telescope. Thanks to the unprecedent high resolution of the map, the team was able to compare the masses of the newly formed stars and gravitationally collapsing dense cores. They found that the mass distributions are similar for the two populations. They also found many smaller cores which don't have strong enough gravity to contract into stars.

One would think that similar mass distributions for prestellar cores and newborn stars would favor the core-collapse model, but actually because it is impossible for a core to impart all of its mass to a new star, this shows that continued gas inflow is an important factor, favoring the competitive accretion model.

Now the team will expand their map using additional data from CARMA and the Nobeyama 45-m Radio Telescope to see if the results from the Orion Nebula Cluster hold true for other regions.

From Science Daily

Not so fast, supernova: Highest-energy cosmic rays detected in star clusters

 For decades, researchers assumed the cosmic rays that regularly bombard Earth from the far reaches of the galaxy are born when stars go supernova -- when they grow too massive to support the fusion occurring at their cores and explode.

Those gigantic explosions do indeed propel atomic particles at the speed of light great distances. However, new research suggests even supernovae -- capable of devouring entire solar systems -- are not strong enough to imbue particles with the sustained energies needed to reach petaelectronvolts (PeVs), the amount of kinetic energy attained by very high-energy cosmic rays.

And yet cosmic rays have been observed striking Earth's atmosphere at exactly those velocities, their passage marked, for example, by the detection tanks at the High-Altitude Water Cherenkov (HAWC) observatory near Puebla, Mexico. Instead of supernovae, the researchers posit that star clusters like the Cygnus Cocoon serve as PeVatrons -- PeV accelerators -- capable of moving particles across the galaxy at such high energy rates.

Their paradigm-shifting research provides compelling evidence for star forming regions to be PeVatrons and is published in two recent papers in Nature Astronomy and Astrophysical Journal Letters.

A characteristic of physics research is how collaborative it is. The research was conducted by Petra Huentemeyer, professor of physics at Michigan Technological University, along with recent graduate Binita Hona '20, doctoral student Dezhi Huang, former MTU postdoc Henrike Fleischhack (now at Catholic University/NASA GSFC/CRESST II), Sabrina Casanova at the Institute of Nuclear Physics Polish Academy of Sciences in Krakow, Ke Fang at the University of Wisconsin and Roger Blanford at Stanford, along with numerous other collaborators of the HAWC Observatory.

Huentemeyer noted that HAWC and physicists from other institutions have measured cosmic rays from all directions and across many decades of energy. It's in tracking the cosmic rays with the highest known energy, PeVs, that their origin becomes so important.

"Cosmic rays below PeV energy are believed to come from our galaxy, but the question is what are the accelerators that can produce them," Huentemeyer said.

Fleischhack said the paradigm shift the researchers have uncovered is that before, scientists thought supernova remnants were the main accelerators of cosmic rays.

"They do accelerate cosmic rays, but they are not able to get to highest energies," she said.

So, what is driving cosmic rays' acceleration to PeV energy?

"There have been several other hints that star clusters could be part of the story," Fleischhack said. "Now we are getting confirmation that they are able to go to highest energies."

Star clusters are formed from the remnants of a supernova event. Known as star cradles, they contain violent winds and clouds of swirling debris -- such as those noted by the researchers in Cygnus OB2 and cluster [BDS2003]8. Inside, several types of massive stars known as spectral type O and type B stars are gathered by the hundreds in an area about 30 parsecs (108 light-years) across.

"Spectral type O stars are the most massive," Hona said. "When their winds interact with each other, shock waves form, which is where acceleration happens."

The researchers' theoretical models suggest that the energetic gamma-ray photons seen by HAWC are more likely produced by protons than by electrons.

"We will use NASA telescopes to search for the counterpart emission by these relativistic particles at lower energies," Fang said.

The extremely high energy at which cosmic rays reach our planet is notable. Specific conditions are required to accelerate particles to such velocities.

The higher the energy, the more difficult it is to confine the particles -- knowledge gleaned from particle accelerators here on Earth in Chicago and Switzerland. To keep particles from whizzing away, magnetism is required.

Stellar clusters -- with their mixture of wind and nascent but powerful stars -- are turbulent regions with different magnetic fields that can provide the confinement necessary for particles to continue to accelerate.

"Supernova remnants have very fast shocks where the cosmic ray can be accelerated; however, they don't have the type of long confinement regions," Casanova said. "This is what star clusters are useful for. They're an association of stars that can create disturbances that confine the cosmic rays and make it possible for the shocks to accelerate them."

But how is it possible to measure atomic interactions on a galactic scale 5,000 light-years from Earth? The researchers used 1,343 days of measurements from HAWC detection tanks.

Huang explained how the physicists at HAWC trace cosmic rays by measuring the gamma rays these cosmic rays produce at galactic acceleration sites: "We didn't measure gamma rays directly; we measured the secondary rays generated. When gamma rays interact with the atmosphere, they generate secondary particles in particle showers."

