Stars Regularly Ripped Apart By Black Holes In Colliding Galaxies

Astronomers based at the University of Sheffield have found evidence that stars are ripped apart by supermassive black holes 100 times more often than previously thought.

Until now, such stellar cannibalism – known as Tidal Distruption Events, or TDEs – had only been found in surveys which observed many thousands of galaxies, leading astronomers to believe they were exceptionally rare: only one event every 10,000 to 100,000 years per galaxy.

However, the pioneering study conducted by leading scientists from the University’s Department of Physics and Astronomy, recorded a star being destroyed by a supermassive black hole in a survey of just 15 galaxies – an extremely small sample size by astronomy standards.

“Each of these 15 galaxies is undergoing a ‘cosmic collision’ with a neighbouring galaxy,” said Dr James Mullaney, Lecturer in Astronomy and co-author of the study.
“Our surprising findings show that the rate of TDEs dramatically increases when galaxies collide. This is likely due to the fact that the collisions lead to large numbers of stars being formed close to the central supermassive black holes in the two galaxies as they merge together.”

The supermassive black holes that lurk in the hearts of all large galaxies can be elusive. This is because they don’t shine in a conventional sense due to their gravity being so strong that nothing can escape, not even light itself. However, the release of energy as stars are ripped apart when they move close to the black holes leads to dramatic flares. The galaxies’ nuclei can then appear as bright as all the billions of stars in a typical galaxy combined. In this way, TDEs can be used to locate otherwise dim black holes and study their strong gravity and how they accrete matter.

“Our team first observed the 15 colliding galaxies in the sample in 2005, during a previous project,” said Rob Spence, University of Sheffield PhD student and co-author of the study.

“However, when we observed the sample again in 2015, we noticed that one galaxy – F01004-2237 – appeared strikingly different. This led us to look at data from the Catalina Sky Survey, which monitors the brightness of objects in the sky over time. We found that in 2010, the brightness of F01004-2237 flared dramatically.”

The particular combination of variability and post-flare spectrum observed in F01004-2237 – which is 1.7 billion light years from Earth – was unlike any known supernova or active galactic nucleus, but characteristic of TDEs.

Clive Tadhunter, Professor of Astrophysics and leader of the study, said: “Based on our results for F01004-2237, we expect that TDE events will become common in our own Milky Way galaxy when it eventually merges with the neighbouring Andromeda galaxy in about 5 billion years.

“Looking towards the centre of the Milky Way at the time of the merger we’d see a flare approximately every 10 to 100 years. The flares would be visible to the naked eye and appear much brighter than any other star or planet in the night sky.”

First Evidence Of Rocky Planet Formation In Tatooine System

Evidence of planetary debris surrounding a double sun, ‘Tatooine-like’ system has been found for the first time by a UCL-led team of researchers.

Published today in Nature Astronomy and funded by the Science and Technology Facilities Council and the European Research Council, the study finds the remains of shattered asteroids orbiting a double sun consisting of a white dwarf and a brown dwarf roughly 1000 light-years away in a system called SDSS 1557.

The discovery is remarkable because the debris appears to be rocky and suggests that terrestrial planets like Tatooine – Luke Skywalker’s home world in Star Wars – might exist in the system. To date, all exoplanets discovered in orbit around double stars are gas giants, similar to Jupiter, and are thought to form in the icy regions of their systems.

In contrast to the carbon-rich icy material found in other double star systems, the planetary material identified in the SDSS 1557 system has a high metal content, including silicon and magnesium. These elements were identified as the debris flowed from its orbit onto the surface of the star, polluting it temporarily with at least 1017 g (or 1.1 trillion US tons) of matter, equating it to an asteroid at least 4 km in size.

Lead author, Dr Jay Farihi (UCL Physics & Astronomy), said: “Building rocky planets around two suns is a challenge because the gravity of both stars can push and pull tremendously, preventing bits of rock and dust from sticking together and growing into full-fledged planets. With the discovery of asteroid debris in the SDSS 1557 system, we see clear signatures of rocky planet assembly via large asteroids that formed, helping us understand how rocky exoplanets are made in double star systems.”

In the Solar System, the asteroid belt contains the leftover building blocks for the terrestrial planets Mercury, Venus, Earth, and Mars, so planetary scientists study the asteroids to gain a better understanding of how rocky, and potentially habitable planets are formed. The same approach was used by the team to study the SDSS 1557 system as any planets within it cannot yet be detected directly but the debris is spread in a large belt around the double stars, which is a much larger target for analysis.

