WHITE DWARF STUDY FINDS STARS AND PLANETS GROW TOGETHER
University of Cambridge
A team of astronomers has found that planet formation in our young Solar System started much earlier than previously thought, with the building blocks of planets growing at the same time as their parent star.A study of some of the oldest stars in the Universe suggests that the building blocks of planets like Jupiter and Saturn begin to form while a young star is growing. It had been thought that planets only form once a star has reached its final size, but new results suggest that stars and planets 'grow up' together. The research, changes our understanding of how planetary systems, including our own Solar System, formed, potentially solving a major puzzle in astronomy. To attempt to answer this question, astronomers studied the atmospheres of white dwarf stars -- the ancient, faint remnants of stars like our Sun -- to investigate the building blocks of planet formation. Normally, the interiors of planets are out of reach of telescopes. But a special class of white dwarfs -- known as 'polluted' systems -- have heavy elements such as magnesium, iron, and calcium in their normally clean atmospheres. These elements must have come from small bodies like asteroids left over from planet formation, which crashed into the white dwarfs and burned up in their atmospheres. As a result, spectroscopic observations of polluted white dwarfs can probe the interiors of those torn-apart asteroids, giving astronomers direct insight into the conditions in which they formed. Planet formation is believed to begin in a protoplanetary disc -- made primarily of hydrogen, helium, and tiny particles of ices and dust -- orbiting a young star. According to the current leading theory on how planets form, the dust particles stick to each other, eventually forming larger and larger solid bodies. Some of these larger bodies will continue to accrete, becoming planets, and some remain as asteroids, like those that crashed into the white dwarfs in the current study.
The researchers analysed spectroscopic observations from the atmospheres of 200 polluted white dwarfs from nearby galaxies. According to their analysis, the mixture of elements seen in the atmospheres of these white dwarfs can only be explained if many of the original asteroids had once melted, which caused heavy iron to sink to the core while the lighter elements floated on the surface. This process, known as differentiation, is what caused the Earth to have an iron-rich core. The cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the earliest stages of the planetary system but decay away in just a million years. In other words, if these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly. The study suggests that the early-formation picture is likely to be correct, meaning that Jupiter and Saturn had plenty of time to grow to their current sizes. Analyses of polluted white dwarfs tell us that this radioactive melting process is a potentially ubiquitous mechanism affecting the formation of all extrasolar planets.
HUNT FOR SECOND-CLOSEST SUPERMASSIVE BLACK HOLE
Harvard-Smithsonian Center for Astrophysics
Two astrophysicists have suggested a way to observe what could be the second-closest supermassive black hole to Earth: a behemoth 3 million times the mass of the Sun, hosted by the dwarf galaxy Leo I. The supermassive black hole, labelled Leo I*, was first proposed by an independent team of astronomers in late 2021. The team noticed stars picking up speed as they approached the centre of the galaxy -- evidence for a black hole -- but directly imaging emission from the black hole was not possible. Now, astrophysicists suggest a new way to verify the supermassive black hole's existence. Rays of light cannot escape their event horizons, but the environment around them can be extremely bright -- if enough material falls into their gravitational well. But if a black hole is not accreting mass, instead, it emits no light and becomes impossible to find with our telescopes. This is the challenge with Leo I -- a dwarf galaxy so devoid of gas available to accrete that it is often described as a "fossil." So, shall we relinquish any hope of observing it? Perhaps not, the astronomers say. It was suggested that a small amount of mass lost from stars wandering around the black hole could provide the accretion rate needed to observe it. Old stars become very big and red -- we call them red giant stars. Red giants typically have strong winds that carry a fraction of their mass to the environment. The space around Leo I* seems to contain enough of these ancient stars to make it observable.
It would be the second-closest supermassive black hole after the one at the centre of our galaxy, with a very similar mass but hosted by a galaxy that is a thousand times less massive than the Milky Way. This fact challenges everything we know about how galaxies and their central supermassive black holes co-evolve. How did such an oversized baby end up being born from a slim parent?" Decades of studies show that most massive galaxies host a supermassive black hole at their centre, and the mass of the black hole is a tenth of a percent of the total mass of the spheroid of stars surrounding it. In the case of Leo I we would expect a much smaller black hole. Instead, Leo I appears to contain a black hole a few million times the mass of the Sun, similar to that hosted by the Milky Way. This is exciting because science usually advances the most when the unexpected happens.
