The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 385 2014 October 19
Here is the latest round-up of news from the Society for Popular Astronomy. The SPA is Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting http://www.popastro.com/
MOON’S HIDDEN VALLEY SYSTEM REVEALED
Scientists have recognized a huge rectangular feature on the Moon that is buried just below the surface. The 2,500-km structure is believed to be the remains of old rift valleys that later became filled with lava. Centred on the Moon's Procellarum region, the feature is really only evident in gravity maps made by the 'Grail' mission in 2012. But with knowledge now of its existence, it is possible to trace its subtle outline even in ordinary photos. Mare Frigoris, for example, a long-recognised dark stripe on the lunar surface, is evidently an edge of the ancient rift system. It covers about 17% of the surface of the Moon -- in terms relative to the size of the Earth, that would be an area equivalent to North America, Europe and Asia combined.
Scientists note that the Procellarum region contains a lot of naturally occurring radioactive elements, such as uranium, thorium and potassium. On the early Moon, they would have heated the crust, which, when it cooled, would have contracted. The shrinking, they propose, would have rifted the surface, opening deep valleys. The geometry is the giveaway. On the Earth, cooling and contraction preferentially produces hexagons containing 120-degree angles. The famous Giant's Causeway in Northern Ireland is a classic example on the small scale, but even in bigger settings, such as in East Africa's rift valleys, geological lines tend to intersect in that way. Procellarum's giant rectangle does the same, too, but because the entire feature is draped over a sphere the angles at the corners are more than 90 degrees. For structures on such a scale, a polygon with 120-degree angles at the corners actually has four sides instead of six. The team cannot tell when the rifting occurred, but the dating of Moon rocks brought back by Apollo would suggest that the valleys were filled by volcanic lavas about 3.5 billion years ago. The new study goes some way to resolving arguments over the origins of Oceanus Procellarum, which looks different from other, more circular, maria (dark regions) on the Moon's surface. For those regions, big asteroid impacts were more important in sculpting their forms.
SWIRLING CLOUD AT TITAN'S POLE IS COLD
NASA/Jet Propulsion Laboratory
Scientists analyzing data from the Cassini mission have discovered that a great cloud is hovering over the south pole of Saturn's largest moon, Titan, after the atmosphere there cooled dramatically. The scientists found that the polar vortex contains frozen particles of hydrogen cyanide, HCN. The discovery suggests that the atmosphere of Titan's southern hemisphere is cooling much faster than was expected. Titan is the only moon in the Solar System that has a dense atmosphere. Like us, Titan experiences seasons. As it makes its 29-year orbit around the Sun along with Saturn, each season lasts about seven Earth years. The most recent seasonal switch occurred about 2009, when winter gave way to spring in the northern hemisphere, and summer turned to autumn in the southern hemisphere. In 2012 May, while Titan's southern hemisphere was experiencing autumn, images from Cassini revealed a swirling cloud, several hundred miles across, taking shape above Titan's south pole. That polar vortex appears to be an effect of the change of season. A puzzling detail about the cloud is its altitude, some 300 kilometres above Titan's surface, where scientists thought the temperature was too high for clouds to form. Keen to understand what could give rise to the cloud, the scientists who were studying Cassini's observations found an important clue in the spectrum of sunlight reflected by Titan's atmosphere. Cassini's visual and infrared mapping spectrometer (VIMS) maps the distribution of chemical compounds in Titan's atmosphere and on its surface. The light coming from the polar vortex showed a remarkable difference from that from other parts of the atmosphere -- it clearly showed a signature of frozen HCN molecules.
As a gas, HCN is present in small amounts in the nitrogen-rich atmosphere of Titan. Finding it in the form of ice was surprising, as HCN can condense to form frozen particles only if the atmospheric temperature is as low as -140 degrees C, about 100 degrees colder than predictions from current theoretical models of Titan's upper atmosphere. To check whether such low temperatures were actually possible, the team looked at observations from Cassini's composite infrared spectrometer (CIRS), which measures atmospheric temperature at different altitudes. Those data showed that the southern hemi-sphere of Titan has been cooling rapidly, making it possible to reach the temperature needed to form the HCN cloud seen at the south pole. Atmospheric circulation has been drawing large masses of gas towards the south since the change of season in 2009. As HCN gas becomes more concentrated there, its molecules radiate a lot of energy at infrared wavelengths, cooling the surrounding atmosphere in the process. Another factor contributing to the cooling is obviously the reduced exposure to sunlight in Titan's southern hemisphere as winter approaches there.