The discovery came as a complete surprise, as the team assumed the dusty white dwarf was a single star but co-author Dr Steven Parsons (University of Valparaíso and University of Sheffield), an expert in double star (or binary) systems noticed the tell-tale signs. “We know of thousands of binaries similar to SDSS 1557 but this is the first time we’ve seen asteroid debris and pollution. The brown dwarf was effectively hidden by the dust until we looked with the right instrument”, added Parsons, “but when we observed SDSS 1557 in detail we recognised the brown dwarf’s subtle gravitational pull on the white dwarf.”

The team studied the binary system and the chemical composition of the debris by measuring the absorption of different wavelengths of light or ‘spectra’, using the Gemini Observatory South telescope and the European Southern Observatory Very Large Telescope, both located in Chile.

Co-author Professor Boris Gänsicke (University of Warwick) analysed these data and found they all told a consistent and compelling story. “Any metals we see in the white dwarf will disappear within a few weeks, and sink down into the interior, unless the debris is continuously flowing onto the star. We’ll be looking at SDSS 1557 next with Hubble, to conclusively show the dust is made of rock rather than ice.”

NASA Telescope Reveals Largest Batch Of Earth-Size, Habitable-Zone Planets Around Single Star

NASA’s Spitzer Space Telescope has revealed the first known system of seven Earth-size planets around a single star. Three of these planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water.

The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside our solar system. All of these seven planets could have liquid water — key to life as we know it — under the right atmospheric conditions, but the chances are highest with the three in the habitable zone.

“This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life,” said Thomas Zurbuchen, associate administrator of the agency’s Science Mission Directorate in Washington. “Answering the question ‘are we alone’ is a top science priority and finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal.”

At about 40 light-years (235 trillion miles) from Earth, the system of planets is relatively close to us, in the constellation Aquarius. Because they are located outside of our solar system, these planets are scientifically known as exoplanets.

This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, researchers using TRAPPIST announced they had discovered three planets in the system. Assisted by several ground-based telescopes, including the European Southern Observatory’s Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.

The new results were published Wednesday in the journal Nature, and announced at a news briefing at NASA Headquarters in Washington.

Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated.

Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces. The mass of the seventh and farthest exoplanet has not yet been estimated — scientists believe it could be an icy, “snowball-like” world, but further observations are needed.

“The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper and the principal investigator of the TRAPPIST exoplanet survey at the University of Liege, Belgium. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”

In contrast to our sun, the TRAPPIST-1 star — classified as an ultra-cool dwarf — is so cool that liquid water could survive on planets orbiting very close to it, closer than is possible on planets in our solar system. All seven of the TRAPPIST-1 planetary orbits are closer to their host star than Mercury is to our sun. The planets also are very close to each other. If a person were standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth’s sky.

The planets may also be tidally locked to their star, which means the same side of the planet is always facing the star, therefore each side is either perpetual day or night. This could mean they have weather patterns totally unlike those on Earth, such as strong winds blowing from the day side to the night side, and extreme temperature changes.

Spitzer, an infrared telescope that trails Earth as it orbits the sun, was well-suited for studying TRAPPIST-1 because the star glows brightest in infrared light, whose wavelengths are longer than the eye can see. In the fall of 2016, Spitzer observed TRAPPIST-1 nearly continuously for 500 hours. Spitzer is uniquely positioned in its orbit to observe enough crossing — transits — of the planets in front of the host star to reveal the complex architecture of the system. Engineers optimized Spitzer’s ability to observe transiting planets during Spitzer’s “warm mission,” which began after the spacecraft’s coolant ran out as planned after the first five years of operations.

“This is the most exciting result I have seen in the 14 years of Spitzer operations,” said Sean Carey, manager of NASA’s Spitzer Science Center at Caltech/IPAC in Pasadena, California. “Spitzer will follow up in the fall to further refine our understanding of these planets so that the James Webb Space Telescope can follow up. More observations of the system are sure to reveal more secrets.”

Following up on the Spitzer discovery, NASA’s Hubble Space Telescope has initiated the screening of four of the planets, including the three inside the habitable zone. These observations aim at assessing the presence of puffy, hydrogen-dominated atmospheres, typical for gaseous worlds like Neptune, around these planets.

In May 2016, the Hubble team observed the two innermost planets, and found no evidence for such puffy atmospheres. This strengthened the case that the planets closest to the star are rocky in nature.

