Category: Astronomy

Spitzer view of the Carina Nebula, a well known nebula containing newborn stars in the Milky Way. Image credit: Spitzer. Click to enlarge.
The saga of how a few monstrous stars spawned a diverse community of additional stars is told in a new image from NASA’s Spitzer Space Telescope.

The striking picture reveals an eclectic mix of embryonic stars living in the tattered neighborhood of one of the most famous massive stars in our Milky Way galaxy, Eta Carinae. Astronomers say that radiation and winds from Eta Carinae and its massive siblings ripped apart the surrounding cloud of gas and dust, shocking the new stars into being.

“We knew that stars were forming in this region before, but Spitzer has shown us that the whole environment is swarming with embryonic stars of an unprecedented multitude of different masses and ages,” said Dr. Robert Gehrz, University of Minnesota, Twin Cities, a member of the team that made the Spitzer observations.

The results were presented yesterday at the 206th meeting of the American Astronomical Society in Minneapolis by Dr. Nathan Smith, lead investigator of the Spitzer findings, University of Colorado, Boulder.

Previous visible-light images of this region, called the Carina Nebula, show cloudy finger-like pillars of dust, all pointing toward Eta Carinae at the center. Spitzer’s infrared eyes cut through much of this dust to expose incubating stars embedded inside the pillars, as well as new star-studded pillars never before seen.

Eta Carinae, located 10,000 light-years from Earth, was once the second brightest star in the sky. It is so massive, more than 100 times the mass of our Sun, it can barely hold itself together. Over the years, it has brightened and faded as material has shot away from its surface. Some astronomers think Eta Carinae might die in a supernova blast within our lifetime.

Eta Carinae’s home, the Carina Nebula, is also quite big, stretching across 200 light-years of space. This colossal cloud of gas and dust not only gave birth to Eta Carinae, but also to a handful of slightly less massive sibling stars. When massive stars like these are born, they rapidly begin to shred to pieces the very cloud that nurtured them, forcing gas and dust to clump together and collapse into new stars. The process continues to spread outward, triggering successive generations of fewer and fewer stars. Our own Sun may have grown up in a similar environment.

The new Spitzer image offers astronomers a detailed “family tree” of the Carina Nebula. At the top of the hierarchy are the grandparents, Eta Carinae and its siblings, and below them are the generations of progeny of different sizes and ages.

“Now we have a controlled experiment for understanding how one giant gas and dust cloud can produce such a wide variety of stars,” said Gehrz.

The false colors in the Spitzer picture correspond to different infrared wavelengths. Red represents dust features and green shows hot gas. Embryonic stars are yellow or white and foreground stars are blue. Eta Carinae itself lies just off the top of image. It is too bright for infrared telescopes to observe.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech. Spitzer’s infrared array camera, which took the picture of the Carina Nebula, was built by NASA Goddard Space Flight Center, Greenbelt, Md.; its development was led by Dr. Giovanni Fazio, Smithsonian Astrophysical Observatory, Cambridge, Mass.

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

Original Source: Spitzer News Release

One small corner of the massive Andromeda galaxy (M31). Image credit: Subaru. Click to enlarge.
The lovely Andromeda galaxy appeared as a warm fuzzy blob to the ancients. To modern astronomers millennia later, it appeared as an excellent opportunity to better understand the universe. In the latter regard, our nearest galactic neighbor is a gift that keeps on giving.

Scott Chapman, from the California Institute of Technology, and Rodrigo Ibata, from the Observatoire Astronomique de Strasbourg in France, have led a team of astronomers in a project to map out the detailed motions of stars in the outskirts of the Andromeda galaxy. Their recent observations with the Keck telescopes show that the tenuous sprinkle of stars extending outward from the galaxy are actually part of the main disk itself. This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated.

At the annual summer meeting of the American Astronomical Society today, Chapman will outline the evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, astronomers looking at the visible evidence thought Andromeda was about 70,000 to 80,000 light-years across. Andromeda itself is about 2 million light-years from Earth.

The new dimensional measure is based on the motions of about 3,000 of the stars some distance from the disk that were once thought to be merely the “halo” of stars in the region and not part of the disk itself. By taking very careful measurements of the “radial velocities,” the researchers were able to determine precisely how each star was moving in relation to the galaxy.

The results showed that the outlying stars are sitting in the plane of the Andromeda disk itself and, moreover, are moving at a velocity that shows them to be in orbit around the center of the galaxy. In essence, this means that the disk of stars is vastly larger than previously known.

Further, the researchers have determined that the nature of the “inhomogeneous rotating disk”-in other words, the clumpy and blobby outer fringes of the disk-shows that Andromeda must be the result of satellite galaxies long ago slamming together. If that were not the case, the stars would be more evenly spaced.

Ibata says, “This giant disk discovery will be very hard to reconcile with computer simulations of forming galaxies. You just don’t get giant rotating disks from the accretion of small galaxy fragments.”

The current results, which are the subject of two papers already available and a third yet to be published, are made possible by technological advances in astrophysics. In this case, the Keck/DEIMOS multi-object spectrograph affixed to the Keck II Telescope possesses the mirror size and light-gathering capacity to image stars that are very faint, as well as the spectrographic sensitivity to obtain highly accurate radial velocities.

A spectrograph is necessary for the work because the motion of stars in a faraway galaxy can only be detected within reasonable human time spans by inferring whether the star is moving toward us or away from us. This can be accomplished because the light comes toward us in discrete frequencies due to the elements that make up the star.

If the star is moving toward us, then the light tends to cram together, so to speak, making the light higher in frequency and “bluer.” If the star is moving away from us, the light has more breathing room and becomes lower in frequency and “redder.”

