SMALLEST DWARF PLANET IN SOLAR SYSTEM
ESO
Astronomers using the SPHERE instrument at the Very Large Telescope (VLT) have revealed that the asteroid Hygiea could be classified as a dwarf planet. The object is the fourth largest in the asteroid belt after Ceres, Vesta and Pallas. For the first time, astronomers have observed Hygiea in sufficiently high resolution to study its surface and determine its shape and size. They found that Hygiea is spherical, potentially taking the crown from Ceres as the smallest dwarf planet in the Solar System. As an object in the main asteroid belt, Hygiea satisfies right away three of the four requirements to be classified as a dwarf planet: it orbits around the Sun, it is not a moon and, unlike a planet, it has not cleared the neighbourhood around its orbit. The final requirement is that it has enough mass for its own gravity to pull it into a roughly spherical shape. This is what VLT observations have now revealed about Hygiea. The team also used the SPHERE observations to constrain Hygiea's size, putting its diameter at just over 430 km. Pluto, the most famous of dwarf planets, has a diameter close to 2400 km, while Ceres is close to 950 km in size.
Surprisingly, the observations also revealed that Hygiea lacks the very large impact crater that scientists expected to see on its surface. Hygiea is the main member of one of the largest asteroid families, with close to 7000 members that all originated from the same parent body. Astronomers expected the event that led to the formation of this numerous family to have left a large, deep mark on Hygiea. The team decided to investigate further. Using numerical simulations, they deduced that Hygiea's spherical shape and large family of asteroids are likely the result of a major head-on collision with a large projectile of diameter between 75 and 150 km. Their simulations show this violent impact, thought to have occurred about 2 billion years ago, completely shattered the parent body. Once the left-over pieces reassembled, they gave Hygiea its round shape and thousands of companion asteroids.
HEAVY ELEMENT BORN FROM NEUTRON-STAR COLLISION
ESO
For the first time, a freshly made heavy element, strontium, has been detected in space, in the aftermath of a merger of two neutron stars. This finding was observed by the X-shooter spectrograph on the Very Large Telescope (VLT). The detection confirms that the heavier elements in the Universe can form in neutron-star mergers, providing a missing piece of the puzzle of chemical element formation. In 2017, following the detection of gravitational waves passing the Earth, ESO pointed its telescopes in Chile, including the VLT, to the source: a neutron-star merger named GW170817. Astronomers suspected that, if heavier elements did form in neutron-star collisions, signatures of those elements could be detected in kilonovae, the explosive aftermaths of such mergers. This is what a team of European researchers has now done, using data from the X-shooter instrument on ESO's VLT. Following the GW170817 merger, ESO's fleet of telescopes began monitoring the emerging kilonova explosion over a wide range of wavelengths. X-shooter in particular took a series of spectra from the ultraviolet to the near infrared. Initial analysis of those spectra suggested the presence of heavy elements in the kilonova, but astronomers could not pinpoint individual elements until now. Astronomers have known the physical processes that create the elements since the 1950s. Over the following decades they have uncovered the cosmic sites of each of these major nuclear forges, except the one known as rapid neutron capture, that created the heavier elements in the periodic table. Rapid neutron capture is a process in which an atomic nucleus captures neutrons quickly enough to allow very heavy elements to be created. Although many elements are produced in the cores of stars, creating elements heavier than iron, such as strontium, requires even hotter environments with lots of free neutrons.
Rapid neutron capture only occurs naturally in extreme environments where atoms are bombarded by vast numbers of neutrons. This is the first time that astronomers can directly associate newly created material formed by neutron capture with a neutron-star merger, confirming that neutron stars are made of neutrons and tying the long-debated rapid-neutron-capture process to such mergers. Scientists are only now starting to understand neutron-star mergers and kilonovae. Because of the limited understanding of those new phenomena and other complexities in the spectra that the VLT's X-shooter took of the explosion, astronomers had not been able to identify individual elements until now.