MIDSIZE BLACK HOLE IN DWARF GALAXY
University of California - Santa Cruz
An intermediate-mass black hole lurking undetected in a dwarf galaxy revealed itself to astronomers when a star strayed too close. The shredding of the star, known as a "tidal disruption event" or TDE, produced a flare of radiation that briefly outshone the combined stellar light of the host dwarf galaxy and could help scientists better understand the relationships between black holes and galaxies. The flare was captured by astronomers with the Young Supernova Experiment (YSE), a survey designed to detect cosmic explosions and transient astrophysical events. This discovery has created widespread excitement because we can use tidal disruption events not only to find more intermediate-mass black holes in quiet dwarf galaxies, but also to measure their masses. Supermassive black holes are found at the centres of all massive galaxies, including our own Milky Way. Astronomers conjecture that these massive beasts, with millions or billions of times the mass of the Sun, could have grown from smaller "intermediate-mass" black holes with thousands to hundreds of thousands of solar masses. One theory for how such massive black holes were assembled is that the early Universe was rampant with small dwarf galaxies with intermediate-mass black holes. Over time, these dwarf galaxies would have merged or been gobbled up by more massive galaxies, their cores combining each time to build up the mass in the centre of the growing galaxy. This merger process would eventually create the supermassive black holes seen today.
Classic black hole hunting techniques, which look for actively feeding black holes, are often not sensitive enough to uncover black holes in the centres of dwarf galaxies. As a result, only a minuscule fraction of dwarf galaxies is known to host intermediate-mass black holes. Finding more midsize black holes with tidal disruption events could help to settle the debate about how supermassive black holes form. Data from the Young Supernova Experiment enabled the team to detect the first signs of light as the black hole began to eat the star. Capturing this initial moment was pivotal to unlocking how big the black hole was, because the duration of these events can be used to measure the mass of the central black hole. This method until now had only been shown to work well for supermassive black holes. This study was based on data from observatories around the world, including the W. M. Keck Observatory in Hawaii, the Nordic Optical Telescope, UC's Lick Observatory, NASA's Hubble Space Telescope, the international Gemini Observatory, the Palomar Observatory, and the Pan-STARRS Survey at Haleakala Observatory.
BRIGHT FLASH IS A BLACK HOLE JET POINTING AT EARTH
University of Birmingham
Astronomers have determined the source of an incredibly bright X-ray, optical and radio signal appearing from halfway across the Universe. The signal, named AT 2022cmc, was discovered earlier this year by the Zwicky Transient Facility in California. Findings suggest that it is likely from a jet of matter, streaking out from a supermassive black hole at close to the speed of light. The team believe the jet is the product of a black hole that suddenly began devouring a nearby star, releasing a huge amount of energy in the process. Their findings could shed new light on how supermassive black holes feed and grow. Astronomers have observed other such "tidal disruption events," or TDEs, in which a passing star is torn apart by a black hole's tidal forces. However AT 2022cmc is brighter than any TDE discovered to date, and is also the farthest TDE ever detected, at some 8.5 billion light years away. The team measured the distance to the AT 2022cmc using the European Southern Observatory's Very Large Telescope, in Chile. The spectrum suggested that the source was hot: around 30,000 degrees, which is typical for a TDE. But we also saw some absorption of light by the galaxy where this event occurred. These absorption lines were highly shifted towards redder wavelengths, telling us that this galaxy was much further away than expected. How could such a distant event appear so bright in our sky? The team says the black hole's jet may be pointing directly toward Earth, making the signal appear brighter than if the jet were pointing in any other direction. The effect is "Doppler boosting," and is similar to the amped-up sound of a passing siren.
AT 2022cmc is the fourth Doppler-boosted TDE ever detected and the first such event that has been observed since 2011. It is also the first boosted TDE discovered using an optical sky survey. As more powerful telescopes start up in the coming years, they will reveal more TDEs, which can shed light on how supermassive black holes grow and shape the galaxies around them. Following AT 2022cmc's initial discovery, the team focused in on the signal using the Neutron star Interior Composition ExploreR (NICER), an X-ray telescope that operates aboard the International Space Station. Things looked pretty normal the first three days then the team looked at it with an X-ray telescope, and found the source was 100 times more powerful than the most powerful gamma-ray burst afterglow. Typically, such bright flashes in the sky are gamma ray bursts -- extreme jets of X-ray emissions that spew from the collapse of massive stars. Gamma-ray bursts are the usual suspects for events like this, however, as bright as they are, there is only so much light a collapsing star can produce. Because AT 2022cmc was so bright and lasted so long, we knew that something truly gargantuan must be powering it -- a supermassive black hole. The extreme X-ray activity is believed to be powered by an "extreme accretion episode" when the shredded star creates a whirlpool of debris as it falls into the black hole. Indeed, the team found that AT 2022cmc's X-ray luminosity was comparable to, though brighter than, three previously detected TDEs. It's probably swallowing the star at the rate of half the mass of the Sun per year. A lot of this tidal disruption happens early on, and we were able to catch this event right at the beginning, within one week of the black hole starting to feed on the star.
