Evidence that Brown Dwarfs are Failed Stars

Image credit: UofM

Researchers from the University of Michigan have gathered evidence that brown dwarf stars have a very similar life to the early stages our own Sun went through when it first formed. The astronomers searched for dusty disks around young brown dwarfs by observing their infrared emissions. They found that most brown dwarfs did have disks at a million years old, which is very similar to young stars at the same age. Other observations showed that they accrete material from the disk in the same way stars do as well.

In cosmic circles, brown dwarfs are something of a flop. Too big to be considered true planets, yet not massive enough to be stars, these free-floating celestial bodies are, in fact, sometimes referred to as failed stars. But do they really form as stars do?from collapsing clouds of gas?or are their origins completely different? A series of publications by University of Michigan astronomer Ray Jayawardhana and collaborators, including a paper in the January 16 issue of Science, offers evidence that brown dwarfs and Sun-like stars are born in much the same way. “They at least have very similar infancies, which may mean that they also have very similar origins,” said Jayawardhana, an assistant professor of astronomy.

Stars form in cold clouds of gas and dust in interstellar space. Dense clumps within these clouds contract under their own gravity, spinning up in the process and gathering material from the surroundings into a disk. Eventually, if a growing protostar accumulates enough mass, its core becomes hot and dense enough for nuclear fusion to occur, and the new star begins to shine. Some scientists have suggested that brown dwarfs form the same way but simply don’t accumulate enough mass to ignite hydrogen fusion, and calculations show that it’s at least theoretically possible for objects with masses as low as those of brown dwarfs to be born this way.

But other scientists have proposed that brown dwarfs are runts kicked out of stellar litters. In this scenario, brown dwarfs are born in multiple star systems and compete with their siblings for matter from the natal cloud. In such systems, the slowest-growing object may be ejected before it gathers enough material to become a star, computer simulations suggest.

One way to distinguish between the two possibilities is by studying disks of dust and gas around young brown dwarfs. If brown dwarfs form as stars do, they should have large, long-lived accretion disks like those found around young stars. But if they have been ejected from multiple star systems, their disks should be shaved down by the gravitational interactions that lead to ejection.

Jayawardhana and colleagues searched for dusty disks around young brown dwarfs by observing their infrared emission with the 8-meter Very Large Telescope (VLT) of the European Southern Observatory in Chile and the 10-meter Keck I telescope in Hawaii. Because dust particles in a disk absorb light and re-radiate the energy at infrared wavelengths, a brown dwarf with a disk will emit more infrared light than one without a disk.

“We found that the majority of brown dwarfs are surrounded by dusty disks at an age of a million years or so,” said Jayawardhana. “That’s similar to young stars at the same age.” Although it’s not possible to directly determine the disks’ sizes, their presence around some brown dwarfs as old as 10 million years suggests that they aren’t pared away in early life.

Other spectroscopic observations, using the twin 6.5-meter Magellan telescopes in Chile (in which the University of Michigan is a partner institution) and the Keck I telescope, showed that brown dwarfs also accrete material from surrounding disks the same way as stars do?although at a slower pace. “We detect telltale signs of gas flowing from the inner edge of the disk onto the brown dwarf at velocities of over a hundred kilometers per second,” said Jayawardhana. In one intriguing case, astronomers have alsofound evidence of material spewing out from the poles of a brown dwarf. Such jets have been seen in young stars of the same age, but not until now in brown dwarfs. “If confirmed, the presence of jets would further strengthen the case for remarkably similar infancies for brown dwarfs and Sun-like stars,” said Jayawardhana, whose collaborators include Subhanjoy Mohanty (Harvard-Smithsonian Center for Astrophysics), Gibor Basri (University of California, Berkeley), David Barrado y Navascues (Laboratory of Space Astrophysics and Fundamental Physics in Madrid, Spain), David Ardila (Johns Hopkins University), Beate Stelzer (Astronomical Observatory of Palermo in Italy), and Karl Haisch, Jr. and Diane Paulson (both at the University of Michigan).

“I wouldn’t say that the story is signed, sealed and delivered,” Jayawardhana said, “but the preponderance of evidence is very much leaning in the direction of these things forming the same way as stars.” And the evidence uncovered so far leads to even more tantalizing prospects. “Now that we know many young brown dwarfs are surrounded by disks,” he said, “I can’t help but wonder if comets and asteroids?if not small planets?could form in these disks.”

This research was supported primarily by a grant from the National Science Foundation.

Original Source: University of Michigan News Release

Spitzer Image of the Tarantula Nebula

Image credit: NASA/JPL

The latest image from the Spitzer Space Telescope is of often-photographed Tarantula Nebula. Spitzer, however, is able to pierce through the dust and material that surrounds the nebula to take a good look inside this active star-forming region. This new photograph has turned up previously hidden stars inside the nebula, as well as empty cavities of space around them – their powerful radiation blows all the dust away. Images like this will help astronomers understand the environments that form stars and get a better sense of where our own Solar System came from.

