Universe Today

Artist’s impression of the explosion of RS Ophiuchi. Image credit: David A. Hardy. Click to enlarge
Astronomers recently noticed that the normally dim star RS Ophiuchi had brightened enough to be visible without a telescope. This white dwarf star has brightened like this 5 times in the last 100 years, and astronomers believe it’s about to collapse into a neutron star. RS Ophiuchi is in a binary system with a much larger red giant star. The two stars are so close that the white dwarf is actually inside the envelope of the red giant, and explodes from within it every 20 years or so.

On 12 February 2006, amateur astronomers reported that a faint star in the constellation of Ophiuchus had suddenly become clearly visible in the night sky without the aid of a telescope. Records show that this so-called recurrent nova, RS Ophiuchi (RS Oph), has previously reached this level of brightness five times in the last 108 years, most recently in 1985. The latest explosion has been observed in unprecedented detail by an armada of space- and ground-based telescopes.

Speaking today (Friday) at the RAS National Astronomy Meeting at Leicester, Professor Mike Bode of Liverpool John Moores University and Dr Tim O’Brien of Jodrell Bank Observatory will present the latest results which are shedding new light on what happens when stars explode.

RS Oph is just over 5,000 light years away from Earth. It consists of a white dwarf star (the super-dense core of a star, about the size of the Earth, that has reached the end of its main hydrogen-burning phase of evolution and shed its outer layers) in close orbit with a much larger red giant star.

The two stars are so close together that hydrogen-rich gas from the outer layers of the red giant is continuously pulled onto the dwarf by its high gravity. After around 20 years, enough gas has been accreted that a runaway thermonuclear explosion occurs on the white dwarf’s surface. In less than a day, its energy output increases to over 100,000 times that of the Sun, and the accreted gas (several times the mass of the Earth) is ejected into space at speeds of several thousand km per second.

Five explosions such as this per century can only be explained if the white dwarf is near the maximum mass it could have without collapsing to become an even denser neutron star.

What is also very unusual in RS Oph is that the red giant is losing enormous amounts of gas in a wind that envelops the whole system. As a result, the explosion on the white dwarf occurs “inside” its companion’s extended atmosphere and the ejected gas then slams into it at very high speed.

Within hours of notification of the latest outburst of RS Oph being relayed to the international astronomical community, telescopes both on the ground and in space swung into action. Among these is NASA’s Swift satellite which, as its name suggests, can be used to react rapidly to things that change in the sky. Included in its armoury of instruments is an X-ray Telescope (XRT), designed and built by the University of Leicester.

“We realised from the few X-ray measurements taken late in the 1985 outburst that this was an important part of the spectrum in which to observe RS Oph as soon as possible,” said Professor Mike Bode of Liverpool John Moores University, who led the observing campaign for the 1985 outburst and now heads the Swift follow-up team on the current explosion.

“The expectation was that shocks would be set up both in the ejected material and in the red giant’s wind, with temperatures initially of up to around 100 million degrees Celsius – nearly 10 times that in the core of the Sun. We have not been disappointed!”

The first observations by Swift, only three days after the outburst began, revealed a very bright X-ray source. Over the initial few weeks, it became even brighter and then began to fade, with the spectrum suggesting that the gas was cooling down, although still at a temperature of tens of millions of degrees. This was exactly what was expected as the shock pushed into the red giant’s wind and slowed down. Then something remarkable and unexpected happened to the X-ray emission.

“About a month after the outburst, the X-ray brightness of RS Oph increased very dramatically,” explained Dr. Julian Osborne of the University of Leicester. “This was presumably because the hot white dwarf, which is still burning nuclear fuel, then became visible through the red giant’s wind.

“This new X-ray flux was extremely variable, and we were able to see pulsations which repeat every 35 seconds or so. Although it is very early days, and data are still being taken, one possibility for the variability is that this is due to instability in the nuclear burning rate on the white dwarf.”

Meanwhile, observatories working at other wavelengths changed their programmes to observe the event. Dr. Tim O’Brien of Jodrell Bank Observatory, who did his PhD thesis work on the 1985 explosion, and Dr. Stewart Eyres of the University of Central Lancashire, lead the team that is securing the most detailed radio observations to date of such an event.

“In 1985, we were not able to begin observing RS Oph until nearly three weeks after the outburst, and then with facilities that were far less capable than those available to us today,” said Dr. O’Brien.

“Both the radio and X-ray observations from the last outburst gave us tantalising glimpses of what was happening as the outburst evolved. In addition, this time, we have developed very much more advanced computer models. The combination of the two now will undoubtedly lead to a greater understanding of the circumstances and consequences of the explosion.

