Best Ultraviolet Image of Andromeda Galaxy

Image credit: NASA

NASA’s Galaxy Evolution Explorer (GALEX) has captured the most sensitive and comprehensive ultraviolet images ever taken of the Andromeda galaxy, M31. By studying the galaxy in the ultraviolet spectrum, astronomers can study some of the fundamental processes that lead the formation of new stars. A new collection of images included Andromeda, as well as the globular cluster M2, and the sky in the constellation of Bootes. GALEX was launched in April, 2003, and will map the sky in the ultraviolet spectrum, looking back to 10 billion years ago.

The most sensitive and comprehensive ultraviolet image ever taken of the Andromeda Galaxy, our nearest large neighbor galaxy, has been captured by NASA?s Galaxy Evolution Explorer. The image is one of several being released to the public as part of the mission?s first collection of pictures.

“The Andromeda image gives us a snapshot of the most recent star formation episode,” said Dr. Christopher Martin, Galaxy Evolution Explorer principal investigator and an astrophysics professor at the California Institute of Technology in Pasadena, which leads the mission. ?By studying this view of the galaxy in the process of forming stars, we can better understand how that fundamental process works, such as where stars form, how fast and why.?

The image of Andromeda, the most distant object the naked eye can see, is a mosaic of nine images taken in September and October of 2003. It combines two ultraviolet colors, one near ultraviolet (red) and one far ultraviolet (blue).

For comparison, a second image shows the Andromeda Galaxy, also called Messier 31, in visible light. Both images, along with other new pictures from the Galaxy Evolution Explorer, are available online at http://www.galex.caltech.edu and http://photojournal.jpl.nasa.gov/mission/GALEX . The new collection of images also includes views of several nearby galaxies; Stephan’s Quintet of Galaxies; an all-sky survey image of the globular star cluster M2; and a deep image of the sky in the constellation Bootes. The Galaxy Evolution Explorer team is also releasing the first batch of scientific data, so the science community can propose additional observations for the mission. These images and data display the power of the Galaxy Evolution Explorer to collect sensitive ultraviolet images of large parts of the sky.

“It?s very rewarding and exciting for the team to see the fruits of their labors,” said Kerry Erickson, the mission?s project manager at NASA?s Jet Propulsion Laboratory, Pasadena, Calif. ?Because people are accustomed to seeing objects in visible light, it?s amazing to see how different the universe looks in ultraviolet and how much information is revealed to us by those observations.?

Scientists are interested in learning more about the Andromeda galaxy, including its brightness, mass, age, and the distribution of young star clusters in its spiral arms. This will provide a tremendous amount of information about the mechanisms of star formation in galaxies, and will help them interpret ultraviolet and infrared observations of other, more distant galaxies.

The Galaxy Evolution Explorer launched on April 28, 2003. Its goal is to map the celestial sky in the ultraviolet and determine the history of star formation in the universe over the last 10 billion years. From its orbit high above Earth, the spacecraft will sweep the skies for up to 28 months using state-of-the-art ultraviolet detectors. Looking in the ultraviolet singles out galaxies dominated by young, hot, short-lived stars that give off a great deal energy at that wavelength. These galaxies are actively creating stars, and therefore provide a window into the history and causes of galactic star formation.

In addition to leading the mission, Caltech is also responsible for science operations and data analysis. JPL, a division of Caltech, manages the mission and led the science instrument development. The mission is part of NASA’s Explorers Program, managed by the Goddard Space Flight Center, Greenbelt, Md. The mission’s international partners are France and South Korea. Caltech manages JPL for NASA

Original Source: NASA News Release

Recycling Extends Ring Lifetimes

Image credit: NASA/JPL

New research from the University of Colorado shows how recycling of material can extend the lifetime of a ring system, such as those around Jupiter, Saturn, Neptune and Uranus. The small moons near the gas giants have long been known to sculpt the shape of the rings. It’s now believed that they’re piles of loosely-collected rubble which pull material out of the rings and then feed it back in when they collide with another object. NASA’s Cassini spacecraft is on its way to Saturn now and should provide more details when it arrives in July 2004.