If stars on one side of Andromeda appear to be coming toward us, while stars on the opposite side appear to be going away from us, then the stars can be assumed to orbit the central object.

The extended stellar disk has gone undetected in the past because stars that appear in the region of the disk could not be known to be a part of the disk until their motions were calculated. In addition, the inhomogeneous “fuzz” that makes up the extended disk does not look like a disk, but rather appears to be a fragmented, messy halo built up from many previous galaxies’ crashing into Andromeda, and it was assumed that stars in this region would be going every which way.

“Finding all these stars in an orderly rotation was the last explanation anyone would think of,” says Chapman.

On the flip side, finding that the bulk of the complex structure in Andromeda’s outer region is rotating with the disk is a blessing for studying the true underlying stellar halo of the galaxy. Using this new information, the researchers have been able to carefully measure the random motions of stars in the stellar halo, probing its mass and the form of the elusive dark matter that surrounds it.

Although the main work was done at the Keck Observatory, the original images that posed the possibility of an extended disk were taken with the Isaac Newton Telescope’s Wide-Field Camera. The telescope, located in the Canary Islands, is intended for surveys, and in the case of this study, served well as a companion instrument.

Chapman says that further work will be needed to determine whether the extended disk is merely a quirk of the Andromeda galaxy, or is perhaps typical of other galaxies.

The main paper with which today’s AAS news conference is concerned will be published this year in The Astrophysical Journal with the title “On the Accretion Origin of a Vast Extended Stellar Disk Around the Andromeda Galaxy.” In addition to Chapman and Ibata, the other authors are Annette Ferguson, University of Edinburgh; Geraint Lewis, University of Sydney; Mike Irwin, Cambridge University; and Nial Tanvir, University of Hertfordshire.

Original Source: Caltech News Release

Artist illustration of a gamma-ray burst. Image credit: NASA. Click to enlarge.
Observations by two of the world’s largest telescopes provide strong evidence that a peculiar type of exploding star may be the origin of elusive gamma-ray bursts that have puzzled scientists for more than 30 years.

A team of astronomers from Italy, Japan, Germany and the United States, including the University of California, Berkeley, conclude from observations with the Keck and Subaru telescopes in Hawaii that naked carbon/oxygen stars that flatten as they collapse into a black hole are good candidates for the source of gamma-ray bursts.

Though astronomers have observed a couple of bursts associated with this type of supernova – a Type Ic supernova sometimes called a hypernova – the theory of how a hypernova produces gamma rays is still speculative. The new observations, though not a smoking gun, provide a major piece of evidence that the theory, called the collapsar model, is correct. The model explains how an asymmetric exploding star produces a tight beam of matter and energy out of each pole that generates an intense burst of gamma rays, while the absence of a hydrogen and helium envelope would allow the blast to escape.

“It appears that to produce a gamma-ray burst, a core-collapse supernova needs to be both asymmetric in its explosion mechanism, so that there is a natural axis along which matter can more easily squirt, and free of a hydrogen envelope, so that the jet doesn’t have to pummel through a lot of material,” said co-author Alex Filippenko, UC Berkeley professor of astronomy.

The team, led by Paolo Mazzali of the Trieste Observatory in Italy and the Max-Planck Institute for Astrophysics in Garching, Germany, reported its findings in a paper appearing in the May 27 issue of Science.

The fact that a gamma-ray burst was not observed in association with this supernova is actually in accord with predictions, said UC Berkeley graduate student Ryan Foley, a member of the team.

“These observations suggest that the collapsar model is probably correct and that some of these Type Ic supernovae appear to be off-axis gamma-ray bursts, in which the gamma-ray burst is pointing in some direction other than Earth,” Foley said.

Gamma-ray bursts are brief but bright flashes of X-rays and gamma rays that seem to go off randomly in the sky about once a day, briefly outshining the sun a million trillion times. It took until 1997 to establish that they originate outside our Milky Way Galaxy, and only within the past few years have astronomers gotten tantalizing hints that the bursts are associated with supernovae.

Because they are so bright, gamma-ray bursts have to be a collimated beam, similar to but tighter than the cone of light emitted by a lighthouse. Otherwise, the energy in the explosion would be equivalent to instantaneously converting the mass of several suns into a fireball of energy.

The most popular scenario is that a collapsing star generates two highly collimated beams or jets of particles and energy that flash outward from the poles. The particles and energy generate a shock wave when they hit gas and dust around the star, which in turn accelerates particles to energies at which they emit high-energy light: gamma rays and X-rays. The initial burst fades over a few seconds, but the resulting shock waves (the “afterglow”) can be visible to optical, radio and X-ray telescopes for days after the explosion.

A possible candidate for the type of supernova that could produce a gamma-ray burst is the Type Ic supernova. Type Ic supernovae result from massive stars whose winds have shed their outer envelopes of hydrogen and often all their helium, or that have lost these outer layers to a binary companion. Only the core is left, composed of the elements produced by fusion in the star’s center – mostly carbon and oxygen but other heavy elements as well, down to a solid iron center.

The collapsar theory proposes that the solid iron sphere at the very core of the star collapses under gravity to a black hole, but that the split-second collapse takes place in a unique way. As the iron and surrounding matter fall inward, the spin of the core increases, flattening the in-falling material into a disk that flows inward along the equator. The congestion of in-falling matter pushes some of it right back out along the path of least resistance – the two blowholes at either pole.

The matter shot out from the poles rams into the other layers of the star, which it may not be able to penetrate. The lack of a hydrogen and helium envelope presumably increases the chances the jet will punch through.