“In 2006, our first observations with the UK’s MERLIN system were made only four days after the outburst and showed the radio emission to be much brighter than expected,” added Dr. Eyres. “Since then it has brightened, faded, then brightened again. With radio telescopes in Europe, North America and Asia now monitoring the event very closely, this is our best chance yet of understanding what is truly going on.”

Optical observations are also being obtained by many observatories around the globe, including the robotic Liverpool Telescope on La Palma. Observations are also being conducted at the longer wavelengths of the infrared part of the spectrum.

“For the first time we are able to see the effects of the explosion and its aftermath at infrared wavelengths from space, with NASA’s Spitzer Space Telescope,” said Professor Nye Evans of Keele University, who heads the infrared follow-up team.

“Meanwhile, the observations we have already obtained from the ground, from the United Kingdom Infrared Telescope on the summit of Mauna Kea in Hawaii, already far surpass the data we had during the 1985 eruption.

“The shocked red giant wind and the material ejected in the explosion give rise to emission not only at X-ray, optical and radio wavelengths, but also in the infrared, via coronal lines (so-called because they are prominent in the Sun’s very hot corona). These will be crucial in determining the abundances of the elements in the material ejected in the explosion and in confirming the temperature of the hot gas.”

26 February 2006 was a highlight of the observational campaign. In what must surely be a unique event, four space satellites, plus radio observatories around the globe, observed RS Oph on the same day.

“This star could not have exploded at a better time for international ground and space based studies of an event which has been changing every time we look at it,” said Professor Sumner Starrfield of Arizona State University, who heads the U.S. side of the collaboration. “We are all very excited and exchanging many emails every day trying to understand what is happening on that day and then predict the behaviour on the next.”

What is apparent is that RS Oph is behaving like a “Type II” supernova remnant. Type II supernovae represent the catastrophic death of a star at least 8 times the mass of the Sun. They also eject very high velocity material which interacts with their surroundings. However, the full evolution of a supernova remnant takes tens of thousands of years. In RS Oph, this evolution is literally occurring before our eyes, around 100,000 times faster.

“In the 2006 outburst of RS Oph, we have a unique opportunity of understanding much more fully such things as runaway thermonuclear explosions and the end-points of the evolution of stars,” said Professor Bode.

“With the observational tools now at our disposal, our efforts 21 years ago look rather primitive by comparison.”

Original Source: RAS News Release

Artist illustration of a planetary disk forming around a pulsar. Image credit: NASA/JPL. Click to enlarge.
Think planets can only form around stars? Well, think again. NASA’s Spitzer Space Telescope has uncovered evidence for a potential planet-forming disk around a pulsar. In a former life, the pulsar would have been a large star 10-20 times bigger than the Sun that eventually consumed its fuel and exploded as a supernova. The remaining debris has started to collect again, and could eventually turn into new planets. This helps explain how planets were discovered around another pulsar in 1992, including one that’s Earth-sized.

NASA’s Spitzer Space Telescope has uncovered new evidence that planets might rise up out of a dead star’s ashes.

The infrared telescope surveyed the scene around a pulsar, the remnant of an exploded star, and found a surrounding disk made up of debris shot out during the star’s death throes. The dusty rubble in this disk might ultimately stick together to form planets.

This is the first time scientists have detected planet-building materials around a star that died in a fiery blast.

“We’re amazed that the planet-formation process seems to be so universal,” said Dr. Deepto Chakrabarty of the Massachusetts Institute of Technology in Cambridge, principal investigator of the new research. “Pulsars emit a tremendous amount of high energy radiation, yet within this harsh environment we have a disk that looks a lot like those around young stars where planets are formed.”

A paper on the Spitzer finding appears in the April 6 issue of Nature. Other authors of the paper are lead author Zhongxiang Wang and co-author David Kaplan, both of the Massachusetts Institute of Technology.

The finding also represents the missing piece in a puzzle that arose in 1992, when Dr. Aleksander Wolszczan of Pennsylvania State University found three planets circling a pulsar called PSR B1257+12. Those pulsar planets, two the size of Earth, were the first planets of any type ever discovered outside our solar system. Astronomers have since found indirect evidence the pulsar planets were born out of a dusty debris disk, but nobody had directly detected this kind of disk until now.

The pulsar observed by Spitzer, named 4U 0142+61, is 13,000 light-years away in the Cassiopeia constellation. It was once a large, bright star with a mass between 10 and 20 times that of our sun. The star probably survived for about 10 million years, until it collapsed under its own weight about 100,000 years ago and blasted apart in a supernova explosion.