A dusty stellar nursery shines brightly in a new image from NASA’s Spitzer Space Telescope, formerly known as the Space Infrared Telescope Facility. Spitzer’s heat-sensing “infrared eyes” have pierced the veiled core of the Tarantula Nebula to provide an unprecedented peek at massive newborn stars.

The new image is available online at http://www.spitzer.caltech.edu and http://photojournal.jpl.nasa.gov/catalog/PIA05062.

“We can now see the details of what’s going on inside this active star-forming region,” said Dr. Bernhard Brandl, principal investigator for the latest observations and an astronomer at both Cornell University, Ithaca, N.Y., and the University of Leiden, the Netherlands.

Launched on August 25, 2003, from Cape Canaveral Air Force Station, Florida, the Spitzer Space Telescope is the fourth of NASA’s Great Observatories, a program that also includes Compton Gamma Ray Observatory, the Chandra X-ray Observatory and the Hubble Space Telescope. Spitzer’s state-of-the-art infrared detectors can sense the infrared radiation, or heat, from the farthest, coldest and dustiest objects in the universe.

One such dusty object is the Tarantula Nebula. Located in the southern constellation of Dorado, in a nearby galaxy called the Large Magellanic Cloud, this glowing cloud of gas and dust is one of the most dynamic star-forming regions in our local group of galaxies. It harbors some of the most massive stars in the universe, up to 100 times more massive than our own Sun, and is the only nebula outside our galaxy visible to the naked eye.

While other telescopes have highlighted the nebula’s spidery filaments and its star-studded core, none was capable of fully penetrating its dust-enshrouded pockets of younger stars.

The new Spitzer image shows, for the first time, a more complete picture of this huge stellar nursery, including previously hidden stars. The image also captures in stunning detail a hollow cavity around the stars, where intense radiation has blown away cosmic dust.

“You can see a hole in the cloud as if a giant hair dryer blew away all the gas and dust,” said Brandl.

By studying this portrait of a family of stars, astronomers can piece together how stars in general, including those like our Sun, form.

JPL manages the Spitzer Space Telescope mission for NASA’s Office of Space Science, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech.

Additional information about the Spitzer Space Telescope is available at http://www.spitzer.caltech.edu.

Original Source: NASA/JPL News Release

Binary Systems Could Create Most Nebulae

Image credit: Hubble

New research from the National Optical Astronomy Observatory may help to explain the formation and shape of many planetary nebulae. The culprit might just be binary star systems, where two stars orbit a common centre of gravity. Astronomers believe that planetary nebulae are caused when white dwarf stars slough off their outer layers, but they couldn’t explain how the nebulae could form jets of material or unusual lobes and prominences. A second star orbiting the dying white dwarf could whip up the outer layers into the strange shapes astronomers see.

Near the end of its lifetime, a star like the Sun ejects its outer layers into space, producing a hazy cloud of material called a planetary nebula. The complex shapes and dazzling colors of planetary nebulae make them some of the most popular objects in the night sky, for both amateur observing and scientific study.

New research suggests that many if not most of the stellar corpses at the centers of these wildly varied cosmic objects have companion stars, a surprising finding that will influence how astronomers explain their origins.

Astronomers used the Wisconsin-Indiana-Yale-NOAO 3.5-meter telescope at the National Science Foundation?s Kitt Peak National Observatory to take radial velocity measurements of 11 central stars of planetary nebulae (PNe), looking for the telltale, repeatable wobble that indicates the presence of a companion’s gravitational influence. This technique is also used to search for extrasolar planets around nearby stars. Ten of the 11 central stars of the PNe in the recent study showed clear evidence for radial velocity oscillations.

?If our current results are confirmed with further observations, we could be at the start of a revolution in the study of the origin of planetary nebulae,? says Howard Bond of the Space Telescope Science Institute in Baltimore, the principal investigator of the results presented today in Atlanta at the 203rd meeting of the American Astronomical Society. ?If these nebulae arise from binary stars, it implies a very different origin for these systems than what most astronomers had thought.?

It might be expected that nebulae ejected from spherical stars would be spherical, but many years of telescope observations show this not to be the case. In fact, most PNe are either elliptical or have pronounced lobes, often accompanied by jet-like structures.

There is general agreement that in order to eject gas with these observed morphologies, single stars would have to rotate rather rapidly or have reasonably strong magnetic fields, which themselves are the product of stellar rotation. However, the stars that most commonly eject PNe are large, bloated giants, indisposed to fast rotation.

?The most direct way to spin up these vast, fluffy stars is by the action of an orbiting companion. In extreme cases, as a red giant star gradually increases in size, it may actually swallow a companion star, which would then spiral down inside the giant and eventually eject its outer layers,? explains Orsola De Marco, an astronomer at the American Museum of Natural History (AMNH) in New York and the lead author of the publication reporting the first results of this project. ?Despite this, the mainstream astronomical view remains rooted in single star theories for the evolution of planetary nebulae, supported by the small percentage of planetary nebulae central stars that that were previously known to be binaries. However, our new research threatens to turn this viewpoint on its head.?