MEGA FLARES FROM NEARBY DWARF STAR
On April 23, the Swift satellite detected the strongest, hottest, and longest-lasting sequence of stellar flares ever seen from a nearby red dwarf star. The initial blast from the series of explosions was as much as 10,000 times more powerful than the largest solar flare ever recorded. Astronomers used to think that major flaring episodes from red dwarfs lasted no more than a day, but Swift detected at least seven powerful eruptions over a period of about two weeks. At its peak, the flare reached temperatures of 200 million C, far hotter than the centre of the Sun. The massive flare was detected from a red dwarf star in the binary system DG CVn, about 60 light-years away. Both stars in the system are dim red dwarfs with masses and sizes about one-third of the Sun's. They orbit one another at about three times the Earth's distance from the Sun, which is too close for Swift to determine which star erupted. The system is poorly studied because it was not on the watch list of stars capable of producing large flares. Most of the stars lying within about 100 light-years of the Solar System are, like the Sun, middle-aged. But a thousand or so young red dwarfs born elsewhere drift through this region, and such stars give astronomers opportunities to study the high-energy activity that typically accompanies stellar youth. Astronomers estimate that DG CVn was born about 30 million years ago, which makes it less than a hundredth the age of the Solar System.
Stars erupt with flares for the same reason that the Sun does. Around active regions of the star's atmosphere, magnetic fields become twisted and distorted. Much like winding up a rubber band, the twisting accumulates energy. Eventually a process called magnetic reconnection destabilizes the fields, resulting in the explosive release of the stored energy that we see as a flare. The outburst emits radiation across the electromagnetic spectrum, from radio waves to visible, ultraviolet and X-ray light. On April 23, the rising tide of X-rays from DG CVn's super-flare triggered Swift's Burst Alert Telescope (BAT). Within several seconds of detecting a strong burst of radiation, the BAT calculates an initial position, decides whether the activity merits investigation by other instruments and, if so, sends the position to the spacecraft. In this case, Swift turned to observe the source in greater detail, and, at the same time, notified astronomers around the globe that a powerful outburst was in progress. For about three minutes after the BAT trigger, the super-flare's X-ray brightness was greater than the combined luminosity of both stars at all wavelengths under normal conditions. Flares so large from red dwarfs are exceedingly rare. The star's brightness in visible and ultraviolet light, measured both by ground-based observatories and Swift's optical/ultraviolet telescope, rose by 10 and 100 times, respectively. The initial flare's X-ray output, as measured by Swift's X-ray telescope, puts even the most intense solar activity recorded to shame. The largest solar explosions are classified as extraordinary, or X class, solar flares on the basis of their X-ray emission. The biggest flare ever seen from the Sun occurred in November 2003 and is rated as X 45. The flare on DG CVn, if viewed from a planet at the same distance as the Earth is from the Sun, would have been roughly 10,000 times greater than this, with a rating of about X 100,000. Three hours after the initial outburst, with X-rays on the downturn, the system exploded with another flare nearly as intense as the first. The two explosions may be an example of 'sympathetic' flaring often seen on the Sun, where an outburst in one active region triggers one in another. Over the next 11 days, Swift detected a series of successively weaker blasts. All told, the star took a total of 20 days to settle back to its normal level of X-ray emission. The reason that a star just a third the size of the Sun can produce such a giant eruption is because of its rapid spin, a crucial ingredient for amplifying magnetic fields. The flaring star in DG CVn rotates in under a day, about 30 or so times faster than the Sun. The Sun, too, rotated much faster in its youth and may well have produced super-flares of its own, but, fortunately for us, it no longer appears capable of doing so.
'COUSIN' PLANETS FOUND AROUND TWIN STARS
European astronomers have found two new Jupiter-sized extra-solar planets, each orbiting one star of a binary-star system. Most known extra-solar planets orbit stars that are alone, like the Sun. Yet many stars exist in binary systems, twin stars formed from the same gas cloud. Now, for the first time, two stars of a binary system are both found to host a 'hot Jupiter' exo-planet. The discoveries, around the stars WASP-94A and WASP-94B, were made by a team of British, Swiss and Belgian astronomers. The British WASP-South survey found tiny dips in the light of WASP-94A, suggesting that a Jupiter-like planet was transiting the star; Swiss astronomers then showed the existence of planets around both WASP-94A and its twin WASP-94B. Hot-Jupiter planets are much closer to their stars than our own Jupiter, with a 'year' lasting only a few days. They are rare, so it would be unlikely to find two hot Jupiters in the same star system by chance. Perhaps WASP-94 has just the right conditions for producing Hot Jupiters? If so WASP-94 could be an important system for understanding why hot Jupiters are so close to the stars that they orbit.