"When particle showers are detected at HAWC, we can measure the shower and the charge of secondary particles," Huang said. "We use the particle charge and time information to reconstruct information from the primary gamma."

In addition to HAWC, the researchers plan to work with the Southern Wide-field Gamma-ray Observatory (SWGO), an observatory currently in the planning stages that will feature Cherenkov light detectors like HAWC but will be located in the southern hemisphere.

"It would be interesting to see what we can see in the southern hemisphere," Huentemeyer said. "We will have a good view of the galactic center that we don't have in the northern hemisphere. SWGO could give us many more candidates in terms of star clusters."

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Most isolated massive stars are kicked out of their clusters

 A pair of University of Michigan studies reveals how some massive stars -- stars eight or more times the mass of our sun -- become isolated in the universe: most often, their star clusters kick them out.

Massive stars typically reside in clusters. Isolated massive stars are called field massive stars. The papers published by U-M students examined most of these stars in the Small Magellanic Cloud, a dwarf galaxy near the Milky Way.

The studies, appearing in the same issue of The Astrophysical Journal, reveal how these field massive stars originate, or become so isolated. Understanding how field massive stars become isolated -- whether they form in isolation or whether they become isolated by being ejected from a star cluster -- will help astronomers probe the conditions in which massive stars are formed. Understanding this and cluster formation is critical for understanding how galaxies evolve.

"About a quarter of all massive stars appear to be isolated, and that's our big question," said recent undergraduate Johnny Dorigo Jones. "How they're found to be isolated, and how they got there."

Dorigo Jones shows in his paper that the vast majority of field massive stars are "runaways," or stars ejected from clusters. Graduate student Irene Vargas-Salazar looked for field massive stars that may have formed in relative isolation by looking for evidence of tiny clusters around them. That means these relatively isolated stars could have formed in conjunction with these smaller stars. But she found very few of these faint clusters.

"Because massive stars require a lot of material to form, there are usually a lot of smaller stars around them," Vargas-Salazar said. "My project asks specifically how many of these field massive stars could have formed in the field."

Dorigo Jones examined how field massive stars are ejected from clusters. He looks at the two different mechanisms that produce runaways: dynamical ejection and binary supernova ejection. In the first, the massive stars are ejected from their clusters -- by up to half a million miles per hour -- because of unstable orbital configurations of stellar groups. In the second, a massive star is ejected when a binary pair has one star that explodes and shoots its companion out into space.

"By having the velocities and the masses of our stars, we're able to compare the distributions of those parameters to the model predictions to determine the certain contributions from each of the ejection mechanisms," Dorigo Jones said.

He found that dynamical ejections -- ejections caused by unstable orbital configurations -- were about 2 to 3 times more numerous than supernova ejections. But Dorigo Jones also found the first observational data that shows a large fraction of the field massive stars came from a combination of both dynamical and supernova ejections.

"These have been studied in the past but we have now set the first observational constraints on the numbers of these two-step runaways," he said. "The way we reach that conclusion is we're essentially seeing that the stars that trace the supernova ejections in our sample are a bit too numerous and too fast compared to the model predictions. You can imagine this being remedied by these stars being reaccelerated upon a supernova kick, having first been dynamically ejected."

The researchers found that potentially up to half of the stars first thought to be from supernova ejections were first dynamically ejected.

Vargas-Salazar's findings also support the idea that most field massive stars are runaways, but she looked at opposite conditions: she looked for field massive stars that formed in relative isolation in tiny clusters of smaller stars, where the massive target star is, called the "tip of the iceberg, or TIB clusters. She did this using two algorithms, "friends-of-friends" and "nearest neighbors," to search for those clusters around 310 field massive stars in the Small Magellanic Cloud.

The "friends-of-friends" algorithm measures the number density of stars by counting how many stars there are at a specific distance from the target star and then doing the same for those stars in turn. The more tightly packed the stars are, the more likely it is to be a cluster. The "nearest neighbors" algorithm measures the number density of stars between the target star and its nearest 20 companions. The more compact and denser the group, the more likely they are to be clusters, Vargas-Salazar said.

Using statistical tests, Vargas-Salazar compared these observations with three random-field datasets and compared the known runaway massive stars to nonrunaways. She found that only a few of the field massive stars appeared to have TIB clusters around them, suggesting that very few actually formed in the field. The balance of the field stars must have originated as runaways.

"In the end, we showed that 5% or less of the stars had TIB clusters. Instead, our findings imply that the majority of stars in field samples could be runaways," Vargas-Salazar said. "Our findings are actually supporting the result that Johnny found, wrapped in a neat little bow."

Vargas-Salazar's findings provide part of the answer to the question of how massive stars form, says Sally Oey, senior author on both of the papers and professor of astronomy at U-M.

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Star clusters are only the tip of the iceberg

 Star clusters have been part of the Imaginarium of human civilization for millennia. The brightest star clusters to Earth, like the Pleiades, are readily visible to the naked eye. A team around astronomer has now revealed the existence of massive stellar halos, termed coronae, surrounding local star clusters.