Astronomers currently believe that the majority of stars?those that begin with no more than eight times the mass of our Sun?end their lives by ejecting a planetary nebula and becoming a cosmic ember called a white dwarf. However, the new results from the WIYN telescope suggest that the story may be more complicated, in that an interaction with a companion star may be required to produce most planetary nebulae.

?We need more data to determine the exact periods of the binary central stars, since this is the only way to be sure of their binarity and eliminate other possible physical sources that could simulate the stellar wobble,? De Marco says. ?We are reasonably sure that these variations are due to binarity, but determination of their precise periods is the only way to be sure. We must also increase the size of our sample.?

Among the objects observed in this initial study are Abell 78, NGC 6891, NGC 6210, and IC 4593. The new radial velocity measurements were taken by the WIYN Hydra spectrographic instrument.

A previously released Hubble Space Telescope image of NGC 6210 is available at: http://hubblesite.org/newscenter/newsdesk/archive/releases/1998/36/image/a

Co-authors of this work are Dianne Harmer of the National Optical Astronomy Observatory (NOAO) in Tucson, AZ, and Andrew Fleming of Michigan Technological University in Houghton, MI, an NSF Research Experiences for Undergraduates (REU) student at AMNH during the summer of 2003.

These results (Abstract 127.03 in the AAS meeting program) will be discussed in an oral session that begins at 10:00 a.m. on Thursday, January 8, in Regency VI. This research has been accepted for publication in the February 1, 2004, issue of Astrophysical Journal Letters.

Images of other planetary nebulae taken by Kitt Peak telescopes are available in the NOAO Image Gallery at:


The Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5-meter telescope is located at Kitt Peak National Observatory, 55 miles southwest of Tucson, AZ. Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation (NSF).

Original Source: NOAO News Release

New Method for Finding Exploding White Dwarf Stars

Image credit: SDSS

Researchers at the University of Washington have developed a new method for studying unusual astronomical pairings: pre-cataclysmic variables – a white dwarf and red dwarf tightly orbiting one another. Before this new method, only 100 of these objects had been discovered, but this new method has turned up another 400 in data from the Sloan Digital Sky Survey. When the two stars get close enough, material from the red dwarf streams onto the white dwarf and deposits on the surface. This heats up the white dwarf and can cause it to explode as a supernova.

Until recently, astrophysicists studying exotic star systems pairing a white dwarf and a red dwarf in very close proximity didn’t have much to go on.

Just five years ago, scientists knew of fewer than 100 such systems, called pre-cataclysmic variables. But today a team of University of Washington astronomers said that, with data from the Sloan Digital Sky Survey (SDSS), the number has now grown to nearly 500.

That is significant because researchers are now able to study white dwarf and red dwarf stars at different stages of their life cycles, giving scientists the ability to compare them and develop an understanding of how the systems evolve and change over the course of billions of years, possibly becoming supernovas.

“We’ve never had the opportunity to study a variety of these systems in detail before now,” said Nicole Silvestri, a University of Washington astronomy researcher. Using this large sample from the SDSS, Silvestri and her colleagues believe they can begin to answer some of the long-standing questions in astronomy about pre-cataclysmic variables and their eventual end products, cataclysmic variable systems.

Silvestri is lead author of a poster presentation on the findings presented today (January 6, 2004) at the American Astronomical Society’s annual meeting in Atlanta. Co-authors of the project are Suzanne Hawley and Paula Szkody of the University of Washington’s Astronomy Department. The National Science Foundation supported the research.

Pre-cataclysmic variable systems pair a red dwarf star about one-tenth the size of our sun and a dense remnant of a star, called a white dwarf, in close orbit around each other. When the two stars are close enough, orbiting one another in less than four hours, the gravity of the denser white dwarf is able to pull material off of the less dense red dwarf. Material from the red dwarf forms a disk around the white dwarf that eventually accumulates on the surface of the white dwarf. (Variability refers to the changing amount of light coming from the stars as they orbit each other).

As the white dwarf gains mass, many small explosions, called cataclysmic events, occur on the surface of the white dwarf. If the white dwarf gravity gets to a critical point, it can collapse catastrophically. This heats up the white dwarf tremendously and may cause it to explode as a supernova.

Pre-cataclysmic variables found so far in the SDSS data have orbital periods of between four and 12 hours and are not close enough to have begun transferring material between the stars.

Silvestri said the evolution of a pre-cataclysmic variable to a cataclysmic variable takes billions of years and studying just one system as it evolves would be impossible. But with nearly 500 pre-cataclysmic variables to study, “A dataset of this size will allow us to take snapshots in time of the evolution of the system,” she said. “This will allow the researchers to study how properties of each star change as the pair draw closer to each other, something that until now, has never been investigated.”

Silvestri and her colleagues are still at a loss to explain one oddity in the research. Thousands of isolated white dwarfs have been observed and hundreds of them have been found to be magnetic. And many white dwarfs in cataclysmic variables are magnetic. But not one of the white dwarfs observed in the pre-cataclysmic variable systems is magnetic.

“This makes the origin of magnetic cataclysmic variables (known as polars), which do contain magnetic white dwarfs, exceedingly mysterious,” added SDSS researcher Suzanne Hawley of the University of Washington.