Although rings around planets like Jupiter, Saturn, Uranus and Neptune are relatively short-lived, new evidence implies that the recycling of orbiting debris can lengthen the lifetime of such rings, according to University of Colorado researchers.

Strong evidence now implies small moons near the giant planets like Saturn and Jupiter are essentially piles of rubble, said Larry Esposito, a professor at CU-Boulder’s Laboratory for Atmospheric and Space Physics. These re-constituted small bodies are the source of material for planetary rings.

Previous calculations by Esposito and LASP Research Associate Joshua Colwell showed the short lifetimes for such moons imply that the solar system is nearly at the end of the age of rings. “These philosophically unappealing results may not truly describe our solar system and the rings that may surround giant extra-solar planets,” said Esposito. “Our new calculations of models explain how inclusion of recycling can lengthen the lifetime of rings and moons.”

The observations from the Voyager and Galileo space missions showed a variety of rings surrounding each of the giant planets, including Jupiter, Saturn, Uranus and Neptune. The rings are mixed in each case with small moons.

“It is clear that the small moons not only sculpt the rings through their gravity, but are also the parents of the ring material,” said Esposito. “In each ring system, destructive processes like grinding, darkening and spreading are acting so rapidly that the rings must be much younger than the planets they circle.”

Numerical models by Esposito and Colwell from the 1990’s showed a “collisional cascade,” where a planet’s moons are broken into smaller moons when struck by asteroids or comets. The fragments then are shattered to form the particles in new rings. The rings themselves are subsequently ground to dust, which is swept away.

But according to Colwell, “Some of the fragments that make up the rings may be re-accreted instead of being ground to dust. New evidence shows some debris has accumulated into moons or moonlets rather than disappearing through collisional erosion.”

“This process has proceeded rapidly,” said Esposito. “The typical ring is younger than a few hundred million years, the blink of an eye compared to the planets, which are 4.5 billion years old. The question naturally arises why rings still exist, to be photographed in such glory by visiting human spacecraft that have arrived lately on the scene,” he said.

“The answer now likely seems to be cosmic recycling,” said Esposito. Each time a moon is destroyed by a cosmic impact, much of the material released is captured by other nearby moons. These recycled moons are essentially collections of rubble, but by recycling material through a series of small moons, the lifetime of the ring system may be longer than we initially thought.”

Esposito and former LASP Research Associate Robin Canup, now with the Southwest Research Institute’s Boulder branch, showed through computer modeling that smaller fragments can be recaptured by other moons in the system. “Without this recycling, the rings and moons are soon gone,” said Esposito.

But with more recycling, the lifetime is longer, Esposito said. With most of the material recycled, as now appears to be the case in most rings, the lifetime is extended by a large factor.

“Although the individual rings and moons we now see are ephemeral, the phenomenon persists for billions of years around Saturn,” said Esposito. “Previous calculations ignored the collective effects of the other moons in extending the persistence of rings by recapturing and recycling ring material.”

Esposito, the principal investigator on a $12 million spectrograph on the Cassini spacecraft slated to arrive at Saturn in July 2004, will look closely at the competing processes of destruction and re-capture in Saturn’s F ring to confirm and quantify this explanation. Esposito discovered the F Ring using data from NASA’s Voyager 2 mission to the outer planets launched in 1978.

Original Source: University of Colorado News Release

Cocoon of Hydrogen Around a Young Star

Image credit: JACH

A young, hot star has been found nestled inside a cocoon of molecular hydrogen gas half a light-year across. The star is called IRAS 07427-2400, and it’s sending out a solar wind so fast (360,000 km/h) that the shock waves are heating up the gas so it’s visible to Earth-based telescopes. Astronomers believe that these massive stars have so much energy that they blast their environment so that planets aren’t able to form the way they do around more “normal” stars, like our own Sun.