“The TRAPPIST-1 system provides one of the best opportunities in the next decade to study the atmospheres around Earth-size planets,” said Nikole Lewis, co-leader of the Hubble study and astronomer at the Space Telescope Science Institute in Baltimore. NASA’s planet-hunting Kepler space telescope also is studying the TRAPPIST-1 system, making measurements of the star’s minuscule changes in brightness due to transiting planets. Operating as the K2 mission, the spacecraft’s observations will allow astronomers to refine the properties of the known planets, as well as search for additional planets in the system. The K2 observations conclude in early March and will be made available on the public archive.

Spitzer, Hubble, and Kepler will help astronomers plan for follow-up studies using NASA’s upcoming James Webb Space Telescope, launching in 2018. With much greater sensitivity, Webb will be able to detect the chemical fingerprints of water, methane, oxygen, ozone, and other components of a planet’s atmosphere. Webb also will analyze planets’ temperatures and surface pressures — key factors in assessing their habitability.

Prediction: More Gas-Giants Will Be Found Orbiting Sun-Like Stars

New planetary formation models from Carnegie’s Alan Boss indicate that there may be an undiscovered population of gas giant planets orbiting around Sun-like stars at distances similar to those of Jupiter and Saturn. His work is published by The Astrophysical Journal.

The population of exoplanets discovered by ongoing planet-hunting projects continues to increase. These discoveries can improve models that predict where to look for more of them.

The planets predicted by Boss in this study could hold the key to solving a longstanding debate about the formation of our Solar System’s giant planets out of the disk of gas and dust that surrounded the Sun in its youth.

One theory holds that gas giants form just like terrestrial planets do — by the slow accretion of rocky material from the rotating disk — until the object contains enough material to gravitationally attract a very large envelope of gas around a solid core. The other theory states that gas giant planets form rapidly when the disk gas forms spiral arms, which increase in mass and density until distinct clumps form that coalesce into baby gas giant planets.

One problem with the first option, called core accretion, is that it can’t explain how gas giant planets form beyond a certain orbital distance from their host stars — a phenomenon that is increasingly found by intrepid planet hunters. However, models of the second theory, called disk instability, have indicated the formation of planets with orbits between about 20 and 50 times the distance between Earth and the Sun.

“Given the existence of gas giant planets on such wide orbits, disk instability or something similar must be involved in the creation of at least some exoplanets,” Boss said. “However, whether or not this method could create closer-orbiting gas giant planets remains unanswered.”

Boss set out to use his modeling tools to learn if gas giant planets can form closer to their host stars by taking a new look at the disk-cooling process. His simulations indicate that there may be a largely unseen population of gas giant planets orbiting Sun-like stars at distances between 6 and 16 times that separating Earth and the Sun. (For context Jupiter is just over five times as distant from the Sun as Earth is, and Saturn is over nine times as distant.)

“NASA’s upcoming Wide Field Infrared Survey Telescope may be ideally suited to test my predictions here,” Boss added.

Mars More Earth-Like Than Moon-Like

Mars’ mantle may be more complicated than previously thought. In a new study published in the Nature-affiliated journal Scientific Reports, researchers at LSU document geochemical changes over time in the lava flows of Elysium, a major martian volcanic province.

LSU Geology and Geophysics graduate researcher David Susko led the study with colleagues at LSU including his advisor Suniti Karunatillake, the University of Rahuna in Sri Lanka, the SETI Institute, Georgia Institute of Technology, NASA Ames, and the Institut de Recherche en Astrophysique et Planétologie in France.

They found that the unusual chemistry of lava flows around Elysium is consistent with primary magmatic processes, such as a heterogeneous mantle beneath Mars’ surface or the weight of the overlying volcanic mountain causing different layers of the mantle to melt at different temperatures as they rise to the surface over time.

Elysium is a giant volcanic complex on Mars, the second largest behind Olympic Mons. For scale, it rises to twice the height of Earth’s Mount Everest, or approximately 16 kilometers. Geologically, however, Elysium is more like Earth’s Tibesti Mountains in Chad, the Emi Koussi in particular, than Everest. This comparison is based on images of the region from the Mars Orbiter Camera, or MOC, aboard the Mars Global Surveyor, or MGS, Mission.

Elysium is also unique among martian volcanoes. It’s isolated in the northern lowlands of the planet, whereas most other volcanic complexes on Mars cluster in the ancient southern highlands. Elysium also has patches of lava flows that are remarkably young for a planet often considered geologically silent.

“Most of the volcanic features we look at on Mars are in the range of 3-4 billion years old,” Susko said. “There are some patches of lava flows on Elysium that we estimate to be 3-4 million years old, so three orders of magnitude younger. In geologic timescales, 3 million years ago is like yesterday.”