“That’s a question we’re still trying to find an answer to,” Silvestri said. “How do you get a magnetic white dwarf in a cataclysmic variable if it doesn’t originate in one of these pairs that is evolving toward being a cataclysmic variable?” The University of Washington team, James Liebert of the University of Arizona and others are preparing a paper on that finding for the Astronomical Journal.

Original Source: SDSS News Release

String of Galaxies Puzzles Astronomers

Image credit: NASA

Wide-field observations of the early Universe have turned up a strange string of galaxies 300 million light-years long that defy current theories about the evolution of the Universe shortly after the Big Bang. The astronomers who discovered the string of galaxies, which are more than 10 billion light-years away, compared it to supercomputer simulations of the early Universe, which wasn’t able to reproduce strings this large this early. The next step of this research will be to map an area of the sky ten times as large to get a better idea of the large scale structure of the Universe.

Wide-field telescope observations of the remote and therefore early Universe, looking back to a time when it was a fifth of its present age (redshift = 2.38), have revealed an enormous string of galaxies about 300 million light-years long. This new structure defies current models of how the Universe evolved, which can’t explain how a string this big could have formed so early.

The string is comparable in size to the “Great Wall” of galaxies found in the nearby Universe by Dr. John Huchra and Dr. Margaret Geller in 1989. This is the first time astronomers have been able to map an area in the early Universe big enough to reveal such a galaxy structure.

The string was discovered by Dr. Povilas Palunas (University of Texas, in Austin, Texas), Dr. Paul Francis (Australian National University, Canberra, Australia), Dr. Harry Teplitz (California Institute of Technology in Pasadena), Dr. Gerard Williger (Johns Hopkins University, Baltimore, Md.), and Dr. Bruce E. Woodgate (NASA Goddard Space Flight Center, Greenbelt, Md.). The initial observations were made with the 4-m (159-inch) Blanco Telescope at the National Science Foundation’s Cerro Tololo Inter-American Observatory in Chile, and confirmed with the 3.9-m (154-inch) Anglo-Australian Telescope at Siding Spring Observatory in eastern Australia. The team presents its finding today at the American Astronomical Society meeting in Atlanta, Georgia, and a paper describing this work will appear in the Astrophysical Journal in February.

The string lies 10,800 million light-years away in the direction of the southern constellation Grus (the Crane). The distance light travels in a year, almost six trillion miles or 9.5 trillion km., is one light-year, so we see the string as it appeared 10.8 billion years ago. It is at least 300 million light-years long and about 50 million light-years wide. (Refer to Movie 1 and Images 3 and 4 for an artist’s concept of the string.) The astronomers have detected 37 galaxies and one quasar in the string, but “there are almost certainly far more than this,” said Palunas. “The string probably contains many thousands of galaxies.” (Refer to Image 1 for an artist’s concept of these galaxies, and to Image 5 for a plot of their locations on the sky.)

“We are seeing this string as it was when the Universe was only a fifth of its present age,” said Woodgate. “That is, we are looking back four-fifths of the way to the beginning of the Universe as a result of the Big Bang.”

The team compared their observations to supercomputer simulations of the early Universe, which could not reproduce strings this large. “The simulations tell us that you cannot take the matter in the early Universe and line it up in strings this large,” said Francis. “There simply hasn’t been enough time since the Big Bang for it to form structures this colossal”.

“Our best guess right now is that it’s a tip-of-the-iceberg effect,” he said. “All we are seeing is the brightest few galaxies. That’s probably far less than 1% of what’s really out there, most of which is the mysterious invisible dark matter. It could be that the dark matter is not arranged in the same way as the galaxies we are seeing.” Recently, evidence has accumulated for the presence of dark matter in the Universe, an invisible form of matter only detectable by the gravitational pull it exerts on ordinary matter (and light). There are many possibilities for what dark matter might be, but its true nature is currently unknown.

In recent years, Francis explained, it had been found that in the local Universe, dark matter is distributed on large scales in very much the same way the galaxies are, rather than being more clumpy, or less. But go back 10 billion years and it could be a very different story. Galaxies probably form in the center of dark matter clouds. But in the early Universe, most galaxies had not yet formed, and most dark matter clouds will not yet contain a galaxy.

“To explain our results,” said Francis, “the dark matter clouds that lie in strings must have formed galaxies, while the dark matter clouds elsewhere have not done so. We’ve no idea why this happened – it’s not what the models predict.”

To follow up this research, the astronomers say, the next step is to map an area of sky ten times larger, to get a better idea of the large-scale structure. Several such surveys are currently under way. The research was funded by NASA and the Australian National University.

Original Source: NASA News Release

Stars of All Ages Have Comets and Planets

Image credit: Harvard CfA

Astronomers from the Harvard Center for Astrophysics studied Comet Kudo-Fujikawa as it swept past the Sun in early 2003, and they noticed it was emitting large amounts of carbon and water vapour. This new view of the comet matches observations of other stars that indicate there could be comets emitting similar material. Since other stars probably have comets, it increases the likelihood that they could also have rocky planets, like the Earth.