Some of the debris, or “fallback,” from that explosion eventually settled into a disk orbiting the shrunken remains of the star, or pulsar. Spitzer was able to spot the warm glow of the dusty disk with its heat-seeking infrared eyes. The disk orbits at a distance of about 1 million miles and probably contains about 10 Earth-masses of material.

Pulsars are a class of supernova remnants, called neutron stars, which are incredibly dense. They have masses about 1.4 times that of the sun squeezed into bodies only 10 miles wide. One teaspoon of a neutron star would weigh about 2 billion tons. Pulsar 4U 0142+61 is an X-ray pulsar, meaning that it spins and pulses with X-ray radiation.

Any planets around the stars that gave rise to pulsars would have been incinerated when the stars blew up. The pulsar disk discovered by Spitzer might represent the first step in the formation of a new, more exotic type of planetary system, similar to the one found by Wolszczan in 1992.

“I find it very exciting to see direct evidence that the debris around a pulsar is capable of forming itself into a disk. This might be the beginning of a second generation of planets,” Wolszczan said.

Pulsar planets would be bathed in intense radiation and would be quite different from those in our solar system. “These planets must be among the least hospitable places in the galaxy for the formation of life,” said Dr. Charles Beichman, an astronomer at NASA’s Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena, Calif.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech. Spitzer’s infrared array camera, which made the pulsar observations, was built by NASA’s Goddard Space Flight Center, Greenbelt, Md. The instrument’s principal investigator is Dr. Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics.

For more information about Spitzer, visit:

http://www.spitzer.caltech.edu/spitzer/

Original Source: NASA/JPL News Release

Evolution of the impact three hours from the time of collision. Image credit: Horner et Al. Click to enlarge
According to current models of planetary formation, Mercury has too much mass. A new explanation proposes that Mercury was created from a much larger parent planet that collided with a giant asteroid 4.5 billion years ago. Astronomers from the University of Bern ran various scenarios modeling early versions of Mercury. This scenario of an early cataclysm best matched the current mass and composition of Mercury. Some of the ejected material would have made it all the way to Venus and even to the Earth.

A New computer simulations of Mercury’s formation show the fate of material blasted out into space when a large proto-planet collided with a giant asteroid 4.5 billion years ago. The simulations, which track the material over several million years, shed light on why Mercury is denser than expected and show that some of the ejected material would have found its way to the Earth and Venus.

“Mercury is an unusually dense planet, which suggests that it contains far more metal than would be expected for a planet of its size. We think that Mercury was created from a larger parent body that was involved in a catastrophic collision, but until these simulations we were not sure why so little of the planet’s outer layers were reaccreted following the impact,” said Dr Jonti Horner, who is presenting results at the Royal Astronomical Society’s National Astronomy Meeting on 5th April.

To solve this problem, Dr Horner and his colleagues from the University of Bern ran two sets of large-scale computer simulations. The first examined the behaviour of the material in both the proto-planet and the incoming projectile; these simulations were among the most detailed to date, following a huge number of particles and realistically modelling the behaviour of different materials inside the two bodies. At the end of the first simulations, a dense Mercury-like body remained along with a large swathe of rapidly escaping debris. The trajectories of the ejected particles were then fed in to a second set of simulations that followed the motion of the debris for several million years. Ejected particles were tracked until either they landed on a planet, were thrown into interstellar space, or fell into the Sun. The results allowed the group to work out how much material would have fallen back onto Mercury and investigate other ways in which debris is cleared up in the Solar System.

The group found that the fate of the debris depended on whereabouts Mercury was hit, both in terms of its orbital position and in terms of the angle of the collision.

Whilst purely gravitational theory suggested that a large fraction of the debris would eventually fall back onto Mercury, the simulations showed that it would take up to 4 million years for 50% of the particles to land back on the planet and in this time many would be carried away by solar radiation. This explains why Mercury retained a much smaller proportion than expected of the material in its outer layers.

The simulations also showed that some of the ejected material made its way to Venus and the Earth. While this is only a small fraction, it illustrates that material can be transferred between the inner planets relatively easily. Given the amount of material that would have been ejected in such a catastrophe, it is likely that there is a reasonable amount (possibly as much as 16 million billion tonnes [1.65×10^19 kg]) of proto-Mercury in the Earth.

Original Source: RAS News Release

The cloud, where OH maser filament are red and extended methanol filaments are green. Image credit: JIVE Click to enlarge
Astronomers have located a gigantic cloud of methyl alcohol surrounding a stellar nursery. The cloud measures half a trillion km across (300 billion miles), and could help astronomers understand how some of the most massive stars in the Universe are formed. It’s methanol, not ethanol, so you wouldn’t want to drink it if you could reach it.