Astronomers have discovered a giant envelope or disk of glowing gas more than half a light year across, illuminated by shockwaves caused by winds travelling at up to 360,000 km/hour (220,000 miles/hour). The disk is orbiting a massive star 20,000 light years from Earth. This is the first time such a disk has been found emitting its own light. The discovery is reported today (8 December 2003) in the journal “Astronomy and Astrophysics”.

The work, led by Dr Nanda Kumar of the Centre for Astrophysics of the University of Porto (CAUP), Portugal, used the United Kingdom Infrared Telescope (UKIRT) in Hawaii, and other telescopes. The team used the new UKIRT Imager Spectrometer (UIST) on UKIRT, to study the young stellar object (YSO) known as IRAS 07427-2400. Their results show that the envelope or disk around the young star is glowing in the light of molecular hydrogen and ionised iron.

Dr Stan Kurtz of the National Autonomous University of Mexico (UNAM), who is an expert in studies of solar system sized disks around massive stars, said “Protostellar disks are known to exist around Sun-like stars, but they are usually seen in silhouette against background light from nebulae. In this case, however, the molecules in the disk are hot enough to shine brightly themselves.”

Dr Kumar adds “This is the first time an envelope like this has been seen in molecular hydrogen emission. It tells us that massive stars form with very different conditions and physical aspects when compared to Sun-like stars.”

The central star itself is very young, at roughly 100,000 years old. By comparison, our middle-aged Sun is about 5 billion years old. The surrounding gas disk is huge – its diameter is one thousand times larger than Pluto’s orbit in our own Solar System. The young star is rapidly changing as gas and dust spiral down onto its surface through the disk, a process called “accretion”. The star is already more than one thousand times more luminous than our Sun.

Dr Amadeu Fernandes of CAUP, Porto states “The UKIRT results show that the glow from the disk is not due to the intense light from the central star, but is instead caused by powerful shock waves”. Dr Chris Davis of the Joint Astronomy Centre in Hawaii explains “The disk is possibly being shocked by supersonic winds driven by the central star. These winds, travelling at hundreds of thousands of kilometres per hour, crash into the disk and heat the gas to thousands of degrees.”

Dr Kumar adds “It is also possible that the shocks are powered by large amounts of gas and dust collapsing through the disk onto the young star. Further investigation is required to understand their origin.”

Disks around young, Sun-like stars are known to be the birth places of planets, which can condense out of the gas and dust after the star has formed. This disk has about 150 times the mass of our Sun – enough gas and dust to make a hundred Sun-like stars, or many thousands of planets. However, the results suggest that it will not produce new planets or stars in the future. The intense shock waves have made the gas far too hot to condense. Dr Davis says “This tells us that massive stars like this may not be able to form planets, as their surrounding gas is too hot.”

Instead of forming a cluster of stars, or a family of orbiting planets, the disk will ultimately be destroyed by the intense ultraviolet radiation from the central star. The radiation is already at work, gnawing at the inner edges of the disk and evaporating the gas. Dr Kumar says “We’ve seen open rings of gas around similar stars, also with UKIRT. We think they may be the remnants of large disks that have been almost completely evaporated.”

The complete destruction of the disk will take many thousands of years. Before this happens, the size and brightness of the disk allow researchers to study it with powerful ground-based telescopes such as UKIRT, without the need for a space telescope.

Dr Davis says “We now have the task of searching for other hot, molecular disks around massive young stars, and of fitting the existence of this super-disk into our theories on the birth of massive stars.”

The disk was first discovered in January 2001 by UKIRT, but further observations were needed to confirm its nature. The team used the Caltech Submillimeter Observatory in Hawaii to provide supporting evidence to prove the rotating nature of the disk. Stan Kurtz used the Very Large Array radio telescope in New Mexico to image the central massive star at radio wavelengths. The team returned to use UKIRT in December 2002.