In early 2003, Comet Kudo-Fujikawa (C/2002 X5) zipped past the Sun at a distance half that of Mercury’s orbit. Astronomers Matthew Povich and John Raymond (Harvard-Smithsonian Center for Astrophysics) and colleagues studied Kudo-Fujikawa during its close passage. Today at the 203rd meeting of the American Astronomical Society in Atlanta, they announced that they observed the comet puffing out huge amounts of carbon, one of the key elements for life. The comet also emitted large amounts of water vapor as the Sun’s heat baked its outer surface.

When combined with previous observations suggesting the presence of evaporating comets near young stars like Beta Pictoris and old stars like CW Leonis, these data show that stars of all ages vaporize comets that swing too close. Those observations also show that planetary systems like our own, complete with a collection of comets, likely are common throughout space.

“Now we can draw parallels between a comet close to home and cometary activity surrounding the star Beta Pictoris, which just might have newborn planets orbiting it. If comets are not unique to our Sun, then might not the same be true for Earth-like planets?” says Povich.

SOHO Sees Carbon
The team’s observations, reported in the December 12, 2003, issue of the journal Science, were made with the Ultraviolet Coronagraph Spectrometer (UVCS) instrument on board NASA’s Solar and Heliospheric Observatory (SOHO) spacecraft.

UVCS can only study a small slice of the sky at one time. By holding the spectrograph slit steady and allowing the comet to drift past, the team was able to assemble the slices into a full, two-dimensional picture of the comet.

The UVCS data revealed a dramatic tail of carbon ions streaming away from the comet, generated by evaporating dust. The instrument also captured a spectacular ‘disconnection event,’ in which a piece of the ion tail broke off and drifted away from the comet. Such events are relatively common, occurring when the comet passes through a region of space where the Sun’s magnetic field switches direction.

Cometary Building Blocks
More remarkable than the morphology of the carbon ion tail was its size. A single snapshot of Kudo-Fujikawa on one day showed that its ion tail contained at least 200 million pounds of doubly ionized carbon. The tail likely held more than 1.5 billion pounds of carbon in all forms.

“That’s a massive amount of carbon, weighing as much as five supertankers,” says Raymond.

Povich adds, “Now, consider that astronomers see evidence for comets like this around newly formed stars like Beta Pictoris. If such stars have comets, then perhaps they have planets, too. And if extrasolar comets are similar to comets in our solar system, then the building blocks for life may be quite common.”

Understanding Our Origins
In 2001, researcher Gary Melnick (CfA) and colleagues found evidence for comets in a very different system surrounding the aging red giant star CW Leonis. The Submillimeter Wave Astronomy Satellite (SWAS) detected huge clouds of water vapor released by a Kuiper Belt-like swarm of comets which are evaporating under the giant’s relentless heat.

“Taken together, the observations of comets around young stars like Beta Pictoris, middle-aged stars like our Sun, all planets, and old stars like CW Leonis strengthen the connection between our solar system and extrasolar planetary systems. By studying our own neighborhood, we hope to learn not only about our origins, but about what we might find out there orbiting other stars,” says Raymond.

Other co-authors on the Science paper reporting these findings are Geraint Jones (JPL), Michael Uzzo and Yuan-Kuen Ko (CfA), Paul Feldman (Johns Hopkins), Peter Smith and Brian Marsden (CfA), and Thomas Woods (University of Colorado).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: Harvard CfA News Release

Double Pulsar System Found

Image credit: RAS

An international team of astronomers have discovered a double pulsar system – the first ever seen. The two objects orbit a common centre of gravity once every 2.4 hours; one rotates at 3000 times a minute, while the other spins at only 22 times a minute. This discovery is important because it will allow astronomers to test various theories of relativity as the two objects interact with each other. The two pulsars will probably merge to become a black hole in 85 million years.

An international team of scientists from the UK, Australia, Italy and the USA have announced in today’s issue of the journal Science Express [ 8th January 2004 ] the first discovery of a double pulsar system.

They have shown that the compact object orbiting the 23-millisecond pulsar PSR J0737-3039A with a period of just 2.4 hours is not only, as suspected, another neutron star but is also a detectable pulsar, PSR J0737-3039B, that is rotating once every 2.8 seconds.

Professor Andrew Lyne of the University of Manchester points out that “While experiments on one pulsar in such an extreme system as this are exciting enough, the discovery of two pulsars orbiting one another opens up new precision tests of general relativity and the probing of pulsar magnetospheres.”

The same team previously reported [Nature 4th December 2003], the discovery of pulsar A in a close binary system which is rapidly losing energy by gravitational radiation. The stars will coalesce in only approximately 85 million years, sending a ripple of gravity waves across the Universe. The discovery of the system shows that such coalescences will occur more frequently than previously thought. “The news has been welcomed by gravitational wave hunters, since it boosts their hopes for detecting the gravitational waves” says Prof. Nichi D’Amico of Cagliari University.