Astronomers based at Jodrell Bank Observatory have discovered a giant bridge of methyl alcohol, spanning approximately 288 billion miles, wrapped around a stellar nursery. The gas cloud could help our understanding of how the most massive stars in our galaxy are formed.

The new observations were taken with the UK’s MERLIN radio telescopes, which have recently been upgraded. The team studied an area called W3(OH), a region in our galaxy where stars are being formed by the gravitational collapse of a cloud of gas and dust. The observations have revealed giant filaments of gas that are emitting as ‘masers’ (molecules in the gas are amplifying and emitting beams of microwave radiation in much the same way as a laser emits beams of light).

The filaments of masing gas form giant bridges between maser ‘spots’ in W3(OH) that had been observed previously. The largest of these maser filaments is 288 billion miles (463 billion km) long. Observations show that the entire gas cloud appears to be rotating as a disc around a central star, in a similar manner to the accretion discs in which planets form around young stars. The maser filaments occur at shock boundaries where large regions of gas are colliding.

“Our discovery is very interesting because it challenges some long-accepted views held in astronomical maser research. Until we found these filaments, we thought of masers as point-like objects or very small bright hotspots surrounded by halos of fainter emission,” said Dr Lisa Harvey-Smith, who is the Principal Investigator for the study and is presenting results at the Royal Astronomical Society’s National Astronomy Meeting on 4th April.

Since the upgrade of the UK’s MERLIN telescope network, astronomers have been able to image methanol masers with a much higher sensitivity and, for the first time, get a complete picture of all the radiation surrounding maser sources. In the new study, the Jodrell Bank team looked at the motion of the W3(OH) star forming region in 3-dimensions and also measured physical properties of the gas such as temperature, pressure and the strength and direction of the magnetic fields. This information is vital when testing theories about how stars are born from the primordial gas in stellar nurseries.

Dr Harvey-Smith said, “There are still many unanswered questions about the birth of massive stars because the formation centres are shrouded by dust. The only radiation that can escape is at radio wavelengths and the upgraded MERLIN network is now giving us the first opportunity to look deep into these star forming regions and see what’s really going on.”

The many different types of interactions between molecules in star forming regions lead to emissions in many different wavelengths. Future observations of masers at other frequencies are planned to complete the complex jigsaw puzzle that has now been revealed.

Dr Harvey-Smith adds, “Although it is exciting to discover a cloud of alcohol almost 300 billion miles across, unfortunately methanol, unlike its chemical cousin ethanol, is not suitable for human consumption!”

Original Source: RAS News Release

Galaxies are not randomly distributed. Image credit: IAC Click to enlarge
Although the galaxies we see in the night sky seem randomly strewn across the heavens, they’re actually organized into large scale structures that look like cosmic filaments. These filaments and walls surround huge bubble-like voids that lack any large structures at all. European astronomers measured the orientation of thousands of galaxies, and found that many are oriented in the direction of these linear filaments.

Astronomers from the University of Nottingham, UK, and the Instituto de Astrofisica de Canarias (Spain), have found the first observational evidence that galaxies are not randomly oriented.

Instead, they are aligned following a characteristic pattern dictated by the large-scale structure of the invisible dark matter that surrounds them.

This discovery confirms one of the fundamental aspects of galaxy formation theory and implies a direct link between the global properties of the Universe and the individual properties of galaxies.

Galaxy formation theories predicted such an effect, but its empirical verification has remained elusive until now. The results of this work were published the 1 April issue of Astrophysical Journal Letters.

Nowadays, matter is not distributed uniformly throughout space but is instead arranged in an intricate “cosmic web” of filaments and walls surrounding bubble-like voids. Regions with high galaxy concentrations are known as galaxy clusters whereas low density regions are termed voids.

This inhomogeneous distribution of matter is called the “Large-scale distribution of the Universe.” When the Universe is considered as whole, this distribution has a similar appearance to a spider’s web or the neural network of the brain. But it was not always like this.

After the Big Bang, when the Universe was much younger, matter was distributed homogeneously. As the Universe was evolving, gravitational pulls began to compress the matter in certain regions of space, forming the large-scale structure that we currently observe.

According to these models and theories a direct consequence of this process is that galaxies should be preferentially oriented perpendicularly to the direction of the linear filaments.

Several observational studies have looked for a preferential spatial orientation (or alignment) of galaxy rotation axes with respect to their surrounding large-scale structures. However, none of them have been successful, due to the difficulties associated with trying to characterise the filaments.