The existence of Jupiter-size planets so near to their stars is a long-standing puzzle, since they cannot form near to the star because it is far too hot there. They must form much further out, where it is cool enough for ices to freeze out of the proto-planetary disc circling the young star, hence forming the core of a new planet. Something must then move the planet into a close orbit, and one likely mechanism is an interaction with another planet or star. Finding hot-Jupiter planets in two stars of a binary pair might allow us to study the processes that move the planets inward. The two stars are relatively bright, making it easy to study their planets, so WASP-94 could be used to discover the compositions of the atmospheres of exo-planets. The WASP survey is the world's most successful search for hot-Jupiter planets that pass in front of (transit) their star. The WASP-South survey instrument scans the sky every clear night, searching hundreds of thousands of stars for transits. The Belgian team selects the best WASP candidates by obtaining high-quality transit light-curves. Geneva Observatory astronomers then show that the transiting body is a planet by measuring its mass, which they do by detecting the planet's gravitational effect on the host star. The collaboration has now found over 100 hot-Jupiter planets, many of them around relatively bright stars that are easy to study.
ULTRA-LUMINOUS PULSAR DETECTED
California Institute of Technology
Astronomers working with the Nuclear Spectroscopic Telescope Array (NuSTAR) have found a pulsating dead star beaming with the energy of about 10 million suns. The object, previously thought to be a black hole because it is so powerful, is in fact a pulsar -- the incredibly dense, rapidly rotating remains of a star. Pulsars are typically between one and two times the mass of the Sun. The new pulsar presumably falls in that same range but shines about 100 times brighter than theory suggests something of its mass ought to be able to do. The finding may help scientists to understand a class of very bright X-ray sources, called ultra-luminous X-ray sources (ULXs). Earlier this year, astronomers detected a spectacular, once-in-a-century supernova (dubbed SN2014J) in a relatively nearby galaxy known as Messier 82 (M82) 12 million light-years away. Because of the rarity of that event, telescopes around the world and in space turned to study the aftermath of the explosion in detail. Besides the supernova, M82 harbours a number of other ULXs. When researchers took a closer look at them in NuSTAR's data, they discovered that something in the galaxy was pulsing, or flashing light. That was a surprise, because for decades people have thought that ultra-luminous X-ray sources had to be black holes. But black holes have no way to create pulsing, which pulsars do. They are like magnets that emit radiation from their magnetic poles. As they rotate, an outside observer with an X-ray telescope, situated at the right angle, would see powerful flashes of light as the beam swept periodically across the observer's field of view, like a lighthouse beacon.
The reason that most astronomers had assumed that black holes were powering ULXs is that those sources are so incredibly bright. Black holes can be up to billions of times the mass of the Sun, making their gravity much stronger than that of a pulsar. As matter falls onto a black hole the gravitational energy turns into heat, which creates X-ray light. The bigger the black hole, the more energy there is to make the object shine. Surprised to see the flashes coming from M82, the NuSTAR team checked and re-checked the data. The flashes were really there, with a pulse showing up every 1.37 seconds. The next step was to work out which X-ray source was producing the flashes. Researchers analyzed the data from NuSTAR and a second X-ray telescope, Chandra, to rule out about 25 different X-ray sources, finally settling on a ULX known as M82X-2 as the source of the flashes. With the pulsar and its location within M82 identified, there are still many questions left to answer. Its energy is many times higher than the Eddington limit, a basic physics guideline that sets an upper limit on the brightness that an object of a given mass should be able to achieve. This is the most extreme violation of that limit ever seen. Astronomers have known that things can go above that by a small amount, but this one blows the limit clean away. Now that the NuSTAR team has shown that that ULX is a pulsar, it seems possible that many other known ULXs may in fact be pulsars as well.
MILKY WAY HAS ONLY HALF THE PREVIOUSLY SUPPOSED AMOUNT OF DARK MATTER
International Centre for Radio Astronomy Research (ICRAR)
A new measurement of dark matter in the Milky Way has indicated that there is half as much of that mysterious substance as was previously thought. Australian astronomers used a method developed almost 100 years ago to discover that the mass of dark matter in our own Galaxy is 8 x 10 to the power 11 times the mass of the Sun. They looked closely, for the first time, at the fringes of the Milky Way. They were able to measure the mass of the dark matter in the Milky Way by studying the speeds of stars throughout the Galaxy, including the edges, which had never been studied to such detail before. The team used a robust technique developed by the British astronomer Sir James Jeans in 1915 -- decades before the discovery of dark matter. The measurement helps to solve a diffculty that has been annoying theoreticians for some time. The current idea of galaxy formation and evolution, called the Lambda Cold Dark Matter theory, says that there should be a handful of big satellite galaxies around the Milky Way that are visible with the naked eye -- but we don't see that. When one uses the new measurement of the mass of the dark matter, then the theory says that there should only be three satellite galaxies out there, which is exactly what we see: the Magellanic Clouds and the Sagittarius Dwarf Galaxy. The study also presented an holistic model of the Milky Way, which allowed the scientists to measure several interesting things such as the speed required to leave the Galaxy, which appears to be about 550 km/s.
