Topic: Early May Astronomy Bulletin

[Clive]

MILLIMETRE-SIZED STONES FORMED OUR PLANET

Lund University

Fragments of asteroids regularly land on Earth as meteorites. If you examine such a find, you can see that it comprises millimetre-sized round stones, known as chondrules. Those small particles are believed to be the original building blocks of the Solar System. However, the research community has not previously been able to explain how the chondrules formed asteroids. A new study shows that asteroids were formed by gravitational capturing of chondrules. The chondrules are of a size to be slowed down by the gas that orbited the young Sun, and they could then be captured by the asteroids' gravity. That would cause them to fall down and accumulate like sand piling up in a sandstorm. Researchers have developed a computer simulation for what the process may have looked like. They assumed that the asteroids were formed in a kind of cosmic ocean of chondrules and that the asteroids started out much smaller than they are today. According to the simulations, the asteroids grew quickly to a diameter of up to 1,000 km, the same size as those found today in the asteroid belt between Mars and Jupiter. The largest asteroids continued to grow to the same mass as the planet Mars, which has 10% of the mass of the Earth.

The research community had previously believed that the Earth was formed through collisions between protoplanets, of the size of Mars, over a period of 100 million years. However, the researchers have not yet understood how the protoplanets themselves were formed. The study shows that protoplanets may have formed very quickly from asteroids, by capturing chondrules in the same way as the asteroids did. The researchers' theory is supported by studies of meteorites from Mars.  Those studies have previously shown that Mars was formed over a period of only 1-3 million years, which is within the same time span as the researchers have obtained in the computer simulation. Traces of that process remain in asteroids that still contain intact chondrules. The terrestrial planets, however, have all melted after their birth and therefore do not show any direct traces of their original building blocks.

TITAN'S ATMOSPHERE FLUCTUATES WITH SOLAR CYCLE

American Geophysical Union

Saturn's moon Titan is the only moon in the Solar System that has an atmosphere as thick as the Earth's; it consists of more than 98% nitrogen, about 1.4% of methane, and small amounts of other gases.  The Cassini satellite has been circling Saturn since 2004, witnessing more than one-third of its 29-year orbit around the Sun, allowing it to observe the changing of the seasons. However, a new study finds that the seasons are not the only thing changing Titan's atmosphere: its chemical makeup fluctuates according to the Sun's 11-year cycle of magnetic activity. Researchers analyzed data from 41 fly-bys of Titan, some at altitudes of less than 1000 km, when Cassini dipped into the upper fringes of its atmosphere. They found that the amount of methane there varied wildly over time -- it dipped from mid-2006 to 2008, then gradually recovered for two years, but crashed to roughly half of its 2006 peak by 2011. Those fluctuations correspond neatly to the 11-year solar cycle, in which the Sun's rotation gradually winds up its magnetic field into contorted coils, giving rise to flares and sunspots that emit ultraviolet and X-ray light. On Titan, that high-energy radiation can tear methane molecules apart.

After reviewing the Cassini data, the authors think that such destruction of methane occurred from 2006 to 2008 during the last phases of the previous solar maximum. Upon reaching solar minimum in 2008, the quietness of the Sun allowed Titan's methane to recover its levels. Then, as the Sun once again began gearing up toward its most recent solar maximum in 2013, methane levels declined. That case is bolstered by data from the previous mission to make such measurements -- the Voyager 1 spacecraft, which swooped by Titan in 1980 during solar-maximum conditions and found similarly depleted levels of methane. By using one- and three-dimensional models, it was possible to trace the movements of the different chemicals through Titan's atmosphere. During solar maximum, the broken-down methane remnants combine to form heavier hydrocarbons that rain down through the atmosphere. During solar minimum, the replenishment of methane in Titan's upper atmosphere comes from its lower layers. Although it takes only weeks for increased solar radiation to deplete Titan's methane level, it takes years for it to recover. It is predicted that the methane level will not reach its previous peak until some time this year.

POSSIBLE POLAR CAP DETECTED ON PLUTO

NASA

Images from the 'New Horizons' spacecraft are beginning to show detail on the surface of Pluto -- the primary target of the close flyby in mid-July. The images were obtained in early to mid-April from about 100 million kilometres with the telescopic 'Long-Range Reconnaissance Imager' (LORRI) camera on New Horizons. They show broad surface markings, including a bright area at one pole that may be a polar cap. Also seen in the images is Pluto's largest moon, Charon, revolving in a 6.4-day orbit. The exposure times used for the images -- a tenth of a second -- were too short for the camera to detect Pluto's four other, much smaller and fainter, moons. Since it was discovered in 1930, Pluto has remained an enigma. It orbits the Sun at a distance of more than 5000 million km, and researchers have struggled to discern any details on its surface. The images that the spacecraft returns will improve dramatically as New Horizons approaches its July rendezvous and passes approximately 12,500 km from the surface.