The double neutron star system was first detected using the 64-m Parkes radio telescope in New South Wales, Australia. Subsequent observations were made both at Parkes and with the 76-m Lovell Telescope of the University of Manchester in Cheshire, UK and revealed the occasional presence of pulsations with a period of 2.8 seconds from the companion pulsar.

Already, four different effects beyond those explained with simple Newtonian gravity have been measured and are completely consistent with Albert Einstein’s theory. Dr. Richard Manchester of the Australia Telescope National Facility says “The fact that both objects are pulsars enables completely new high-precision tests of gravitational theories. This system is really extreme.” Future observations of the two stars will measure their slow spiral in towards each other as they radiate gravitational radiation – a dance of death leading to their ultimate fusion into what may become a black hole. General relativity predicts that the two stars will slowly wobble like spinning tops allowing new tests of the theory.

Another unique aspect of the new system is the strong interaction between radiation from the two stars. By chance, the orbit is seen nearly edge on to us, and the signal from one pulsar is eclipsed by the other. Dr. Andrea Possenti of Cagliari Astronomical Observatory says “This provides us with a wonderful opportunity to probe the physical conditions of a pulsar’s outer atmosphere, something we’ve never been able to do before.”

The surveys designed by the team to discover new pulsars at the Parkes Telescope have been extraordinarily successful. They have discovered over 700 pulsars in the last 5 years, nearly as many as were discovered in the preceding 30 years. The discovery of this double pulsar system will be the major jewel in the crown.

Original Source: RAS News Release

Astronomers See a Magnetar Form

Image credit: NASA

A team of astronomers were lucky enough to observe the rare event of a neutron star turning into a magnetic object called a magnetar. Ten magnetars have been seen to date, but this object, a transient magnetar, is brand new. A normal neutron star is the rapidly spinning remnant of a star that went supernova; they typically possess a very strong magnetic field. A magnetar is similar, but it has a magnetic field up to 1,000 times as strong as a neutron star. This new discovery could indicate that magnetars are more common in the Universe than previously thought.

In a lucky observation, scientists say they have discovered a neutron star in the act of changing into a rare class of extremely magnetic objects called magnetars. No such event has been witnessed definitively until now. This discovery marks only the tenth confirmed magnetar ever found and the first transient magnetar.

The transient nature of this object, discovered in July 2003 with NASA’s Rossi X-ray Timing Explorer, may ultimately fill in important gaps in neutron star evolution. Dr. Alaa Ibrahim of George Washington University and NASA Goddard Space Flight Center in Greenbelt, Md., presents this result today at the meeting of the American Astronomical Society in Atlanta.

A neutron star is the core remains of a star at least eight times more massive than the Sun that exploded in a supernova event. Neutron stars are highly compact, highly magnetic, fast-spinning objects with about a Sun’s worth of mass compressed into a sphere roughly ten miles in diameter.

A magnetar is up to a thousand times more magnetic than ordinary neutron stars. At a hundred trillion (10^14) Gauss, they are so magnetic that they could strip a credit card clean at a distance of 100,000 miles. The Earth’s magnetic field, in comparison, is about 0.5 Gauss, and a strong refrigerator magnet is about 100 Gauss. Magnetars are brighter in X rays than they are in visible light, and they are the only stars known that shine predominantly by magnetic power.

The observation presented today supports the theory that some neutron stars are born with these ultrahigh magnetic fields, but they may be at first too dim to see and measure. In time, however, these magnetic fields act to slow the neutron star’s spin. This act of slowing releases energy, making the star brighter. Additional disturbances in the star’s magnetic field and crust can make it brighter yet, leading to the measurement of its magnetic field. The newly discovered star, dim as recent as a year ago, is named XTE J1810-197.

“The discovery of this source came courtesy of another magnetar that we were monitoring, named SGR 1806-20,” said Ibrahim. He and his colleagues detected XTE J1810-197 with the Rossi Explorer about a degree to the northeast of SGR 1806-20, within the Milky Way galaxy about 15,000 light years away in the constellation Sagittarius.

Scientists pinpointed the location of the source with NASA’s Chandra X-ray Observatory, which provides more accurate positioning than Rossi. Checking archive data from the Rossi Explorer, Dr. Craig Markwardt of NASA Goddard estimated that XTE J1810-197 became active (that is, 100 times brighter than before) around January 2003. Looking back even further with archived data from ASCA and ROSAT, two decommissioned international satellites, the team could spot XTE J1810-197 as a very dim, isolated neutron star as early as 1990. Thus, the history of XTE J1810-197 emerged.

The inactive state of XTE J1810-197, Ibrahim said, was similar to that of other puzzling objects called Compact Central Objects (CCOs) and Dim Isolated Neutron Stars (DINSs). These objects are thought to be neutron stars created in the hearts of star explosions, and some still reside there, but they are too dim to study in detail.

One mark of a neutron star is its magnetic field. But to measure this, scientists need to know the neutron star’s spin period and the rate that it is slowing down, called the “spin down”. When XTE J1810-197 lit up, the team could measure its spin (1 revolution per 5 seconds, typical of magnetars), its spin down, and thus its magnetic field strength (300 trillion Gauss).