“It has so much energy that it pushes through these outer layers of the star, which are of relatively small density compared to the disk of in-falling material in the center of the star,” said Foley. “Eventually, if it punches out, you have a gamma-ray jet. Some Type Ic supernovae may be failed gamma-ray bursts, which means the jet tried to push out, but there was too much material in the way, and it never actually broke out. That would explain why we don’t see gamma-ray bursts associated with some of these objects.”

If the theory is true, astronomers should see different things depending on whether the jet is aimed toward Earth or away from it. If the jet is coming out perpendicular to our line of sight, for example, no gamma-ray burst would be visible, but other aspects of the expanding supernova blast wave should be observable. In particular, the spectrum of the supernova a year or so after its explosion should show emission lines of elements, such as oxygen, that are split, one shifted slightly to lower wavelengths and the other shifted to higher wavelengths. The two lines would come from opposite sides of the expanding disk around the equatorial region of the remnant black hole, one Doppler shifted toward the red because it is moving away from us, the other blueshifted because it is moving toward us. Such split or double lines would not be visible from a polar perspective.

About two years ago, on Oct. 25, 2003, UC Berkeley researchers had discovered a Type Ic supernova using Filippenko’s automated supernova search telescope, the Katzman Automatic Imaging Telescope (KAIT) at the University of California’s Lick Observatory. Called SN 2003jd, the supernova was about 260 million light years away in the constellation Aquarius. Though no associated gamma-ray burst was recorded, the supernova appeared to be as bright as the supernovae previously associated with gamma-ray bursts, so the international team reporting this week in Science decided to look again at the supernova, taking its spectrum in search of double-peaked emission lines.

“These observations were actually guided by our theoretical predictions,” Mazzali said. “The idea was that a bright Type Ic supernova, not accompanied by a gamma-ray burst, could be just what we were looking for: an off-axis event which could confirm our predictions.”

Koji Kawabata from Hiroshima University, Ken’ichi Nomoto of the University of Tokyo and his colleagues observed the remnant nebula with the 8.2-meter Subaru telescope on Sept. 12, 2004, about 330 days after it blew. Subsequently, Filippenko and Foley turned the 10-meter Keck telescope on the nebula on Oct. 19, 2004, about 370 days after the initial explosion, to obtain spectral images with the Low Resolution Imaging Spectrometer (LRIS). Both telescopes sit atop Mauna Kea volcano on the island of Hawaii. Subaru is operated by the National Astronomical Observatory of Japan, while the Keck Observatory is operated by the California Association for Research in Astronomy, whose board of directors includes representatives from the California Institute of Technology (Caltech) and UC.

Kawabata, Mazzali and his team analyzed the spectra, revealing that they exhibit split oxygen and magnesium emission lines exactly as would be expected if the collapsar model of gamma-ray production were correct. This was the first Type Ic supernova to show split oxygen lines.

“Jets are a signature of the model, which means that not all explosions will be pointed directly at us. If every time we looked at these objects they appeared to be pointing at us, that would mean the model is probably flawed,” Foley said. “The model predicts that a certain percentage of these objects should look like this supernova (SN 2003jd). Now that we’ve found one of these, the credibility of the model has increased.”

To see such double oxygen lines, the supernova nebula would have to be viewed within 20 degrees of the expanding disk, a rare situation that could explain why other Type Ic supernovae, including some associated with a gamma-ray burst, do not show the split oxygen line.

“(Our observations) strengthen the connection between gamma-ray bursts and Type Ic supernovae by showing that the Type Ic SN 2003jd appears to indeed have been an asymmetric explosion whose main axis of ejection happened not to be pointing at us,” Filippenko said.

Other coauthors of the paper are Keiichi Maeda, Jinsong Deng and Nozomu Tominaga of the University of Tokyo; Enrico Ramirez-Ruiz of the Institute for Advanced Study in Princeton, New Jersey; Stefano Benetti of the Astronomical Observatory of Padova, Italy; Elena Pian of the Trieste Observatory; Youichi Ohyama of the Subaru Telescope; Masanori Iye of Japan’s National Astronomical Observatory; Thomas Matheson of the National Optical Astronomy Observatory in Tuscon, Ariz.; Lifan Wang of Lawrence Berkeley National Laboratory; and Avishay Gal-Yam of Caltech.

The work was supported in part by the National Science Foundation, the Japan Society for the Promotion of Science and Japan’s Ministry of Education, Culture, Sports, Science and Technology.

Original Source: Berkeley News Release

Stellar nursery in the Orion Nebula. Image credit: ESO. Click to enlarge.
The most detailed measurements to date of the dusty disks around young stars confirm a new theory that the region where rocky planets such as Earth form is much farther away from the star than originally thought.

These first definitive measurements of planet-forming zones offer important clues to the initial conditions that give birth to planets. Understanding planet formation is key to understanding Earth’s origins, yet this remains a mysterious process, said John Monnier, assistant professor of astronomy at the University of Michigan and lead author on the paper, “The near-infrared size luminosity relations for Herbig Ae/Be disks” in a recent edition of Astrophysical Journal.

Very young stars are surrounded by thick, rotating disks of gas and dust, which are expected to eventually disappear as material is either pulled into the star, is blown from the disk, or collects into larger pieces of debris. This transition marks the leap from star formation to planet formation.

The scientists examined the innermost region of such disks where the star’s energy heats the dust to extremely high temperatures. These dusty disks are where the seeds of planets form, where dusty particles stick together and eventually grow to large masses.

However, if the dust orbits too close to the star, it evaporates, shutting off any hope of planet formation. It’s important to know where the evaporation begins since it has a dramatic effect on planet formation, Monnier said. The initial temperature and density of dust surrounding young stars are critical ingredients for advanced computer models of planet formation.