FIRST EXOPLANET LIGHT SPECTRUM

ESO

Astronomers using the HARPS instrument on the ESO 3.6-m telescope at the La Silla Observatory in Chile have made the first-ever direct detection of the spectrum of visible light reflected off an exoplanet.  The object concerned is 51 Pegasi b, some 50 light-years away in the constellation Pegasus. It was discovered in 1995 and will always be remembered as the first confirmed exoplanet to be found orbiting an ordinary star like the Sun. It is also regarded as the archetypal 'hot Jupiter' -- a class of planets now known to be relatively commonplace, which are comparable in size and mass with Jupiter, but orbit much closer to their parent stars. Since that landmark discovery, more than 1900 exoplanets in 1200 planetary systems have been confirmed, but now 51 Pegasi b returns to the spotlight in another advance in exoplanet studies. Currently, the most widely used method to examine an exoplanet's atmosphere is to observe the host star's spectrum as it is filtered through the planet's atmosphere during transit; another approach is to observe the system when the star passes in front of the planet, which primarily provides information about the exoplanet's temperature. Of course, such observations are possible only if we happen to see the system so nearly in the plane of the planet's orbit that transits take place.

The new technique does not depend on finding a planetary transit, and so can potentially be used to study many more exoplanets. It allows the planetary spectrum to be detected directly in visible light, so characteristics of the planet that are inaccessible to other techniques can be inferred. The host star's spectrum is used as a template to guide a search for a similar signature of light that is expected to be reflected off the planet as it describes its orbit.  That is an exceedingly difficult task, as planets are very dim in comparison with their parent stars. The signal from the planet is also easily swamped by other tiny effects and sources of noise. In the face of such adversity, the success of the technique when applied to the HARPS data collected on 51 Pegasi b provides a valuable proof of concept. The technique is of great scientific importance, as it allows astronomers to measure the planet's mass and orbital inclination, which is essential to an understanding of the system. Italso allows an estimate of the planet's reflectivity, or albedo, which can be used to infer the composition of both the planet's surface and atmosphere. 51 Pegasi b was found to have a mass about half that of Jupiter and an orbit with an inclination of about 9 degrees to the direction to the Earth. The planet also seems to be larger than Jupiter in diameter and to be highly reflective. Those are typical properties for a hot Jupiter that is very close to its parent star and exposed to intense starlight. [But if this is the first one to be deternined, how does the author of this article know what is typical?  -- ED.] HARPS was essential to the team's work, but the fact that the result was obtained with a 3.6-m telescope, which has a limited range of application with this technique, is exciting news for astronomers, who may look forward to more advanced instruments on larger telescopes, such as the future European 'Extremely Large Telescope'.

NEARBY PLANETARY SYSTEM DISCOVERED ROBOTICALLY

University of Hawaii at Manoa

Astronomers using telescopes in Hawaii, California and Arizona recently discovered a planetary system orbiting a star that is 'only' 54 light-years away. There are three planets that all orbit their star at a distance closer than Mercury orbits the Sun, completing their orbits in just 5, 15 and 24 days. Astronomers found them from measurements by the Automated Planet Finder (APF) telescope at Lick Observatory in California, the Keck Observatory on Mauna Kea,  Hawaii, and the Automatic Photometric Telescope (APT) at Fairborn Observatory in Arizona. The team detected the wobble of the star HD 7924 as the planets orbited and pulled on the star gravitationally.  APF and Keck Observatory traced out the planets' orbits over several years using the Doppler technique that has successfully found hundreds of mostly larger planets orbiting nearby stars. APT made measurements of the brightness of HD 7924 to assure the validity of the planet discoveries. The new APF facility offers a way to speed up the planet search. Planets can be discovered and their orbits traced much more quickly because APF is a dedicated facility that robotically searches for planets every clear night. Training computers to run the observatory all night, without human oversight, took years of effort by the University of California staff and graduate students on the discovery team.