HUGE OUTFLOW OF GAS EXTENDING BEYOND GALAXY
University of California - San Diego
Exploring the influence of galactic winds from a distant galaxy called Makani, a group of collaborators from around the world made a novel discovery. Their study's findings provide direct evidence for the first time of the role of galactic winds -- ejections of gas from galaxies - in creating the circumgalactic medium (CGM). It exists in the regions around galaxies, and it plays an active role in their cosmic evolution. The unique composition of Makani -- meaning wind in Hawaiian -- uniquely lent itself to the breakthrough findings. Makani is not a typical galaxy. It's what's known as a late-stage major merger -- two recently combined similarly massive galaxies, which came together because of the gravitational pull each felt from the other as they drew nearer. Galaxy mergers often lead to starburst events, when a substantial amount of gas present in the merging galaxies is compressed, resulting in a burst of new star births. Those new stars, in the case of Makani, probably caused the huge outflows -- either in stellar winds or at the end of their lives when they exploded as supernovae. Most of the gas in the Universe inexplicably appears in the regions surrounding galaxies -- not in the galaxies. Typically, when astronomers observe a galaxy, they are not witnessing it undergoing dramatic events -- big mergers, the rearrangement of stars, the creation of multiple stars or driving huge, fast winds. While those events may occur at some point in a galaxy's life, they'd be relatively brief. Here, astronomers are actually catching it all right as it's happening through these huge outflows of gas and dust.
The team used data collected from the W. M. Keck Observatory's new Keck Cosmic Web Imager (KCWI) instrument, combined with images from the Hubble Space Telescope and the Atacama Large Millimeter Array (ALMA), to draw their conclusions. The KCWI data provided what the researchers call the "stunning detection" of the ionized oxygen gas to extremely large scales, well beyond the stars in the galaxy. It allowed them to distinguish a fast gaseous outflow, launched from the galaxy a few million years ago, from a gas outflow launched hundreds of millions of years earlier that has since slowed significantly. The earlier outflow has flowed to large distances from the galaxy, while the fast, recent outflow has not had time to do so. From Hubble, the researchers procured images of Makani's stars, showing it to be a massive, compact galaxy that resulted from a merger of two once-separate galaxies. From ALMA, they could see that the outflow contains molecules as well as atoms. The data sets indicated that with a mixed population of old, middle-age and young stars, the galaxy might also contain a dust-obscured accreting supermassive black hole. That suggested to the scientists that Makani's properties and timescales are consistent with theoretical models of galactic winds. In terms of both their size and speed of travel, the two outflows are consistent with their creation by the past starburst events; they're also consistent with theoretical models of how large and fast winds should be if created by starbursts. So observations and theory are agreeing well. The hourglass shape of Makani's nebula is strongly reminiscent of similar galactic winds in other galaxies, but Makani's wind is much larger than observed in other galaxies. That means astronomers can confirm it's actually moving gas from the galaxy into the circumgalactic regions around it, as well as sweeping up more gas from its surroundings as it moves out. It's moving a lot of it -- at least one to 10 percent of the visible mass of the entire galaxy -- at very high speeds, thousands of kilometres per second. While astronomers are converging on the idea that galactic winds are important for feeding the CGM, most of the evidence has come from theoretical models or observations that don't encompass the entire galaxy. Here we have the whole spatial picture for one galaxy, which is a remarkable illustration of what people expected. Makani's existence provides one of the first direct windows into how a galaxy contributes to the ongoing formation and chemical enrichment of its CGM.
NEW CLASS OF BLACK HOLES
Ohio State University
Black holes are an important part of how astrophysicists make sense of the Universe -- so important that scientists have been trying to build a census of all the black holes in the Milky Way galaxy. But new research shows that their search might have been missing an entire class of black holes that they didn't know existed. In a study, astronomers offer a new way to search for black holes, and show that it is possible that there is a class of black holes smaller than the smallest known black holes in the Universe. Imagine a census of a city that only counted people 5'9" and taller -- and imagine that the census takers didn't even know that people shorter than 5'9" existed. Data from that census would be incomplete, providing an inaccurate picture of the population. That is essentially what has been happening in the search for black holes. Astronomers have long been searching for black holes, which have gravitational pulls so fierce that nothing -- not matter, not radiation -- can escape. Black holes form when some stars die, shrink into themselves, and explode. Astronomers have also been looking for neutron stars -- small, dense stars that form when some stars die and collapse. Both could hold interesting information about the elements on Earth and about how stars live and die. But in order to uncover that information, astronomers first have to determine where the black holes are. And to figure out where the black holes are, they need to know what they are looking for. One clue: Black holes often exist in binary systems. That simply means that two stars are close enough to one another to be locked together by gravity in a mutual orbit around one another. When one of those stars dies, the other can remain, still orbiting the space where the dead star -- now a black hole or neutron star -- once lived, and where a black hole or neutron star has formed.