The work described is published on 8th December 2003 in “Astronomy and Astrophysics” volume 412.

Original Source: JACH News Release

Distant Galaxy is Furiously Making Stars

Image credit: NRAO

One of the most distant galaxies ever seen seems to be in the midst of extremely active star formation. The galaxy has been dubbed the Cloverleaf, and it’s 11 billion light-years away, so astronomers are seeing it when the Universe was less than 3 billion years old. It has a rate of star formation 300 times greater than our own Milky Way – 1,000 new stars are being formed each year. The discovery was made using the National Science Foundation’s Very Large Array radio telescope.

Astronomers have discovered a key signpost of rapid star formation in a galaxy 11 billion light-years from Earth, seen as it was when the Universe was only 20 percent of its current age. Using the National Science Foundation’s Very Large Array (VLA) radio telescope, the scientists found a huge quantity of dense interstellar gas — the environment required for active star formation — at the greatest distance yet detected.

A furious spawning of the equivalent of 1,000 Suns per year in a distant galaxy dubbed the Cloverleaf may be typical of galaxies in the early Universe, the scientists say.

“This is a rate of star formation more than 300 times greater than that in our own Milky Way and similar spiral galaxies, and our discovery may provide important information about the formation and evolution of galaxies throughout the Universe,” said Philip Solomon, of Stony Brook University in New York.

While the raw material for star formation has been found in galaxies at even greater distances, the Cloverleaf is by far the most distant galaxy showing this essential signature of star formation. That essential signature comes in the form of a specific frequency of radio waves emitted by molecules of the gas hydrogen cyanide (HCN).

“If you see HCN, you are seeing gas with the high density required to form stars,” said Paul Vanden Bout of the National Radio Astronomy Observatory (NRAO).

Solomon and Vanden Bout worked with Chris Carilli of NRAO and Michel Guelin of the Institute for Millimeter Astronomy in France. They reported their results in the December 11 issue of the scientific journal Nature.

In galaxies like the Milky Way, dense gas traced by HCN but composed mainly of hydrogen molecules is always associated with regions of active star formation. What is different about the Cloverleaf is the huge quantity of dense gas along with very powerful infrared radiation from the star formation. Ten billion times the mass of the Sun is contained in dense, star-forming gas clouds.

“At the rate this galaxy is seen to be forming stars, that dense gas will be used up in only about 10 million years,” Solomon said.

In addition to giving astronomers a fascinating glimpse of a huge burst of star formation in the early Universe, the new information about the Cloverleaf helps answer a longstanding question about bright galaxies of that era. Many distant galaxies have super-massive black holes at their cores, and those black holes power “central engines” that produce bright emission. Astronomers have wondered specifically about those distant galaxies that emit large amounts of infrared light, galaxies like the Cloverleaf which has a black hole and central engine.

“Is this bright infrared light caused by the black-hole-powered core of the galaxy or by a huge burst of star formation? That has been the question. Now we know that, in at least one case, much of the infrared light is produced by intense star formation,” Carilli said.

The rapid star formation, called a starburst, and the black hole are both generating the bright infrared light in the Cloverleaf. The starburst is a major event in the formation and evolution of this galaxy.

“This detection of HCN gives us a unique new window through which we can study star formation in the early Universe,” Carilli said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Astronomers Find a Pair of Neutron Stars

Image credit: CSIRO

Astronomers have discovered a pair of neutron stars that could assist in the search for the long theorized “gravity waves”, first predicted by Einstein. Separated by only 800,000 kilometres, the twin objects take only two hours to rotate each other. The theory is that the pair is losing energy in the form of gravity waves, and will eventually slow down and merge with a blast of energy. This new discovery tells astronomers that these twin neutron stars are more common than previously believed, and new gravity wave detectors should locate a merger every year or two, and not once a decade.

Neutron star pairs may merge and give off a burst of gravity waves about six times more often than previously thought, scientists report in today?s issue of the journal Nature [4 December]. If so, the current generation of gravity-wave detectors might be able to register such an event every year or two, rather than about once a decade ? the most optimistic prediction until now.