For the study, scientists looked at young stars that are about one and a half times the mass of the sun. “We can study these stars more in-depth because they are brighter and easier to see,” Monnier said.

In the last decade or so, beliefs about the systems that build planets have changed drastically with the onset of powerful observatories that can take more precise measurements, Monnier said.

They found that measurements thought to be accurate were actually very different than originally thought.

For this work, scientists used the two largest telescopes in the world linked together to form the Keck Interferometer. This ultra-powerful duo acts as the ultimate zoom lens allowing astronomers to peer into planetary nurseries with 10X the detail of the Hubble Space Telescope. By combining the light from the two Keck Telescopes, researchers were able to achieve the capabilities of a single telescope that spans a football field, but for a fraction of the cost, Monnier said.

Other key authors were Rafael Millan-Gabet and Rachel Akeson of the Michelson Science Center. Other key institutions included the Caltech-run, NASA Jet Propulsion Laboratory and the W.M. Keck Observatory in Kamuela, Hawaii.

The Keck Interferometer was funded by NASA and developed and operated by Jet Propulsion Lab, W.M. Keck Observatory, and the Michelson Science Center.

Original Source: U of Michigan News Release

Low-frequency radio sky at the time of a cosmic ray hit. Image credit: MPIFR. Click to enlarge.
Using the LOPES experiment, a prototype of the new high-tech radio telescope LOFAR to detect ultra-high energy cosmic ray particles, a group of astrophysicists, in collaboration of Max-Planck-Gesellschaft and Helmholtz-Gemeinschaft, has recorded the brightest and fastest radio blasts ever seen on the sky. The blasts, whose detection are reported in this week’s issue of the journal Nature, are dramatic flashes of radio light that appear more than 1000 times brighter than the sun and almost a million times faster than normal lightning. For a very short moment these flashes – which had gone largely unnoticed so far – become the brightest light on the sky with a diameter twice the size of the moon.

The experiment showed that the radio flashes are produced in the Earth atmosphere, caused by the impact of the most energetic particles produced in the cosmos. These particles are called ultra-high energy cosmic rays and their origin is an ongoing puzzle. The astrophysicists now hope that their finding will shed new light on the mystery of these particles.

The scientists used an array of radio antennas and the large array of particle detectors of the KASCADE-Grande experiment at Forschungszentrum Karlsruhe. They showed that whenever a very energetic cosmic particle hit the Earth atmosphere a corresponding radio pulse was recorded from the direction of the incoming particle. Using imaging techniques from radio astronomy the group even produced digital film sequences of these events, yielding the fastest movies ever produced in radio astronomy. The particle detectors provided them with basic information about the incoming cosmic rays.

The researchers were able to show that the strength of the emitted radio signal was a direct measure of the cosmic ray energy. “It is amazing that with simple FM radio antennas we can measure the energy of particles coming from the cosmos” says Prof. Heino Falcke from the Netherlands Foundation for Research in Astronomy (ASTRON) who is the spokesperson of the LOPES collaboration. “If we had sensitive radio eyes, we would see the sky sparkle with radio flashes”, he adds.

The scientists used pairs of antennas similar to those used in ordinary FM radio receivers. “The main difference to normal radios is the digital electronics and the broad-band receivers, which allow us to listen to many frequencies at once”, explains Dipl. Phys. Andreas Horneffer, a graduate student of the University of Bonn and the International Max-Planck Research School (IMPRS), who installed the antennas as part of his PhD project.

In principle some of the detected radio flashes are in fact strong enough to wipe out conventional radio or TV reception for a short time. To demonstrate this effect the group has converted their radio reception of a cosmic ray event into a sound track (see below). However, since the flashes only last for some 20-30 nanoseconds and bright signals happen only once a day, they would be hardly recognisable in everyday life.

The experiment also showed that the radio emission varied in strength relative to the orientation of the Earth magnetic field. This and other results verified basic predictions that had been made in theoretical calculations earlier by Prof. Falcke and his former PhD student Tim Huege, as well as by calculations of Prof. Peter Gorham from the University of Hawaii.

Cosmic ray particles constantly bombard the earth causing little explosions of elementary particles which form a beam of matter and anti-matter particles rushing through the atmosphere. The lightest charged particles, electrons and positrons, in this beam will be deflected by the geomagnetic field of the Earth which causes them to emit radio emission. This type of radiation is well known from particle accelerators on Earth and is called synchrotron radiation. In analogy, the astrophysicists now speak of “geosynchrotron” radiation due to the interaction with the Earth magnetic field.

The radio flashes were detected by the LOPES antennas installed at the KASCADE-Grande cosmic ray air shower experiment at Forschungszentrum Karlsruhe, Germany. KASCADE-Grande is a leading experiment for measuring cosmic rays. “This shows the strength of having a major astroparticle physics experiment directly in our neighbourhood – this gave us the flexibility to also explore unusual ideas as this one” says Dr. Andreas Haungs, spokesperson of KASCADE-Grande.

The radio telescope LOPES (LOFAR Prototype Experimental Station) uses prototype antennas of the largest radio telescope of the world, LOFAR, to be built after 2006 in the Netherlands and parts of Germany. LOFAR has a radical new design, combining a multitude of cheap low-frequency antennas which collect the radio signals from the entire sky at once. Connected by high-speed internet a supercomputer then has the ability to detect unusual signals and make images of interesting regions on the sky without moving any mechanical parts. “LOPES achieved the first major scientific results of the LOFAR project already in the development phase. This makes us confident that LOFAR will indeed be as revolutionary as we had hoped it will be.” explains Prof. Harvey Butcher, director of the Netherlands Foundation for Research in Astronomy (ASTRON) in Dwingeloo, The Netherlands, where LOFAR is currently being developed.

