Distance from Brightest Stars Is Key to Preserving Primordial Discs

The NASA/ESA Hubble Space Telescope was used to conduct a three-year study of the crowded, massive and young star cluster Westerlund 2. The research found that the material encircling stars near the cluster's centre is mysteriously devoid of the large, dense clouds of dust that would be expected to become planets in a few million years. Their absence is caused by the cluster's most massive and brightest stars that erode and disperse the discs of gas and dust of neighbouring stars. This is the first time that astronomers have analysed an extremely dense star cluster to study which environments are favourable to planet formation. This time-domain study from 2016 to 2019 sought to investigate the properties of stars during their early evolutionary phases and to trace the evolution of their circumstellar environments. Such studies had previously been confined to the nearest, low-density, star-forming regions. Astronomers have now used the Hubble Space Telescope to extend this research to the centre of one of the few young massive clusters in the Milky Way, Westerlund 2, for the first time. Astronomers have now found that planets have a tough time forming in this central region of the cluster. The observations also reveal that stars on the cluster's periphery do have immense planet-forming dust clouds embedded in their discs. To explain why some stars in Westerlund 2 have a difficult time forming planets while others do not, researchers suggest this is largely due to location.

The most massive and brightest stars in the cluster congregate in the core. Westerlund 2 contains at least 37 extremely massive stars, some weighing up to 100 solar masses. Their blistering ultraviolet radiation and hurricane-like stellar winds act like blowtorches and erode the discs around neighbouring stars, dispersing the giant dust clouds.

"Basically, if you have monster stars, their energy is going to alter the properties of the discs," explained lead researcher Elena Sabbi, of the Space Telescope Science Institute in Baltimore, USA.

"You may still have a disc, but the stars change the composition of the dust in the discs, 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."

Westerlund 2 is a unique laboratory in which to study stellar evolutionary processes because it's relatively nearby, is quite young, and contains a rich stellar population. The cluster resides in a stellar breeding ground known as Gum 29, located roughly 14,000 light-years away in the constellation of Carina (The Ship's Keel).

The stellar nursery is difficult to observe because it is surrounded by dust, but Hubble's Wide Field Camera 3 can peer through the dusty veil in near-infrared light, giving astronomers a clear view of the cluster. Hubble's sharp vision was used to resolve and study the dense concentration of stars in the central cluster.

"With an age of less than about two million years, Westerlund 2 harbours some of the most massive, and hottest, young stars in the Milky Way," said team member Danny Lennon of the Instituto de Astrofisica de Canarias and the Universidad de La Laguna.

"The ambient environment of this cluster is therefore constantly bombarded by strong stellar winds and ultraviolet radiation from these giants that have masses of up to 100 times that of the Sun."

Sabbi and her team found that of the nearly 5,000 stars in Westerlund 2 with masses between 0.1 and 5 times the Sun's mass, 1,500 of them show dramatic fluctuations in their luminosity, which is commonly accepted as being due to the presence of large dusty structures and planetesimals.

Orbiting material would temporarily block some of the starlight, causing fluctuations in brightness. However, Hubble only detected the signature of dust particles around stars outside the central region. They did not detect these dips in brightness in stars residing within four light-years of the centre.

"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 centre."

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.

"Westerlund 2 gives us much better statistics on how mass affects the evolution of stars, how rapidly they evolve, and we see the evolution of stellar discs and the importance of stellar feedback in modifying the properties of these systems," said Sabbi. "We can use all of this information to inform models of planet formation and stellar evolution."

This cluster will also be an excellent target for follow-up observations with the upcoming NASA/ESA/CSA James Webb Space Telescope, an infrared observatory. Hubble has helped astronomers identify the stars that have possible planetary structures. With the Webb telescope, researchers will be able to study which discs around stars are not accreting material and which discs still have material that could build up into planets. Webb will also study the chemistry of the discs in different evolutionary phases and watch how they change, to help astronomers determine what role the environment plays in their evolution.

"A major conclusion of this work is that the powerful ultraviolet radiation of massive stars alters the discs around neighbouring stars," said Lennon. "If this is confirmed with measurements by the James Webb Space Telescope, this result may also explain why planetary systems are rare in old massive globular clusters."

Some planetary systems just aren't into heavy metal

Small planetary systems with multiple planets are not fans of heavy metal - think iron, not Iron Maiden - according to a new Yale University study. Researchers at Yale and the Flatiron Institute have discovered that compact, multiple-planet systems are more likely to form around stars that have lower amounts of heavy elements than our own Sun. This runs counter to a good deal of current research, which has focused on stars with higher metallicity. The research team looked at 700 stars and their surrounding planets for the study, which appears in The Astrophysical Journal Letters. The researchers considered any element heavier than helium - including iron, silicon, magnesium, and carbon - as a heavy metal. "We used iron as a proxy," said lead author John Michael Brewer, a postdoctoral researcher at Yale who works with astronomy professor Debra Fischer. "These are all elements that make up the rocks in small, rocky planets." Brewer said an abundance of compact, multi-planet systems around low-metallicity stars suggests several things. First, he said, it may indicate that there are many more of these systems than previously assumed. Until recently, research instruments have not had the necessary precision to detect smaller planets and instead focused on detecting larger planets. Now, with the advent of technology such as the Extreme Precision Spectrometer (EXPRES) developed by Fischer's team at Yale, researchers will be able to find smaller planets.