Gravity waves were predicted by Einstein?s general theory of relativity. Astronomers have indirect evidence of their existence but have not yet detected them directly.

The revised estimate of the neutron-star merger rate springs from the discovery of a double neutron-star system, a pulsar called PSR J0737-3039 and its neutron-star companion, by a team of scientists from Italy, Australia, the UK and the USA using the 64-m CSIRO Parkes radio telescope in eastern Australia.

Neutron stars are city-sized balls of a highly dense, unusual form of matter. A pulsar is a special type ? a spinning neutron star that emits radio waves.

PSR J0737-3039 and its companion are just the sixth known system of two neutron stars. They lie 1600-2000 light-years (500-600 pc) away in our Galaxy.

Separated by 800,000 km ? about twice the distance between the Earth and Moon ? the two stars orbit each other in just over two hours.

Systems with such extreme speeds have to be modelled with Einstein?s general theory of relativity.

?That theory predicts that the system is losing energy in the form of gravity waves,? said lead author Marta Burgay, a PhD student at the University of Bologna.

?The two stars are in a ?dance of death?, slowly spiralling together.?

In 85 million years the doomed stars will fuse, rippling spacetime with a burst of gravity waves.

?If the burst happened in our time, it could be picked up by one of the current generation of gravitational wave detectors, such as LIGO-I, VIRGO or GEO? said team leader Professor Nicol? D?Amico, Director of the Cagliari Astronomical Observatory in Sardinia.

The previous estimate of the neutron-star merger rate was strongly influenced by the characteristics of just one system, the pulsar B1913+16 and its companion. PSR B1913+16 was the first relativistic binary system discovered and studied, and the first used to show the existence of gravitational radiation.

PSR J0737-3039 and its companion are an even more extreme system, and now form the best laboratory for testing Einstein?s prediction of orbital shrinking.

The new pulsar also boosts the merger rate, for two reasons.

It won?t live as long as PSR B1913+16, the astronomers say. And pulsars like it are probably more common than ones like PSR B1913+16.

?These two effects push the merger rate up by a factor of six or seven,? said team member Dr Dick Manchester of CSIRO.

But the actual numerical value of that rate depends on assumptions about how pulsars are distributed in our Galaxy.

?Under the most favourable distribution model, we can say at the 95% confidence level that this first generation of gravitational wave detectors could register a neutron star merger every one to two years,? said Dr Vicky Kalogera, Assistant Professor of Physics and Astronomy at Northwestern University in Illinois, USA.

Dr Kalogera and colleagues Chunglee Kim and Duncan Lorimer have modelled binary coalescence rates using a range of assumptions.

The new result is ?good news for gravity-wave astronomers,? according to team member Professor Andrew Lyne, Director of the Jodrell Bank Observatory of the University of Manchester in the UK.

?They may get to study one of these cosmic catastrophes every few years, instead of having to wait half a career,? he said.

Original Source: CSIRO News Release

Stardust Approaches Comet Wild 2

Image credit: NASA/JPL

NASA’s Stardust spacecraft took this photograph of its target, Comet Wild 2, while it was still 25 million kilometers away. The spacecraft is on track to reach the comet on January 2, 2004 when it will pass only 300 km away and capture particles of its tail to return to Earth for analysis – the best photographs are still to come. Mission planners will use these early images to help fine-tune Startdust’s trajectory to give it the closest possible approach to Wild 2’s centre.

Forty-nine days before its historic rendezvous with a comet, NASA’s Stardust spacecraft successfully photographed its quarry, comet Wild 2 (pronounced Vilt-2), from 25 million kilometers (15.5 million miles) away. The image, the first of many comet portraits it will take over the next four weeks, will aid Stardust?s navigators and scientists as they plot their final trajectory toward a Jan. 2, 2004 flyby and collection of samples from Wild 2.