“This is indeed an unusual combination, where nuclear physicists and radio astronomers work together to create a unique and highly original astroparticle physics experiment”, states Dr. Anton Zensus, director at the Max-Planck-Institut f?r Radioastronomie (MPIfR) in Bonn. “It paves the way for new detection mechanisms in particle physics as well as demonstrating the breathtaking capabilities of the next generation telescopes such as LOFAR and later the Square Kilometer Array (SKA). Suddenly major international experiments in different research areas come together”

As a next step the astrophysicists want to use the upcoming LOFAR array in the Netherlands and Germany for radio astronomy and cosmic ray research. Test are under way to integrate radio antenna into the Pierre Auger Observatory for cosmic rays in Argentina and possibly later in the second Auger Observatory in the Northern hemisphere. “This may be a major breakthrough in detection technology. We hope to use this novel technique for detecting and understanding the nature of the highest energy cosmic rays and also to detect ultra-high energy neutrinos from the cosmos”, says Prof. Johannes Bl?mer, Astroparticle Physics programme director of the Helmholtz Association and at Forschungszentrum Karlsruhe.

The detection has been confirmed in part by a French group using the large radio telescope of the Paris observatory at Nan?ay. Historically, work on radio emission from cosmic rays was first done in the late 1960ies with the first claims of detections. However, no useful information could be extracted with the technology of these days, and the work ceased quickly. The main shortcomings were the lack of imaging capabilities (now implemented by software), the low time resolution, and the lack of a well-calibrated particle detector array. All of this has been overcome with the LOPES experiment.

Original Source: MPI News Release

Hubble deep field view. Image credit: Hubble. Click to enlarge.
Cosmologists from Princeton University announced a new method to understand why the expansion of the universe is speeding up. The proposed technique will be able to determine if the cosmic acceleration is due to a yet unknown form of Dark Energy in the universe or if it is a signature of a breakdown of Einstein’s theory of General Relativity at very large scales of the universe. The result is being presented today by the principal investigator, Dr. Mustapha Ishak-Boushaki, a research associate at Princeton University in New Jersey, to the Canadian Astronomical Society meeting in Montreal, QC.

“The accelerating expansion of the universe constitutes one of the most intriguing and challenging problems in astrophysics. Moreover, it is related to problems in many other fields of physics. Our research work is focused on constraining different possible causes of this acceleration.” says Dr. Ishak-Boushaki.

During the last 8 years, several independent astronomical observations have demonstrated that the expansion of the universe has entered a phase of acceleration. The discovery of this acceleration came as a surprise to astrophysicists who were expecting to measure a slowing down of the expansion caused by the gravitational attraction of ordinary matter in the universe.

In order to explain the cosmic acceleration, theoretical cosmologists introduced the notion of a new energy component that would constitute two thirds of the entire energy density of the universe and that is gravitationally repulsive rather than attractive. This component has been termed Dark Energy.

Is Dark Energy real? “We don’t know,” comments Professor David Spergel from Princeton. “It could be a whole new form of energy or the observational signature of the failure of Einstein’s theory of General Relativity. Either way, its existence will have profound impact on our understanding of space and time. Our goal is to be able to distinguish the two cases.”

The simplest case of Dark Energy is the cosmological constant that Einstein introduced 80 years ago in order to reconcile his theory of General Relativity with his prejudice that the universe is static. He had to withdraw the cosmological constant a few years later when the expansion of the universe was discovered. The discovery of the cosmic acceleration has revived the debate about the cosmological constant in a new context.

Another fundamentally different possibility is that the cosmic acceleration is a signature of a new theory of gravity that enters at very large scales of the universe rather than the product of Dark Energy. Some of the recently proposed modified gravity models are inspired by Superstring theory and extra dimensional physics.

Could we distinguish between these two possibilities? The proposed procedure shows that the answer is yes. The general idea is as follows. If the acceleration is due to Dark Energy then the expansion history of the universe should be consistent with the rate at which clusters of galaxies grow. Deviations from this consistency would be a signature of the breakdown of General Relativity at very large scales of the universe. The procedure proposed implements this idea by comparing the constraints obtained on Dark Energy from different cosmological probes and allows one to clearly identify any inconsistencies.

As an example, a universe described by a 5-dimensional modified gravity theory was considered in this study and it was shown that the procedure can identify the signature of this theory. Importantly, it was shown that future astronomical experiments can distinguish between modified gravity theories and Dark Energy models.

The research work on the results presented was led by Dr. Mustapha Ishak-Boushaki in collaboration with Professor David Spergel, both from the Department of Astrophysical Sciences at Princeton University, and Amol Upadhye, a graduate student at the Physics Department at Princeton University.

Original Source: Princeton News Release

Gemini North image of stellar nursery RY Tau. Image credit: Gemini. Click to enlarge.
Using a giant telescope on Mauna Kea Hawaii is a dream for most amateur sky watchers. Recently a Canadian amateur astronomy group took advantage of a rare opportunity and used one of the largest telescopes in the world, the Gemini 8-meter telescope, to look more deeply into the remains of a particular stellar nursery than anyone ever has.

The observations of a star emerging from its cocoon were the result of a proposal submitted as part of a nationwide contest in Canada. The winning group from Quebec received its data/images during a special ceremony at the annual meeting of the Canadian Astronomical Society at the University of Montreal on May 15, 2005.

“Our group knew that this object was unique and hadn’t been observed in detail with a big telescope like Gemini,” said Gilbert St-Onge, the club member who submitted the proposal. “I feel like we’ve not only made a pretty picture, but probably provided some new and valuable data for the pros!”