In fact, Elysium’s volcanoes hypothetically could still erupt, Susko said, although further research is needed to confirm this. “At least, we can’t yet rule out active volcanoes on Mars,” Susko said. “Which is very exciting.”

Susko’s work in particular reveals that the composition of volcanoes on Mars may evolve over their eruptive history. In earlier research led by Karunatillake, assistant professor in LSU’s Department of Geology and Geophysics, researchers in LSU’s Planetary Science Lab, or PSL, found that particular regions of Elysium and the surrounding shallow subsurface of Mars are geochemically anomalous, strange even relative to other volcanic regions on Mars. They are depleted in the radioactive elements thorium and potassium. Elysium is one of only two igneous provinces on Mars where researchers have found such low levels of these elements so far.

“Because thorium and potassium are radioactive, they are some of the most reliable geochemical signatures that we have on Mars,” Susko said. “They act like beacons emitting their own gamma photons. These elements also often couple in volcanic settings on Earth.”

In their new paper, Susko and colleagues started to piece together the geologic history of Elysium, an expansive volcanic region on Mars characterized by strange chemistry. They sought to uncover why some of Elysium’s lava flows are so geochemically unusual, or why they have such low levels of thorium and potassium. Is it because, as other researchers have suspected, glaciers located in this region long ago altered the surface chemistry through aqueous processes? Or is it because these lava flows arose from different parts of Mars’ mantle than other volcanic eruptions on Mars?

Perhaps the mantle has changed over time, meaning that more recent volcanic eruption flows differ chemically from older ones. If so, Susko could use Elysium’s geochemical properties to study how Mars’ bulk mantle has evolved over geologic time, with important insights for future missions to Mars. Understanding the evolutionary history of Mars’ mantle could help researchers gain a better understanding of what kinds of valuable ores and other materials could be found in the crust, as well as whether volcanic hazards could unexpectedly threaten human missions to Mars in the near future. Mars’ mantle likely has a very different history than Earth’s mantle because the plate tectonics on Earth are absent on Mars as far as researchers know. The history of the bulk interior of the red planet also remains a mystery.

Susko and colleagues at LSU analyzed geochemical and surface morphology data from Elysium using instruments on board NASA’s Mars Odyssey Orbiter (2001) and Mars Reconnaissance Orbiter (2006). They had to account for the dust that blankets Mars’ surface in the aftermath of strong dust storms, to make sure that the shallow subsurface chemistry actually reflected Elysium’s igneous material and not the overlying dust.

Through crater counting, the researchers found differences in age between the northwest and the southeast regions of Elysium — about 850 million years of difference. They also found that the younger southeast regions are geochemically different from the older regions, and that these differences in fact relate to igneous processes, not secondary processes like the interaction of water or ice with the surface of Elysium in the past.

“We determined that while there might have been water in this area in the past, the geochemical properties in the top meter throughout this volcanic province are indicative of igneous processes,” Susko said. “We think levels of thorium and potassium here were depleted over time because of volcanic eruptions over billions of years. The radioactive elements were the first to go in the early eruptions. We are seeing changes in the mantle chemistry over time.”

“Long-lived volcanic systems with changing magma compositions are common on Earth, but an emerging story on Mars,” said James Wray, study co-author and associate professor in the School of Earth and Atmospheric Sciences at Georgia Tech.

Wray led a 2013 study that showed evidence for magma evolution at a different martian volcano, Syrtis Major, in the form of unusual minerals. But such minerals could be originating at the surface of Mars, and are visible only on rare dust-free volcanoes.

“At Elysium we are truly seeing the bulk chemistry change over time, using a technique that could potentially unlock the magmatic history of many more regions across Mars,” he said.

Susko speculates that the very weight of Elysium’s lava flows, which make up a volcanic province six times higher and almost four times wider than its morphological sister on Earth, Emi Koussi, has caused different depths of Mars’ mantle to melt at different temperatures. In different regions of Elysium, lava flows may have come from different parts of the mantle. Seeing chemical differences in different regions of Elysium, Susko and colleagues concluded that Mars’ mantle might be heterogeneous, with different compositions in different areas, or that it may be stratified beneath Elysium.

Overall, Susko’s findings indicate that Mars is a much more geologically complex body than originally thought, perhaps due to various loading effects on the mantle caused by the weight of giant volcanoes.