In the alphabet soup of neutron stars, there are also Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs). Both of these are now considered to be the same kind of objects, magnetars; and another presentation at today’s meeting by Dr. Peter Woods et al. supports this connection. These objects periodically but unpredictably erupt with X-ray and gamma-ray light. CCOs and DINSs appear not to have a similar active state.

Although the concept is still speculative, an evolutionary pattern may be emerging, Ibrahim said. The same neutron star, endowed with an ultrahigh magnetic field, may pass through each of these four phases during its lifetime. The proper order, however, remains unclear. “Discussion of such a pattern has surfaced in the scientific community in recent years, and XTE J1810-197’s transient nature provides the first tangible evidence in favor of such a kinship,” Ibrahim said. “With a few more examples of stars showing a similar trend, a magnetar family tree may emerge.”

“The observation implies that magnetars could be more common than what is seen but exist in a prolonged dim state,” said team member Dr. Jean Swank of NASA Goddard.

“Magnetars seem now to be in a perpetual carnival mode; SGRs are turning into AXPs and AXPs can start behaving like SGRs anytime and without warning,” said team member Dr. Chryssa Kouveliotou of NASA Marshall, who is receiving the Rossi Award at the AAS meeting for her work on magnetars. “What started with a few odd sources, may soon be proven to encompass a huge number of objects in our Galaxy.”

Additional supporting data came from the Interplanetary Network and the Russian-Turkish Optical Telescope. Ibrahim’s colleagues on this observation also include Dr. William Parke of George Washington University; Drs. Scott Ransom, Mallory Roberts and Vicky Kaspi of McGill University; Dr. Peter Woods of NASA Marshall; Dr. Samar Safi-Harb of the University of Manitoba; Dr. Solen Balman of the Middle East Technical University in Ankara; and Dr. Kevin Hurley of University of California at Berkeley. Drs. Eric Gotthelf and Jules Halpern of Columbia University provided important data from Chandra.

Original Source: NASA News Release

Supernova’s Companion Star Found

Image credit: ESA

When the second brightest supernova seen in modern times, SN 1993J, blew up several years ago, it did leave a survivor. Using the Hubble Space Telescope, and several ground-based observatories, an international team of astronomers discovered a massive companion star that must have been orbiting the supernova at the time it exploded. This discovery is very important because it will allow astronomers watch what the remnant of SN 1993J does to its companion star. They might even be able to detect a neutron star or black hole forming in real time.

A joint European/University of Hawaii team of astronomers has for the first time observed a stellar ?survivor? to emerge from a double star system involving an exploded supernova.

Supernovae are some of the most significant sources of chemical elements in the Universe, and they are at the heart of our understanding of the evolution of galaxies.

Supernovae are some of the most violent events in the Universe. For many years astronomers have thought that they occur in either solitary massive stars (Type II supernovae) or in a binary system where the companion star plays an important role (Type I supernovae). However no one has been able to observe any such companion star. It has even been speculated that the companion stars might not survive the actual explosion…

The second brightest supernova discovered in modern times, SN 1993J, was found in the beautiful spiral galaxy M81 on 28 March 1993. From archival images of this galaxy taken before the explosion, a red supergiant was identified as the mother star in 1993 – only the second time astronomers have actually seen the progenitor of a supernova explosion (the first was SN 1987A, the supernova that exploded in 1987 in our neighbouring galaxy, the Large Magellanic Cloud).

Initially rather ordinary, SN 1993J began to puzzle astronomers as its ejecta seemed too rich in the chemical element helium and instead of fading normally it showed a bizarre sharp increase in brightness. The astronomers realised that a normal red supergiant alone could not have given rise to such a weird supernova. It was suggested that the red supergiant orbited a companion star that had shredded its outer layers just before the explosion.

Ten years after this cataclysmic event, a European/University of Hawaii team of astronomers has now peered deep into the glowing remnants of SN 1993J using the NASA/ESA Hubble Space Telescope?s Advanced Camera for Surveys (ACS) and the giant Keck telescope on Mauna Kea in Hawaii. They have discovered a massive star exactly at the position of the supernova that is the long sought companion to the supernova progenitor.

This is the first supernova companion star ever to be detected and it represents a triumph for the theoretical models. In addition, this observation allows a detailed investigation of the stellar physics leading to supernova explosions. It is now clear that during the last 250 years before the explosion 10 solar masses of gas were torn violently from the red supergiant by its partner. By observing the companion closely in the coming years it may even be possible to detect a neutron star or black hole emerge from the remnants of the explosion ?in real time?.

Given the paucity of observations of supernova progenitor systems this result, published in Nature on 8 January 2004, is likely to ‘be crucial to understanding how very massive stars explode and why we see such peculiar supernovae’ according to first author Justyn R. Maund from the University of Cambridge, UK.

The team is composed of Stephen J. Smartt and Justyn R. Maund (University of Cambridge, UK), Rolf. P. Kudritzki (University of Hawaii, USA), Philipp Podsiadlowski (University of Oxford, UK) and Gerry F. Gilmore (University of Cambridge, UK).