"Clusters form big families of stars that can stay together for large parts of their lifetime. Today, we know of roughly a few thousand star clusters in the Milky Way, but we only recognize them because of their prominent appearance as rich and tight groups of stars. Given enough time, stars tend to leave their cradle and find themselves surrounded by countless strangers, thereby becoming indistinguishable from their neighbours and hard to identify" says Stefan Meingast, lead author of the paper published in Astronomy & Astrophysics. "Our Sun is thought to have formed in a star cluster but has left its siblings behind a long time ago" he adds.

Thanks to the ESA Gaia spacecraft's precise measurements, astronomers at the University of Vienna have now discovered that what we call a star cluster is only the tip of the iceberg of a much larger and often distinctly elongated distribution of stars.

"Our measurements reveal the vast numbers of sibling stars surrounding the well-known cores of the star clusters for the first time. It appears that star clusters are enclosed in rich halos, or coronae, more than 10 times as large as the original cluster, reaching far beyond our previous guesses. The tight groups of stars we see in the night sky are just a part of a much larger entity" says Alena Rottensteiner, co-author and master student at the University of Vienna. "There is plenty of work ahead revising what we thought were basic properties of star clusters, and trying to understand the origin of the newfound coronae."

To find the lost star siblings, the research team developed a new method that uses machine learning to trace groups of stars which were born together and move jointly across the sky. The team analyzed 10 star clusters and identified thousands of siblings far away from the center of the compact clusters, yet clearly belonging to the same family. An explanation for the origin of these coronae remains uncertain, yet the team is confident that their findings will redefine star clusters and aid our understanding of their history and evolution across cosmic time.

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Strange dismembered star cluster found at Galaxy's edge

An international team of astronomers has discovered the remnant of an ancient collection of stars that was torn apart by our own galaxy, the Milky Way, more than two billion years ago.

The extraordinary discovery of this shredded 'globular cluster' is surprising, as the stars in this galactic archaeological find have much lower quantities of heavier elements than in other such clusters. The evidence strongly suggests the original structure was the last of its kind, a globular cluster whose birth and life were different to those remaining today.

Our Galaxy is home to about 150 globular clusters, each a ball of a million or so stars that orbit in the Galaxy's tenuous stellar halo. These globular clusters are old and have witnessed the growth of the Milky Way over billions of years.

The study, published in Nature, was led by University of Sydney PhD student, Zhen Wan, and his supervisor, Professor Geraint Lewis, as part of the S5 international collaboration.

Using the Anglo-Australian Telescope in outback New South Wales, this collaboration measured the speeds of a stream of stars in the Phoenix constellation, revealing them to be remnants of a globular cluster that was pulled apart by the gravity of the Milky Way about two billion years ago.

Mr Wan said: "Once we knew which stars belonged to the stream, we measured their abundance of elements heavier than hydrogen and helium; something astronomers refer to as metallicity. We were really surprised to find that the Phoenix Stream has a very low metallicity, making it distinctly different to all of the other globular clusters in the Galaxy.

"Even though the cluster was destroyed billions of years ago, we can still tell it formed in the early Universe from the composition of its stars."

HEAVY METALS

After the Big Bang, only hydrogen and helium existed in any substantial amount in the Universe. These elements formed the first generation of stars many billions of years ago. It is within these and later stellar generations that heavier elements were formed, such as the calcium, oxygen and phosphorus that in part make up your bones.

Observations of other globular clusters have found that their stars are enriched with heavier elements forged in earlier generations of stars. Current formation theories suggest that this dependence on previous stars means that no globular cluster should be found unenriched and that there is a minimum metallicity 'floor' below which no cluster can form.

But the metallicity of the Phoenix Stream progenitor sits well below this minimum, posing a significant problem for our ideas of globular cluster origins.

"This stream comes from a cluster that, by our understanding, shouldn't have existed," said co-author Associate Professor Daniel Zucker from Macquarie University.

S5 team leader, Dr Ting Li from Carnegie Observatories, said: "One possible explanation is that the Phoenix Stream represents the last of its kind, the remnant of a population of globular clusters that was born in radically different environments to those we see today."

While potentially numerous in the past, this population of globular clusters was steadily depleted by the gravitational forces of the Galaxy, which tore them to pieces, absorbing their stars into the main body of the galactic system. This means that the stream is a relatively temporary phenomenon, which will dissipate in time.

"We found the remains of this cluster before it faded forever into the Galaxy's halo," Mr Wan said.

As yet, there is no clear explanation for the origins of the Phoenix Stream progenitor cluster and where it sits in the evolution of galaxies remains unclear.

Professor Lewis said: "There is plenty of theoretical work left to do. There are now many new questions for us to explore about how galaxies and globular clusters form, which is incredibly exciting."

Is the Phoenix Stream unique? "In astronomy, when we find a new kind of object, it suggests that there are more of them out there," said co-author Dr Jeffrey Simpson from the University of New South Wales. While globular clusters like the progenitor of the Phoenix Stream may no longer exist, their remnants may live on as faint streams."

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