“It’s more Earth-like than moon-like,” Susko said. “The moon is cut and dry. It often lacks the secondary minerals that occur on Earth due to weathering and igneous-water interactions. For decades, that’s also how we envisioned Mars, as a lifeless rock, full of craters with a number of long inactive volcanoes. We had a very simple view of the red planet. But the more we look at Mars, the less moon-like it becomes. We’re discovering more variety in rock types and geochemical compositions, as seen across the Curiosity Rover’s traverse in Gale Crater, and more potential for viable resource utilization and capacity to sustain a human population on Mars. It’s much easier to survive on a complex planetary body bearing the mineral products of complex geology than on a simpler body like the moon or asteroids.”

Susko plans to continue clarifying the geologic processes that cause the strange chemistry found around Elysium. In the future, he will study these chemical anomalies through computational simulations, to determine if recreating the pressures in Mars’ mantle caused by the weight of giant volcanoes could affect mantle melting to yield the type of chemistry observed within Elysium.

Dating The Milky Way’s Disc

When a star like our sun gets to be very old, after another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of its mass remaining, it will shrink to a fraction of its radius and become a white dwarf star. White dwarfs are common, the most famous one being the companion to the brightest star in the sky, Sirius. As remnants of some of the oldest stars in the galaxy, white dwarfs offer an independent means of dating the lifetimes of different galactic populations.

A globular cluster is a roughly spherical ensemble of stars (as many as several million) that are gravitationally bound together and typically located in the outer regions of galaxies. The white dwarf stars in the Milly Way’s globular clusters reveal an age spread of between eleven and thirteen billion years. By contrast, the thick disk of the galaxy is thought to be older than ten billion years but that figure is not very well constrained. White dwarfs in the disc can be used to refine those age estimates and, since they are closer and brighter to us than those in globular clusters, they can provide more detailed information. However, they are not located in well-defined regions like clusters and so they are also harder to spot.

CfA astronomer Warren Brown and his colleagues used the 6.5-m Multiple Mirror Telescope (MMT) to obtain spectra of fifty-seven white dwarf candidate stars in the disk first discovered in all-sky surveys. Modeling the spectra of these stars revealed a mixture of types (for example, some stars had atmospheres of pure helium and others of pure hydrogen) and also an age for the disc of eleven billion years. The result is consistent with the current age estimates for the thick disc but also suggests that the current minimum age estimate might be increased. Additional measurements are needed to refine the age range, and the scientists predict that large-scale sky surveys now underway will significantly increase the number of non-cluster white dwarfs and enable the determination of their parameters.

What Drives Universe’s Expansion?

Astronomy experiments could soon test an idea developed by Albert Einstein almost exactly a century ago, scientists say.

Tests using advanced technology could resolve a longstanding puzzle over what is driving the accelerated expansion of the Universe.

Researchers have long sought to determine how the Universe’s accelerated expansion is being driven. Calculations in a new study could help to explain whether dark energy- as required by Einstein’s theory of general relativity — or a revised theory of gravity are responsible.

Einstein’s theory, which describes gravity as distortions of space and time, included a mathematical element known as a Cosmological Constant. Einstein originally introduced it to explain a static universe, but discarded his mathematical factor as a blunder after it was discovered that our Universe is expanding.

Research carried out two decades ago, however, showed that this expansion is accelerating, which suggests that Einstein’s Constant may still have a part to play in accounting for dark energy. Without dark energy, the acceleration implies a failure of Einstein’s theory of gravity across the largest distances in our Universe.

Scientists from the University of Edinburgh have discovered that the puzzle could be resolved by determining the speed of gravity in the cosmos from a study of gravitational waves -space-time ripples propagating through the universe.

The researchers’ calculations show that if gravitational waves are found to travel at the speed of light, this would rule out alternative gravity theories, with no dark energy, in support of Einstein’s Cosmological Constant. If however, their speed differs from that of light, then Einstein’s theory must be revised.

Such an experiment could be carried out by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, whose twin detectors, 2000 miles apart, directly detected gravitational waves for the first time in 2015.

Experiments at the facilities planned for this year could resolve the question in time for the 100th anniversary of Einstein’s Constant.

The study, published in Physics Letters B, was supported by the UK Science Technology Facilities Council, the Swiss National Science Foundation, and the Portuguese Foundation of Science and Technology.

Dr Lucas Lombriser, of the University of Edinburgh’s School of Physics and Astronomy, said: “Recent direct gravitational wave detection has opened up a new observational window to our Universe. Our results give an impression of how this will guide us in solving one of the most fundamental problems in physics.”