In addition, Brewer said, the new study suggests that small planetary systems may be the earliest type of planetary system, making them an ideal place to search for life on other planets. "Low-metallicity stars have been around a lot longer," Brewer said. "That's where we'll find the first planets that formed."

Fischer, who is a co-author of the study, demonstrated in 2005 that higher metallicity in stars increased the probability of forming large, Jupiter-like planets. This provided strong support to the core-accretion model for gas giant planet formation and established this as the leading mechanism for planet formation.

Understanding the formation of smaller planets has been more elusive.

"Our surprising result, that compact systems of multiple, small planets are more likely around lower metallicity stars suggests a new, important clue in understanding the most common type of planetary system in our galaxy," said co-author Songhu Wang, a 51 Pegasi b Fellow at Yale.

Another tantalizing possibility to explore, according to the researchers, is the connection between iron and silicon in the birth of planets. The new study shows a high silicon-to-iron ratio in stars with lower metallicity.

"Silicon could be the secret ingredient," Fischer said. "The ratio of silicon to iron is acting as a thermostat for planet formation. As the ratio increases, nature is dialing up the formation of small, rocky planets."

Astrophysicists measure precise rotation pattern of Sun-like stars for the first time

Sun-like stars rotate up to two and a half times faster at the equator than at higher latitudes, a finding by researchers at NYU Abu Dhabi that challenges current science on how stars rotate. Until now, little was known about the precise rotational patterns of Sun-like stars, only that the equator spins faster than at higher latitudes, similar to the Sun. Scientists at the NYU Abu Dhabi Center for Space Science used observations from NASA's Kepler mission and asteroseismology - the study of sound waves traveling inside stars - to determine with precision how Sun-like stars rotate, which no other scientific method has been able to achieve. Their study found that Sun-like stars, characterized as being like the Sun in mass and age, do indeed rotate in a similar manner as the Sun in that their equatorial regions rotate more rapidly than at mid- to high latitudes. But there's a key difference.The equator of the Sun rotates about 10 percent faster than its mid latitudes, while equators of Sun-like stars spin up to two and a half times faster than their mid latitudes. "This is very unexpected, and challenges current numerical simulations, which suggest that stars like these should not be able to sustain differential rotation of this magnitude," said Othman Benomar, research associate at the NYU Abu Dhabi Center for Space Science and lead author of the study published in Science.

"Understanding differential rotation - how fast one part of a star spins compared to the rest - is not only important for a complete understanding of how a star works, it will help us gain deeper insights about their magnetic fields," explained Katepalli Sreenivasan, principal investigator of the NYU Abu Dhabi Center for Space Science.

Magnetic fields on the Sun have been known to cause enormous solar storms that frequently disrupt orbiting space satellites and have knocked out power grids on Earth.

Scientists agree that the rotation of the Sun plays a crucial role in the generation of the solar magnetic field, but the exact details still remain a mystery, despite the Sun having been observed and studied in great detail.

Sreenivasan added, "learning more about how stars rotate and generate their own magnetic fields could help us gain further insight into the solar dynamo, the physical process that generates the Sun's magnetic field."

Magnetic waves are main force in star formation, researchers say

Magnetic waves are the main force in star formation in space, according to new research. This birth process leads to the formation of planets orbiting the sun and, ultimately, life on those planets, according Stella Offner, an assistant professor of astronomy at the University of Texas Austin. The new findings, which included using a supercomputer for models of the multitude of processes happening inside a cloud where stars are forming, was published this week in the journal Nature Astronomy. "These clouds are violent places," Stella Offner, assistant professor of astronomy, said in a press release. "It's an extreme environment with all kinds of different physics happening at once." This includes gravity and turbulence as well as radiation and winds from forming stars, which are called stellar feedback. Offner wanted to know: "Why are the motions in these clouds so violent?" The answer, according to some astronomers, are observed motions to gravitational collapse, or possibly turbulence and stellar feedback. Offner said it's virtually impossible to use telescopes to observe these clouds to find the influence of the various processes, she said.

"That's why we need computer models," Offner said.

Using the computer models, she noticed extra motions when comparing clouds with gravity, magnetic fields and stars.

Stellar winds interacting with the cloud magnetic field generated energy and influenced gas at great distances across the cloud more than previously thought.

"Think of the magnetic fields like rubber bands that stretch across the cloud," Offner said. "The winds push the field -- it's like rubber bands being plucked. The waves outrun the wind and cause distant motions."

This study focused on one area within star-forming clouds but Offner said she plans to study this process on larger scales in time and space.