THE ELUSIVE GLOW BETWEEN DISTANT GALAXIES
University of New South Wales
An international team of astronomers have turned a new technique onto a group of galaxies and the faint light between them -- known as 'intra-group light' -- to characterise the stars that dwell there. The brightest parts of the intra-group light are ~50 times fainter than the darkest night sky on Earth. It is extremely hard to detect, even with the largest telescopes on Earth -- or in space. Using their sensitive technique, which eliminates light from all objects except that from the intra-group light, the researchers not only detected the intra-group light but were able to study and tell the story of the stars that populate it. They looked at the age and abundance of the elements that composed them and then compared those features with the stars still belonging to galaxy groups. The intra-group light is younger and less metal-rich than the surrounding galaxies. Not only were the orphan stars in the intra-group light 'anachronistic' but they appeared to be of a different origin to their closest neighbours. The researchers found the character of the intra-group stars appeared similar to the nebulous 'tail' of a further away galaxy. The combination of these clues allowed the researchers to rebuild the history -- the story -- of the intra-group light and how its stars came to be gathered in their own stellar orphanage. It’s thought these individual stars were at some points stripped from their home galaxies and now they float freely, following the gravity of the group. The stripping, called tidal stripping, is caused by the passage of massive satellite galaxies -- similar to the Milky Way -- that pull stars in their wake. This is the first time the intra-group light of these galaxies has been observed. The galaxies are so far away, that we're observing them as they were 2.5 billion years ago.
DETECTING DARK MATTER WITH NEUTRON SPIN CLOCKS
University of Bern
Astronomers have succeeded in significantly narrowing the scope for the existence of dark matter. The experiment was carried out at the European Research Neutron Source at the Institute Laue-Langevin in France, and makes an important contribution to the search for these particles, of which little is known. Cosmological observations of the orbits of stars and galaxies enable clear conclusions to be drawn about the attractive gravitational forces that act between the celestial bodies. The astonishing finding: visible matter is far from sufficient for being able to explain the development or movements of galaxies. This suggests that there exists another, so far unknown, type of matter. Accordingly, in the year 1933, the Swiss physicist and astronomer Fritz Zwicky inferred the existence of what is known now as dark matter. Dark matter is a postulated form of matter which isn't directly visible but interacts via gravity, and consists of approximately five times more mass than the matter with which we are familiar. Recently, following a precision experiment developed at the Albert Einstein Center for Fundamental Physics (AEC) at the University of Bern, an international research team succeeded in significantly narrowing the scope for the existence of dark matter. What dark matter is actually made of is still completely unclear. What is certain, however, is that it is not made from the same particles that make up the stars, planet Earth or us humans. Worldwide, increasingly sensitive experiments and methods are being used to search for possible dark matter particles -- until now, however, without success.
Certain hypothetical elementary particles, known as axions, are a promising category of possible candidates for dark matter particles. An important advantage of these extremely lightweight particles is that they could simultaneously explain other important phenomena in particle physics which have not yet been understood. If the elusive axions actually exist, they should leave behind a characteristic signature in the measurement apparatus. The experiment enables the team to determine the rotational frequency of neutron spins, which move through a superposition of electric and magnetic fields. The spin of each individual neutron acts as a kind of compass needle, which rotates due to a magnetic field similarly to the second hand of a wristwatch -- but nearly 400,000 times faster. This rotational frequency was precisely measured and examined for the smallest periodic fluctuations which would be caused by the interactions with the axions.. The results of the experiment were clear: The rotational frequency of the neutrons remained unchanged, which means that there is no evidence of axions in the measurement. The measurements, which were carried out at the European Research Neutron Source at the Institute Laue-Langevin, allowed for the experimental exclusion of a previously completely unexplored parameter space of axions. It also proved possible to search for hypothetical axions which would be more than 1,000 times heavier than was previously possible with other experiments. Although the existence of these particles remains mysterious, astronomers have successfully excluded an important parameter space of dark matter.
Topic: Mid December Astronomy Bulletin (Read 350 times)