The Keck Observatory found the first evidence of planets orbiting HD 7924, discovering the innermost planet in 2009 with the HIRES instrument installed on the 10-m Keck I telescope. The Kepler space telescope has discovered thousands of extrasolar planets and demonstrated that they are common in our Galaxy. However, nearly all of those planets are relatively large and far from our Solar System.  Most 'nearby' stars have not been thoroughly searched for the small 'super-Earth' planets (larger than the Earth but smaller than Neptune) that Kepler found in great abundance. The new discovery shows the type of planetary system that astronomers expect to find around many 'nearby' stars in the coming years. The three planets are unlike anything in the Solar System, with masses 7-8 times the mass of the Earth and orbits very close to their host star. The robotic observations of HD 7924 are the start of a systematic survey for super-Earth planets orbiting nearby stars. When the survey is complete we will have a census of small planets orbiting Sun-like stars within approximately 100 light-years of the Sun.

ASTRONOMERS FIND RUNAWAY GALAXIES

Harvard-Smithsonian Center for Astrophysics

We know of about two dozen runaway stars, and have even found one runaway star cluster escaping from its galaxy for ever. Now, astronomers have observed 11 runaway galaxies that have been flung out of their clusters into intergalactic space. An object is a runaway if it is moving faster than the escape velocity, so it leaves its home never to return. In the case of a runaway star in our Galaxy, that speed is about 500 km/s. To run away from a cluster of galaxies, a runaway galaxy has to go faster, travelling at up to 3,000 km/s.  The team initially set out to identify new members of a class of galaxies called compact ellipticals, which are bigger than star clusters but smaller than a typical galaxy, spanning 'only' a few hundred light-years. (For comparison, the Milky Way is 100,000 light-years across.) Compact ellipticals also have masses 1000 times less than that of a galaxy like our Milky Way. Before this study, only about 30 compact elliptical galaxies were known, all of them residing in galaxy clusters. To locate new examples researchers sorted through public archives of data from the Sloan Digital Sky Survey and the GALEX satellite. Their search identified almost 200 previously unknown compact ellipticals. Of those, 11 were completely isolated and found far from any large galaxy or cluster of galaxies.

Such isolated compact galaxies were unexpected because theorists thought that they originated from larger galaxies that had been stripped of most of their stars through interactions with an even bigger galaxy. So the compact galaxies should all be found near big galaxies. Not only were the new-found compact ellipticals isolated, but also they were moving faster than galaxies in clusters.  Astronomers asked what else could explain them. The answer was a classic three-body interaction. A hyper-velocity star can be created if a binary-star system passes close to the black hole at the centre of our Galaxy. One star gets captured while the other is thrown out at tremendous speed. Similarly, a compact elliptical could be paired with the big galaxy that stripped it of its stars. Then a third galaxy passes and flings the compact elliptical away, before getting accreted by the remaining big galaxy. This discovery represents a prominent success of the Virtual Observatory -- a project to make data from large astronomical surveys easily available to researchers.  So-called 'data mining' can result in finds never anticipated when the original data were collected.

SUPERNOVA IS 'MISSING LINK'

National Radio Astronomy Observatory

Astronomers using the Very Large Array (VLA) radio telescope have found a long-sought 'missing link' between supernova explosions that generate gamma-ray bursts (GRBs) and those that don't. The scientists found that a stellar explosion seen in 2012 has many characteristics expected of one that generates a powerful burst of gamma rays, yet no such burst occurred. The object, called SN 2012ap, is what astronomers term a core-collapse supernova. That type of blast occurs when the nuclear fusion reactions at the core of a very massive star no longer can provide the energy needed to hold up the core against the weight of the outer parts of the star. The core then collapses catastrophically into a super-dense neutron star or a black hole. The rest of the star's material is blasted into space in a supernova explosion.  The most common type of such a supernova blasts the star's material outward in a nearly-spherical bubble that expands rapidly, but at speeds far less than that of light. Such explosions produce no burst of gamma rays, but in a small percentage of cases, the infalling material is drawn into a short-lived swirling disc surrounding the new neutron star or black hole. The disc generates jets of material that move outwards from its poles at speeds approaching that of light, and it is that type of explosion that produces gamma-ray bursts. The disc and jets together have been called the supernova's 'engine'.