The research conducted by the astrophysical group formed by Ignacio Trujillo (University of Nottingham, UK), Conrado Carretero and Santiago G. Patiri, (both from the Instituto de Astrofisica de Canarias, Spain) has been able to measure this effect, confirming theoretical predictions.

To achieve this goal, they used a new technique based on the analysis of the huge voids that are found in the large-scale structure of the Universe. These voids have been detected by searching for large regions of space depleted of bright galaxies.

In addition, they took advantage of information provided by the two largest sky surveys yet undertaken: the Sloan Digital Sky Survey and the Two Degree Field Survey. These surveys contain positional information for more than half a million galaxies located within a distance of one billion light-years of the Earth.

Other parameters provided by the surveys, such as the position angle and the ellipticity of the objects, were used to estimate the orientation of the disk galaxies.

“We found that there is an excess of disk galaxies that are highly inclined relative to the plane defined by the large-scale structure surrounding them,” explained Dr. Trujillo. “Their rotation axes are mainly oriented in the direction of the filaments.

“Our work provides important confirmation of the tidal torque theory which explains how galaxies have acquired their current spin,” said Trujillo.

“The spin of the galaxies is believed to be intrinsically linked to their morphological shapes. So, this work is a step forward on our understanding of how galaxies have reached their current shapes.”

Dr. Ignacio Trujillo has a research assistant position, funded by PPARC, in the School of Physics and Astronomy at the University of Nottingham.

An abstract of the paper is available on the web at:
http://xxx.lanl.gov/abs/astro-ph/0511680

Original Source: RAS News Release

Deep Impact. Image credit: NASA. Click to enlarge
When Deep Impact collided with Tempel 1, it released an amazing amount of water vapour from the comet – as much as 250,000 tonnes were blasted into space. These measurements were made by NASA’s Swift satellite, which normally locates and observes gamma ray bursts. Swift, like almost every other telescope on Earth and in space was pointed at Comet Tempel 1 when Deep Impact smashed into it last July. Swift monitored the X-ray emissions before and after the collision, and used that to measure the amount of water vapour ejected.

Over the weekend of 9-10 July 2005 a team of UK and US scientists, led by Dr. Dick Willingale of the University of Leicester, used NASA’s Swift satellite to observe the collision of NASA’s Deep Impact spacecraft with comet Tempel 1. Reporting today (Tuesday) at the UK 2006 National Astronomy Meeting in Leicester, Dr. Willingale revealed that the Swift observations show that the comet grew brighter and brighter in X-ray light after the impact, with the X-ray outburst lasting a total of 12 days.

“The Swift observations reveal that far more water was liberated and over a longer period than previously claimed,” said Dick Willingale.

Swift spends most of its time studying objects in the distant Universe, but its agility allows it to observe many objects per orbit. Dr. Willingale used Swift to monitor the X-ray emission from comet Tempel 1 before and after the collision with the Deep Impact probe.

The X-rays provide a direct measurement of how much material was kicked up after the impact. This is because the X-rays were created by the newly liberated water as it was lifted into the comet’s thin atmosphere and illuminated by the high-energy solar wind from the Sun.

“The more material liberated, the more X-rays are produced,” explained Dr. Paul O’Brien, also from the University of Leicester.

The X-ray power output depends on both the water production rate from the comet and the flux of subatomic particles streaming out of the Sun as the solar wind. Using data from the ACE satellite, which constantly monitors the solar wind, the Swift team managed to calculate the solar wind flux at the comet during the X-ray outburst. This enabled them to disentangle the two components responsible for the X-ray emission.

Tempel 1 is usually a rather dim, weak comet with a water production rate of 16,000 tonnes per day. However, after the Deep Impact probe hit the comet this rate increased to 40,000 tonnes per day over the period 5-10 days after impact. Over the duration of the outburst, the total mass of water released by the impact was 250,000 tonnes.

One objective of the Deep Impact mission was to determine what causes cometary outbursts. A simple theory suggests that such outbursts are caused by the impact of meteorites on the comet nucleus. If this is the case, Deep Impact should have initiated an outburst.

Although the impact was observed across the electromagnetic spectrum, most of what was seen was directly attributable to the impact explosion. After 5 days, optical observations showed that the comet was indistinguishable from its state prior to the collision. This was in stark contrast to the X-ray observations.

The analysis of the X-ray behaviour by the Swift team indicates that the collision produced an extended X-ray outburst largely because the amount of water produced by the comet had increased.

“A collision such as Deep Impact can cause an outburst, but apparently something rather different from the norm can also happen,” said Dr. Willingale. “Most of the water seen in X-rays came out slowly, possibly in the form of ice-covered dust grains.”

Original Source: RAS News Release