Gemini Astronomer Tracy Beck, who studies these stellar incubators, agrees. “This object is a classic, and one of the first-known examples of this type of young star,” she said. “I believe this is by far the deepest and most detailed image ever taken of this object and scientists will no doubt use these data for important research in the future.”

The object, known as RY Tau is part of a class of objects known as T Tauri stars. These stars represent the very youngest of low-mass stellar specimens that have only recently emerged from the cocoon of gas and dust in which they formed. The new Gemini image of RY Tau displays a striking array of wispy gas filaments that glow from scattering caused by radiation from the nearby star. Over the next few million years this gas will be blown away by the central star leaving a normal star and perhaps a family of planets that also formed from gas and dust in the cloud.

The observations, which took a total of about one hour using the Gemini Multi-Object Spectrograph (GMOS), were challenging to make. The central star is so bright that it can overwhelm the faint glowing clouds around it. To overcome this, a series of many short exposures were obtained and stacked to produce the final image. A selection of four filters were also used to bring out specific color features in the dynamic cloud.

The program was sponsored by the team of scientists who coordinate Gemini observations for Canada (through the Canadian Gemini Office) at the National Research Council of Canada’s Herzberg Institute of Astrophysics (HIA) in Victoria. B.C. The contest, which began in 2004, solicited proposals from more than a hundred amateur astronomy clubs throughout Canada as a way to thank them for the work they do to support and excite the public about astronomy. The winning proposal was selected by a process similar to that used by professional astronomers, where selection criteria include scientific merit and an assessment of the uniqueness of the observation.

“When we first worked on scheduling these observations, we jokingly referred to the program as the “amateur hour” since it allows amateur astronomers to get an hour of time on a large telescope,” said Doug Welch, Canadian Gemini Project Scientist. “However, the caliber of the proposals and scientific potential of this data has shown that it is more like a pro-am golf tournament where the hobbyists work directly with the pros!”

The contest also included an hour of time on Gemini’s neighbor on Mauna Kea, the Canada-France-Hawaii Telescope (CFHT). The winning observation at CFHT was from a group in Alberta, Canada who used the wide-field capability of the telescope to image a large field of the Pleiades star cluster with the MegaPrime imager.

Original Source: Gemini News Release

A negative image of the Sun showing the active region. Image credit: PPARC. Click to enlarge.
Solar physicists have observed the smallest ever coronal mass ejection (CME) – a type of explosion where plasma from the Sun is thrown out into space, sometimes striking the Earth and damaging orbiting satellites. The observation has come as a great surprise to scientists and has turned previous ideas up-side-down.

To date studies of these phenomena have focussed on large explosions which are easier to detect and which have massive footprints on the Sun, sometimes covering thousands of millions of square miles. But in a paper published in the May edition of Astronomy and Astrophysics, an international team from the UK, Argentina, Finland, France and Hungary showed that CMEs can also be produced from regions as small as the Earth, around 10,000 miles across. This still may sound large but it is tiny by cosmic standards.

CMEs are believed to be caused by the destabilisation of twisted loops in the Sun’s magnetic field, which contain lots of energy, settling into more stable positions (like a twisted rubber band unwinding suddenly). Until now, the events have been traced back to large areas of magnetic activity on the Sun, but the new observations relate to an area much smaller than anything seen before. However, even though the event was small it was still energetic enough to reach the Earth and amazingly the magnetic field lines were ten times more twisted than is usually seen in the larger areas.

Understanding CMEs and the mechanisms that power them is important because the plasma and accelerated particles they throw into space can damage satellites, cause harm to astronauts and even affect the Earth itself, causing beautiful aurora but also power black outs and problems to radio signals. This is the science of space weather.

Dr Lucie Green of UCL’s Mullard Space Science Laboratory said “Previously coronal mass ejections were thought to be huge, involving massive portions of the Sun’s magnetic field and all the theoretical models are based around this assumption. However, this one was amazing in that it came from a tiny magnetic region on the Sun which would normally have been overlooked in the search for CME source regions. This will be an exciting area for further study.”

Existing models for CMEs are based on the type of large event previously observed and the team cannot yet say how frequent such mini CMEs are or whether they represent a significant part of space weather. The event was so small that is was almost at the limit of what we can see with current instruments. Future missions studying the Sun will be able to ‘see’ in much better detail, such as the UK-US-Japanese mission called Solar-B.

The research used data from NASA/ESA’s SOHO spacecraft, NASA’s TRACE satellite and from the now defunct Japanese/US/UK Yohkoh satellite. UK involvement was funded by PPARC.

Original Source: PPARC News Release

An artist’s impression of merging neutron stars, one of the theoretical progenitors of gamma-ray bursts. Image credit: NASA E/PO, Sonoma State University, Aurore Simonnet. Click to enlarge.
Gamma Ray bursters (GRBs) are almost a daily occurrence – so you’d think follow-up on such things would be routine by now – but that’s just not the case. Imagine a fire department having to respond to a new fire every day: they’d still have to pull on hats and heavy raincoats, slide down shiny metallic poles, and jump onto their fire engine as fast as possible.

And on Monday, May 9th, 2005 there were two fires; one reported earlier in the day by NASA’s HETE-2, and then another by its even more sophisticated associate SWIFT.

So even as we were working on a previous story for Universe Today readers about a dark Gamma Ray Burster detected by HETE-2 in August 2002, two new bursts occurred overhead. Both events sounded the alarm and astronomers worldwide scrambled to capture the rapidly diminishing optical afterglow (OAs) that might help explain one of the deepest mysteries in the Universe: how can so much energy be packed into those tiny, massless photons?