YOUNG STELLAR BRIDGE IN MAGELLANIC CLOUDS
Polish astronomers from the Optical Gravitational Lensing Experiment (OGLE) have discovered a young stellar bridge that forms a continuous connection between the Magellanic Clouds. The finding is based on number-density maps for stellar populations found in data gathered by OGLE in the most extensive optical survey of that region to date. The young population is present mainly in the western half of the Magellanic Bridge area, which, together with the newly discovered young population in the eastern Bridge, forms a continuous stream of stars connecting the two galaxies. The young-population distribution is clumped, with one of the major densities close to the Small Magellanic Cloud; the other, which is fairly isolated and is located approximately midway between the Clouds, is called the OGLE Island.
The observations were made with the 1.3-m Warsaw telescope at the Las Campanas Observatory in Chile. The OGLE project started regular observations in 1992 as one of the first-generation micro-lensing projects dedicated to detecting and characterizing micro-lensing events. During its 22-year operation OGLE has gradually evolved and conducted numerous projects that contribute to many fields of modern astrophysics. The current, fourth, phase of the OGLE survey started in 2010 March; the Galactic bulge and disc, the Magellanic Clouds and the Magellanic Bridge, including the areas around them, are the primary fields being observed. The OGLE scientists present density maps of stellar populations in the entire Magellanic Bridge region which show in detail the extent of those populations, which should provide valuable input information for models of past interaction between the Milky Way and the Magellanic Clouds. Density maps have been constructed for three key stellar populations -- young, intermediate-age, and old populations, the last being represented by the 'red clump' and the red-giant-branch stars. The density map confirms that the majority of young stars are found in the western part of the classical Bridge, but, importantly, shows that the young population is also present in the eastern part of the Bridge region, which was not observed before, so there is a continuous stream of young stars connecting the two galaxies.
UNUSUAL DEATH FOR ANCIENT STARS
DOE/Lawrence Berkeley National Laboratory
Certain primordial stars, vastly more massive than any existing now, may have died unusually. In death, those objects -- among the Universe's first-generation of stars -- would have exploded as supernovae and burned completely, leaving no remnant black hole behind. First-generation stars are especially interesting because they produced the first heavy elements, or chemical elements other than hydrogen and helium. In death, they sent their chemical creations out into space, paving the way for subsequent generations of stars and planetary systems. With a greater understanding of how the first stars died, scientists hope to glean some insights about how the Universe, as we know it today, came to be. The researchers found that there is a narrow window where super-massive stars could explode completely instead of becoming super-massive black holes.
To model the evolution of a primordial super-massive star, scientists used a one-dimensional stellar-evolution code called KEPLER. It takes into account key processes like nuclear burning and stellar convection and, relevant for massive stars, photo-disintegration of elements, electron-positron pair production and special-relativistic effects. The team also included general-relativistic effects, which are important for stars above 1,000 solar masses. They found that primordial stars between 55,000 and 56,000 solar masses live about 1.69 million years before becoming unstable owing to general-relativ-istic effects and then start to collapse. As the star collapses, it rapidly synthesizes heavy elements like oxygen, neon, magnesium and silicon, starting with helium in its core. The process releases more energy than the binding energy of the star, halting the collapse and causing a massive explosion -- a supernova. To model the death mechanisms of such stars, the team used CASTRO -- a multi-dimensional compressible astrophysics code developed at Berkeley. The simulations show that once the collapse is reversed, instabilities mix heavy elements produced in the star's final moments throughout the star itself. The researchers say that that mixing should create a distinct observational signature that could be detected by upcoming near-infra-red experiments such as ESA's Euclid and NASA's Wide-Field Infrared Survey Telescope. Depending on the intensity of the supernovae, some super-massive stars could, when they explode, enrich their entire host galaxies and even some 'nearby' galaxies with elements ranging from carbon to silicon. In some cases, supernovae might even trigger bursts of star formation in their host galaxies, which would make them visually distinct from other young galaxies.
Bulletin compiled by Clive Down
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