Stephen Smartt, also from the University of Cambridge, says, ?Supernova explosions are at the heart of our understanding of the evolution of galaxies and the formation of chemical elements in the Universe. It is essential that we know what type of stars produce them.?

For the last ten years astronomers have believed that they could understand the very peculiar behaviour of 1993J by invoking the existence of a binary companion star and now this picture has proved correct.

According to Rolf Kudritzki, from the University of Hawaii, ?The combination of the outstanding spatial resolution of Hubble and the huge light gathering power of the Keck 10- metre telescope in Hawaii has made this fantastic discovery possible.?

Supernovae occur when a star of more than about eight times the mass of the Sun reaches the end of its nuclear fuel reserves and can no longer produce enough energy to keep the star from collapsing under its own immense weight. The core of the star collapses, and the outer layers are ejected in a fast-moving shock wave.

This huge energy release causes the visible supernova we see. While astronomers are convinced that observations will match this theoretical model, they are in the embarrassing position that they have confidently identified only two stars that later exploded as supernovae ? the precursors of supernovae 1987A and 1993J.

There have been more than 2000 supernovae discovered in galaxies beyond the Milky Way and there appear to be about eight distinct sub-classes. However identifying which stars produce which flavours has proved incredibly difficult. This team has now embarked on a parallel project with the Hubble Space Telescope to image a large number of galaxies and then wait patiently for a supernova to explode.

Supernovae appear in spiral galaxies like M81 on average once every 100 years or so. The team, led by Stephen Smartt, hope to increase the numbers of supernova progenitors known from 2 to 20 over the next five years.

Original Source: ESA News Release

Lifeless Suns in the Early Universe

Image credit: Harvard CfA

New calculations by a pair of Harvard astronomers predict that the first “Sun-like” stars in the Universe were alone; devoid of planets or life. The very first generation of stars was hot and massive; they lived hard and died young. After they exploded as supernovae and seeded the Universe with heavier materials, other stars formed in stellar nurseries. The next generation of stars was probably similar in mass and composition to our own Sun, but there weren’t enough minerals to create rocky planets like the Earth. It took a succession of supernovae before there was enough heavy material that planets could form – probably 500 million to 2 billion years after the Big Bang.

To most people, the phrase “Sun-like star” calls to mind images of a friendly, warm yellow star accompanied by a retinue of planets possibly capable of nurturing life. But new calculations by Harvard astronomers Volker Bromm and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics), which were announced today at the 203rd meeting of the American Astronomical Society in Atlanta, show that the first Sun-like stars were lonely orbs moving through a universe devoid of planets or life.

“The window for life opened sometime between 500 million and 2 billion years after the Big Bang” says Loeb. “Billions of years ago, the first low-mass stars were lonely places. The reason for that youthful solitude is embedded in the history of our universe.”

In The Beginning
The very first generation of stars were not at all like our Sun. They were white-hot, massive stars that were very short-lived. Burning for only a few million years, they collapsed and exploded as brilliant supernovae. Those very first stars began the seeding process in the universe, spreading vital elements like carbon and oxygen, which served as planetary building blocks.

“Previously, with Lars Hernquist and Naoki Yoshida (also at the CfA), I have simulated those first supernova explosions to calculate their evolution and how much heavy elements (elements heavier than hydrogen or helium) they produced,” says Bromm. “Now, in this work, Avi Loeb and I have determined that a single first-generation supernova could produce enough heavy elements to enable the first Sun-like stars to form.”

Bromm and Loeb showed that many second-generation stars had sizes, masses, and hence temperatures similar to our Sun. Those properties resulted from the cooling influence of carbon and oxygen when the stars formed. Even elemental abundances as low as one-ten thousandth those found in the Sun proved sufficient to allow smaller, low-mass stars like our Sun to be born.

Yet those same low abundances prohibited rocky planets from forming around those first Sun-like stars due to a lack of raw materials. Only as further generations of stars lived, died, and enriched the interstellar medium with heavy elements did the birth of planets, and life itself, become possible.

“Life is a recent phenomenon,” Loeb states unequivocally. “We know that it took many supernova explosions to make all the heavy elements we find here on Earth and in our Sun and our bodies.”

Recent observational evidence corroborates their finding. Studies of known extrasolar planets have found a strong correlation between the presence of planets and the abundance of heavy elements (“metals”) in their stars. That is, a star with higher metallicity and more heavy elements is more likely to possess planets. Conversely, the lower a star’s metallicity, the less likely it is to have planets.

“We’re now just beginning to investigate the metallicity threshold for planet formation, so it’s hard to say when exactly the window for life opened. But clearly, we’re fortunate that the metallicity of the matter that birthed our solar system was high enough for the Earth to form,” says Bromm. “We owe our existence in a very direct way to all the stars whose life and death preceded the formation of our Sun. And this process began right after the Big Bang with the very first stars. As the universe evolved, it progressively seeded itself with all the heavy elements necessary for planets and life to form. Thus, the evolution of the universe was a step-by-step process that resulted in a stable G-2 star capable of sustaining life. A star we call the Sun.”

Original Source: Harvard CfA News Release