GRB events themselves are of two types. Some are “hard and fast” while others are “soft and slow”. When a “hard-fast” event occurs, astronomical fire fighters have to work quickly.

It all begins when HETE-2 or SWIFT detects an outburst from their lonely perches in low Earth orbit. Once a burst is detected, instruments aboard the craft swing around to make a determination of just where in space that event has occurred and capture whatever lingering x-ray data may still be available. Here SWIFT has the advantage over HETE-2 due to its smaller “error circle” – the satellite’s best guess as to the location of the event in the heavens. Data related to location, burst strength, and duration is then beamed to NASA instrument teams on Earth where the data is integrated and advisories are published to enlist available astronomers and instruments.

All this is comparable to an “all points bulletin” going out so astronomers anywhere can scramble to capture what is now the rapidly diminishing afterglow of the main event. Typically only small instruments associated with major observatories respond first, since larger instruments are scheduled months in advance for other projects. You’ll never see the mighty Hubble Space Telescope swing around for one of these events.

Even as available scopes turn to the sky, astronomers look for detailed survey charts of the region associated with the error circle. Why? Because images and data may already be available to simplify afterglow identification along with baseline spectra, and recessionary distances of host galaxies to assist their analysis.

As time passes, larger and larger instruments are made available for use. This is essential since dark GRBs fade quickly and more and more optical depth is needed to pick up the trail. In the case of GRB020819 detected by HETE-2 in 2002 not even the 10-meter Keck ESI instrument (capturing 26th magnitude light) was able to image an afterglow even though it swung around within 15 days of the initial high energy burst. But with SWIFT’s rapid response and tighter error circles its now becoming possible to catch a dark GRB in the act of disappearing for good.

Of the two GRB events of the day, GRB050509B is most promising because its optical light afterglow is relatively dim (fainter than the 20th magnitude) and galaxy is relatively bright (the 15th). And it is because roughly 1 in 10 GRBs are optically impoverished that makes them specially interesting to astrophysicists. Basically current models used to explain dark GRB events predict that a broad range of photon-energy levels should be seen to accompany any gamma-ray outburst. The fact that some GRBs don’t happen to map well against theory is troublesome, and therein lies the itch behind GRB020819’s (and now GRB050509B’s) scratch.

One theory of GRBs is that gamma rays somehow “muscle their way” through intervening matter in space while optical light – the “afterglow” of such an event – does not. This optical extinction scenario has yet to be demonstrated and several dark GRBs show no sign of the kind of massive clouds of intervening matter needed to support it.

Meanwhile there are three theories of the fireball-shock type postulated by astrophysicists to account for all rapidly diminishing short-time scale GRBs. The fireball scenario typically involves an explosion that hurls extremely hot gases at exceedingly high speeds into space after some kind of a cataclysmic event. These gases then interact with other material already present in the interstellar medium (ISM) or material previously ejected into space by the same object at lower velocities. One theory postulates a “relativistic jet” of hot gases penetrating either a stratified “onion” of other slower moving gases or a simple homogeneously filled-“bubble” of gases. The others assume either a rapidly expanding sphere of hot gases into the same type “bubble” or “onion”.

According to an international team of astronomers, data analyzed across the EM spectrum from GRB020819 supports the expanding sphere into homogeneous medium fireball-shock model. Those findings have been documented in a paper entitled “The Radio Afterglow and Host Galaxy of the Dark GRB020819” published May 2, 2005. The related Universe Today article is an online companion to this article entitled “Shedding Light on Dark Gamma Ray Bursters“.

Meanwhile – at this very moment – astronomical “firefighters and fire marshals” are gathering data associated with SWIFT’s detection of GRB 050509B. This process may continue in one form or another for as long as six months. Several years hence a team of investigators – such as that associated with GRB020819 – may attempt to make sense of how a little understood paroxysm of matter and energy occuring some 2.5 billion years ago in the direction of the springtime constellation Coma Berenices could ever have happened in the first place.

To make all this possible numerous and ever larger “eyes on the skies” will have to collect and focus radio, near-infrared, optical, and near-ultraviolet light. All of that information will be sifted through with infinite patience in an effort to forensically determine not “who done it” but “what”.

Written by Jeff Barbour

Dark gamma ray burst GRB020819. Image credit: Keck. Click to enlarge.
Virtually everything we know about the Universe comes to us through the agency of light. Unlike matter, light is uniquely suited to travel the vast distances across space to our instruments. Most astronomical phenomena however are persistent and repeatable – we can rely on them to “hang around” for long-term observation or “come back around” on a regular basis. But this isn’t so for gamma ray bursts (GRB’s) – those mysterious cosmological events that supercharge photons (and sub-atomic particles) with absurdly high energy levels.

The first detected celestial GRB occurred during nuclear arms treaty monitoring in 1967. That event required years of analysis before its extraterrestrial origin was confirmed. After this discovery, primitive triangulation methods were put in place using detectors located on various space probes within the Interplanetary Network (IPN). Such methods required a great deal of number crunching and made instant follow-up using Earth-based instruments impossible. Despite the delays involved, hundreds of gamma ray sources were catalogued. Today – even using the Internet – it would still require several days to respond using an IPN-type detection approach.

All this began to change in 1991 when NASA put the Compton Gamma Ray Observatory (CGRO) into space using space shuttle Atlantis as part of its “Great Observatories” program. Within four months of scanning the sky, CGRO made it clear to astronomers that the Universe underwent sporadic and widely distributed gamma ray paroxysms on an almost daily basis – paroxysms caused by cataclysmic events that hurl vast quantities of gamma and other high-energy radiation across the abyss of space-time.