The new research shows, however, that not all such supernova explosions produce gamma-ray bursts. SN2012ap had jets moving at nearly the speed of light, and those jets were quickly slowed down, just like the jets we see in gamma-ray bursts. An earlier supernova seen in 2009 also had fast jets, but its jets expanded freely, without experiencing the slowdown characteristic of those that generate gamma-ray bursts. The free expansion of the 2009 object, the scientists said, is more like what is seen in  supernova explosions with no engine, and probably indicates that its jet contained a large percentage of heavy particles, as opposed to the lighter particles in gamma-ray-burst jets. The heavy particles more easily punch through the material surrounding the star. What we see is that there is a wide diversity in the engines in this type of supernova explosion.  Those with strong engines and lighter particles produce gamma-ray bursts, and those with weaker engines and heavier particles don't.  2012ap tends to show that the nature of the engine plays a central role in determining the characteristics of such a supernova explosion.

CLUSTERS OF MONSTER STARS LIT UP EARLY UNIVERSE

RAS

The first stars in the Universe were born several hundred million years after the Big Bang, ending a period known as the cosmological 'dark ages' -- when atoms of hydrogen and helium had formed, but nothing shone in visible light. Now two Canadian researchers have calculated what the first stars were like: they find that they could have clustered together in phenomenally bright groups, with periods when they were as luminous as 100 million Suns. The scientists modelled how the luminosity of the stars would have changed as they formed from the gravitational collapse of discs of gas. The early evolution turns out to be chaotic, with clumps of material forming and spiralling into the centre of the discs, creating bursts of luminosity a hundred times brighter than average. Those first stars would have been at their brightest when they were 'protostars', still forming and pulling in material. In a small cluster of even 10 to 20 protostars, the ongoing bursts would mean that the cluster would have long periods of enhanced brightness. According to the simulation, every so often a cluster of 16 protostars could have its luminosity increase by a factor of up to 1000, to an extraordinary 100 million times the brightness of the Sun.

The earliest stars had very short lives and produced the first heavy elements such as carbon and oxygen. Light from such stars has travelled towards us for almost 13 billion years, so to observers on Earth they look very faint and also have the wavelengths of their light stretched out into the infrared by the expansion of the Universe.  That makes those stars very hard to observe, but the next-generation James Webb Space Telescope (JWST) will survey the skies to look for them. Although the luminosity of an individual first star is probably too faint for JWST to spot it, the new work suggests that clusters of the first protostars could be prominent beacons in the early Universe.

GIANT COSMIC TSUNAMI WAKES UP GALAXIES

RAS

Galaxies are often found in clusters, which contain many 'red and dead' members that stopped forming stars in the distant past.  Now an international team of astronomers has discovered that such comatose galaxies can sometimes come back to life. If clusters of galaxies merge, a huge shock-wave can drive the birth of a new generation of stars -- the sleeping galaxies get a new lease of life.  Galaxy clusters are like cities, where thousands of galaxies can be packed together, at least in comparison to the sparsely-populated space around them. Over billions of years, they build up structure in the Universe -- merging with adjacent clusters, as growing cities absorb nearby towns. When that happens, there is a huge release of energy as the clusters collide. The resulting shock-wave travels through the cluster like a tsunami, but until now there was no evidence that the galaxies themselves were affected very much. The team observed the merging galaxy cluster CIZA J2242.8+5301, located 2.3 billion light-years away in the direction of the constellation Lacerta. They used the Isaac Newton and William Herschel telescopes on La Palma, and the Subaru, CFHT and Keck telescopes on Hawaii, and found that the cluster galaxies were transformed by the shock-wave, triggering a new wave of star formation.

The new work implies that the merger of galaxy clusters has a major impact on the formation of stars. Much like a teaspoon stirring a mug of coffee, the shocks lead to turbulence in the galactic gas. That then triggers an avalanche-like collapse, which eventually leads to the formation of very dense, cold gas clouds, which are vital for the formation of new stars. Star formation at that rate leads to a lot of massive, short-lived stars coming into being, which explode as supernovae a few million years later. The explosions drive huge amounts of gas out of the galaxies, and, with most of the rest consumed in star formation, the galaxies soon run out of fuel. After a long enough time, the cluster mergers make the galaxies even more red and dead -- they slip back into a coma and have little prospect of a second resurrection. Every cluster of galaxies in the 'nearby' Universe has experienced a series of mergers during its lifetime, so they should all have passed through a period of extremely vigorous production of stars. Given that the shocks will, however, lead to only a brief (in astronomical terms) increase in star formation, astronomers have to be very lucky to catch the cluster at a time in its evolution when the galaxies are still being 'lit up' by the shock.