Falling stars hold clue for understanding dying stars

An international team of researchers has proposed a new method to investigate the inner workings of supernovae explosions. This new method uses meteorites and is unique in that it can determine the contribution from electron anti-neutrinos, enigmatic particles which can't be tracked through other means. Supernovae are important events in the evolution of stars and galaxies, but the details of how the explosions occur are still unknown. This research, led by Takehito Hayakawa, a visiting professor at the National Astronomical Observatory of Japan, found a method to investigate the role of electron anti-neutrinos in supernovae. By measuring the amount of 98Ru (an isotope of Ruthenium) in meteorites, it should be possible to estimate how much of its progenitor 98Tc (a short-lived isotope of Technetium) was present in the material from which the Solar System formed. The amount of 98Tc in turn is sensitive to the characteristics, such as temperature, of electron anti-neutrinos in the supernova process; as well as to how much time passed between the supernova and the formation of the Solar System. The expected traces of 98Tc are only a little below the smallest currently detectable levels, raising hopes that they will be measured in the near future. Hayakawa explains, "There are six neutrino species. Previous studies have shown that neutrino-isotopes are predominantly produced by the five neutrino species other than the electron anti-neutrino. By finding a neutrino-isotope synthesized predominantly by the electron anti-neutrino, we can estimate the temperatures of all six neutrino species, which are important for understanding the supernova explosion mechanism."

At the end of its life, a massive star dies in a fiery explosion known as a supernova. This explosion blasts most of the mass in the star out into outer space. That mass is then recycled into new stars and planets, leaving distinct chemical signatures which tell scientists about the supernova.

Meteorites, sometimes called falling stars, formed from material left over from the birth of the Solar System, thus preserving the original chemical signatures.

Quantum vacuum may allow stars to exist in unconventional configurations

A new kind of star comes up from a study by SISSA's postdoctoral researcher Raul Carballo-Rubio. In a piece of research recently published in Physical Review Letters, Carballo-Rubio has developed a novel mathematical model that combines general relativity with the repulsive effect of quantum vacuum polarization. The inclusion of this repulsive force allows describing ultracompact configurations of stars, which were previously considered by scientists not to exist in equilibrium. "As a consequence of the attractive and repulsive forces at play, a massive star can either become a neutron star, or turn into a black hole" says Carballo-Rubio. In neutron stars, stellar equilibrium is the result of the "fight" between gravity, which is an attractive force, and a repulsive force called degeneracy pressure, of quantum mechanical origin. "But if the star's mass becomes higher than a certain threshold, about 3 times the solar mass, the equilibrium would be broken and the star collapses due to the overwhelming pull of the gravitational force".

In this study, the researcher has investigated the possibility that additional quantum mechanical forces that are largely expected to be present in nature, permit new equilibrium configurations for stars above this threshold. The additional force that has been taken into account is a manifestation of the effect known as "quantum vacuum polarization", which is a robust consequence of mixing gravity and quantum mechanics in a semiclassical framework.

"The novelty in this analysis is that, for the first time, all these ingredients have been assembled together in a fully consistent model. Moreover, it has been shown that there exist new stellar configurations, and that these can be described in a surprisingly simple manner".

There are still several important issues that remain to be studied, including the observational applications of these results. "It is not clear yet whether these configurations can be dynamically realized in astrophysical scenarios, or how long would they last if this is the case".

From an observational perspective, these "semiclassical relativistic stars" would be very similar to black holes. However, even minute differences would be perceptible in the next generation of gravitational wave observatories: "If there are very dense and ultracompact stars in the Universe, similar to black holes but with no horizons, it should be possible to detect them in the next decades".

The Most Powerful Magnets in the Universe Are Collapsed Stars

When a large star dies, sometimes it becomes a neutron star, a tiny, 12 mile across ball that's made almost entirely out of neutrons. These dead stars are incredibly dense, and spin incredibly fast. Just one thimbleful of neutron star would weigh 100 million tons. Magnetars are a variation of neutron stars, and they somehow manage to be even scarier. Neutron stars already have extremely strong magnetic fields--about 2 trillion times more powerful than Earth's. Yet magnetars have magnetic fields 1,000 times stronger than that. Yeah, that's a pretty intense field. Magnetars are not just insanely powerful--they're also very, very dangerous. If you were a mere 1,000 kilometers from a magnetar, your entire body would dissolve as the magnetic field rearranged the sequence of atoms in your body. 

In addition to their terrifying magnetic powers, magnetars also have something called starquakes, which function similarly to earthquakes here on Earth--except with much more intense results. A crack in the crust of a magnetar is responsible for the brightest light we've ever observed from space. And if a magnetar was closer to us, like 10 light years away, and blasted us with the radiation from a starquake, it would destroy our ozone layer and probably kill all life on Earth.

But don't worry--thankfully, there aren't any magnetars near Earth. The closest one is about 9,000 light years away. Let's pray that it stays that way.