But CGRO had one main limitation – although it could detect gamma rays and alert astronomers quickly, it wasn’t particularly accurate as to where such events happened in space. Because of this large “error circle”, astronomers were unable to locate the visible light “afterglow” of such events. Despite this limitation, CGRO went on to detect hundreds of continuous, periodic, and episodic gamma ray sources – including supernovae, pulsars, black holes, quasars, and even the Earth itself! Meanwhile CGRO also discovered something unsuspected – certain pulsars acted as narrow band transmitters of gamma rays without accompanying visible light – and therein lay astronomer?s first sense of “dark” GRBs.

Today we know that “dark pulsars” are not the only “dark” sources of gamma rays in the Universe. Astronomers have determined that some small portion of episodic (one-time-only) GRBs are also low in visible light, and they – like anyone tickled by the unusual and inexplicable – want to know why. In fact GRB’s are so unique that aficionados may often be heard saying “When you’ve seen one GRB, you’ve seen one GRB”.

The first satellite to simplify optical detection of GRB afterglows was BeppoSAX. Developed by the Italian Space Agency in the mid 1990’s, BeppoSAX launched April 30, 1996 from Cape Canaveral and continued to detect and pinpoint X-ray emission sources until 2002. BeppoSax’s error circle was small enough to enable optical astronomers to rapidly track down many GRB afterglows for detailed study in visible light using earth-based instruments.

BeppoSAX re-entered the Earth’s atmosphere April 29, 2003, but by this time NASA’s replacement (HETE-2 the High Energy Transient Explorer-2) was already several years on station in low-earth orbit. Instrument’s on HETE-2 (its first incarnation HETE failed to separate from the third stage of its Pegasus rocket in 1996) expanded the range of X-ray detection and provided even tighter error circles – just the thing astronomers needed to improve their response time in locating GRB afterglows.

Two years and a few months later (Monday, August 19, 2002) HETE-2 set off the bells and whistles as a strong gamma ray source was detected somewhere near the head of the constellation Pisces the Fishes. That event (designated GRB 020819) caused a series of astronomical observatories to begin capturing radio-frequency, near-infrared, and visible light photons in an effort to determine just where the event occurred and help make sense of the phenomenon driving it.

According to the paper “The Radio Afterglow and Host Galaxy of the Dark GRB 020819” published May 2, 2005 by an international team of investigators (including Pall Jakobsson of the Niels Bohr Institute, Copenhagen Denmark who proofed this article), within 4 hrs of detection the 1 meter Siding Spring Observatory (SSO) telescope in Australia was turned to a region of space less than 1/7th the apparent diameter of the Moon. 13 hours later, a second, slightly larger instrument – the 1.5 meter P60 unit at Mt. Palomar – also joined the chase. Neither instrument – despite capturing light as faint as magnitude 22 – caught anything unusual for that region of space. However a large and extremely photogenic 19.5 magnitude face-on barred spiral galaxy fell nicely within the grasp of their instruments.

Fifteen days later, the 10 meter Keck ESI instrument on Mauna Kea, Hawaii imaged the same region in blue and red light down to magnitude 26.9. At this optical depth, a distinct 24th magnitude “blob” (suspected to be an HII star-formation region) could be seen 3 arc seconds north of the spiral galaxy. A final attempt to detect anything further was made January 1, 2003 – again using the Keck 10 meter. No change was seen in optical light emanating from the region of GRB 020819. All this confirmed that no visible afterglow accompanied the gamma ray outburst detected by HETE-2 some 134 days earlier. The investigating team had their “dark gamma ray burster”. Later would come the task of figuring out just what the heck it was – or at least was not…

Periodically throughout the cycle of optical and near-infrared inspection, the region of the burst was monitored in radio-wave frequencies. Using the VLA (Very Large Array – consisting of 27 Y-configured 25 meter dishes located fifty miles west of Socorro, New Mexico) the team succeeded in capturing a dwindling trail of 8.48 Ghz radiation and identified its locale.

First radio waves from GRB 020819 were collected 1.75 days after the HETE-2 alert. By day 157, rf energy levels flattened to the point where the source could no longer be seen with confidence. However by this time, its location had been pinpointed to the “blob” three arc-seconds north of the core of the previously uncharted spiral galaxy. Unfortunately – due to its faintness – the distance to the blob itself could not be determined spectrographically – however the galaxy was found to lie some 6.2 BLY away and enjoys “high-confidence” in terms of having a relationship with the source.

As a result of such investigations astronomers are now learning more and more about a class of cataclysmic events that results in massive fluxes of high and low energy photons while almost completely skipping intermediate frequencies – such as ultraviolet, visible, and near-infrared of light. Is there anything that could account for this?

Based on learning from GRB 020819, the team explored three fireball-shock models of how dark GRBs might occur. Of the three (an even expansion of high energy gases into a homogenous medium, even expansion into a stratified medium, and a collimated jet penetrating either type medium), the best fit against GRB 020819 behaviors was that of an even expansion of high energy gases into a homogenous medium of other gases (a model first proposed by the astrophysicist R. Sari et al in 1998). The virtue of this isotropic-expansion model being (in the words of the investigating team) that “only a modest amount of extinction must be invoked” to account for the absence of visible light.

In addition to narrowing the range of possible scenarios associated with dark GRBs, the team concluded that “GRB 020819, a relatively nearby burst, is only one of two of the 14 GRB’s localized to within (2 arc minutes using) HETE-2 that does not have a reported OA. This lends support to the recent proposition that the dark burst fraction is far lower than previously suggested, perhaps as small as 10%.”

Written by Jeff Barbour