For years, the black holes scientists knew about were all between about five and 15 times the mass of the Sun. The known neutron stars are generally no bigger than about 2.1 times the mass of the Sun -- if they were above 2.5 times the Sun's mass, they would collapse to a black hole. But in the summer of 2017, a survey called LIGO -- the Laser Interferometer Gravitational-Wave Observatory -- saw two black holes merging together in a galaxy about 1.8 million light years away. One of those black holes was about 31 times the mass of the Sun, the other about 25 times the mass of the Sun. Astrophysicists had long suspected that black holes might come in sizes outside the known range, and LIGO's discovery proved that black holes could be larger. But there remained a window of size between the biggest neutron stars and the smallest black holes. Scientists began combing through data from APOGEE, the Apache Point Observatory Galactic Evolution Experiment, which collected light spectra from around 100,000 stars across the Milky Way. The spectra could show whether a star might be orbiting around another object: changes in spectra -- a shift toward bluer wavelengths, for example,
followed by a shift to redder wavelengths -- could show that a star was orbiting an unseen companion. The team began combing through the data, looking for stars that showed that change, indicating that they might be orbiting a black hole. Then, the APOGEE data was narrowed to 200 stars that might be most interesting. Data crunching found a giant red star that appeared to be orbiting something, but that something, based on their calculations, was probably much smaller than the known black holes in the Milky Way, but way bigger than most known neutron stars. After more calculations and additional data from the Tillinghast Reflector Echelle Spectrograph and the Gaia satellite, the team realized that they had found a low-mass black hole, probably about 3.3 times the mass of the Sun. In conclusion, scientists have come up with a new way to search for black holes and also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn't previously known about. The masses of things tell us about their formation and evolution, and they tell us about their nature.
VOYAGER 2 REACHES INTERSTELLAR SPACE
University of Iowa
Researchers at the University of Iowa report that the spacecraft Voyager 2 has entered the interstellar medium (ISM), the region of space outside the bubble-shaped boundary produced by wind streaming outward from the Sun. Voyager 2, thus, becomes the second human-made object to journey out of our Sun's influence, following Voyager 1's solar exit in 2012. In a new study, the researchers confirm Voyager 2's passage on 2018 Nov. 5, into the ISM by noting a definitive jump in plasma density detected by the plasma-wave instrument on the spacecraft. The marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space. It is also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space. Voyager 2's entry into the ISM occurred at 119.7 astronomical units, or more than 11 billion miles from the Sun. Voyager 1 passed into the ISM at 122.6 AU. The spacecraft were launched within weeks of each other in 1977, with different mission goals and trajectories through space. Yet they crossed into the ISM at basically the same distances from the Sun. That gives valuable clues to the structure of the heliosphere -- the bubble, shaped much like a wind sock, created by the Sun's wind as it extends to the boundary of the Solar System. It implies that the heliosphere is symmetrical, at least at the two points where the Voyager spacecraft crossed.
Data from instrumentation on Voyager 2 also gives additional clues to the thickness of the heliosheath, the outer region of the heliosphere and the point where the solar wind piles up against the approaching wind in interstellar space. Researchers say the heliosheath has varied thickness, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause, a boundary where the solar wind and the interstellar wind are in balance and considered the crossing point to interstellar space. Some had thought Voyager 2 would make that crossing first, based on models of the heliosphere. The last measurement obtained from Voyager 1 was when the spacecraft was at 146 AU, or more than 13.5 billion miles from the Sun. The plasma-wave instrument is recording that the plasma density is rising, in data feeds from a spacecraft now so far away that it takes more than 19 hours for information to travel from the spacecraft to Earth. The two Voyagers will outlast Earth. They're in their own orbits around the Galaxy for five billion years or longer. And the probability of them running into
anything is almost zero.
Topic: Early November Astronomy Bulletin (Read 1493 times)