?Christmas came early this year,? said Project Manager Tom Duxbury at NASA?s Jet Propulsion Laboratory, Pasadena, Calif. ?Our job is to aim a 5 meter (16 foot) long spacecraft at a 5.4 kilometer (3.3 mile) wide comet that is closing on it at six times the speed of a bullet. We plan to ?miss the comet? by all of 300 kilometers (188 miles), and all this will be happening 389 million kilometers (242 million miles) away from home. By finding the comet as early and as far away as we did, the complexity of our operations leading up to encounter just dropped drastically.?

The ball of dirty ice and rock, about as big as three Brooklyn Bridges laid end-to-end, was detected on November 13 by the spacecraft?s optical navigation camera on the very first attempt. The set of images was stored in Stardust?s onboard computer and downloaded the next day where mission navigator Dr. Shyam Bhaskaran processed them and noticed a white blob of light bisecting the base of a triangle made by three stars Stardust uses for deep space navigation.

?When I first looked at the picture I didn?t believe it,? said Bhaskaran. ?We were not expecting to observe the comet for at least another two weeks. But there it was, very close to where we thought it would be.?

The Wild 2 sighting was verified on November 18 using the second set of optical navigation images downloaded from Stardust. To make this detection, the spacecraft?s camera saw stars as dim as 11th visual magnitude, more than 1,500 times dimmer than a human can see on a clear night.

The early detection of Wild 2 provides mission navigators critical information on the comet?s position and orbital path. Future optical navigation images will allow them to do more fine-tuning. In turn, these new orbital plots will be used to plan the spacecraft?s approach trajectory correction maneuver. Stardust?s first such maneuver is planned for December 3.

Unlike other orbiting bodies, the paths of comets cannot be precisely predicted because their orbits about the Sun are not solely determined by gravity. The escape of gas, dust and rock from comets provides a “rocket effect” that causes them to stray from a predictable orbital path. The actual orbital path cannot be precisely determined from Earth-based telescopes because the comet is shrouded in a cloud of escaping gas and dust. What is seen from Earth is not the actual 5.4 kilometer (3.3 mile) wide body composed of rock and ice, but the cloud of debris and gas that envelops it.

?With these images we anticipate we will flyby comet Wild 2 at an altitude of 300 kilometers, give or take about 16 kilometers,? added Bhaskaran. ?Without them, we wouldn?t be able to safely get any closer to the comet than several thousand kilometers.?

Stardust will return to Earth in Jan. 2006 to make a soft landing at the U.S. Air Force Utah Test and Training Range. Its sample return capsule, holding microscopic particles of comet and interstellar dust, will be taken to the planetary material curatorial facility at NASA’s Johnson Space Center, Houston, where the samples will be carefully stored and examined.

Stardust?s cometary and interstellar dust samples will help provide answers to fundamental questions about the origins of the solar system. More information on the Stardust mission is available at http://stardust.jpl.nasa.gov .

Stardust, a part of NASA’s Discovery Program of low-cost, highly focused science missions, was built by Lockheed Martin Astronautics and Operations, Denver, Colo., and is managed by JPL for NASA’s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena. The principal investigator is astronomy professor Donald E. Brownlee of the University of Washington in Seattle.

Original Source: NASA/JPL News Release

The Next Supernova?

Image credit: ESO

The European Southern Observatory has released new images of a relatively nearby star, Eta Carina, which could be in the final stages of its life and could explode as a supernova in the near future (astronomically-speaking) – within the next 10-20,000 years or so. The star is 7,500 light years away, 100 times the mass of the Sun, and the most luminous object in the Milky Way. Since 1841, it has created a beautiful nebula around itself by continuously shedding outside layers while it spins quickly. By watching how Eta Carina changes, astronomers will gain valuable insights into the final stages of a supermassive star’s life.

Ever since 1841, when the until then inconspicuous southern star Eta Carinae underwent a spectacular outburst, astronomers have wondered what exactly is going on in this unstable giant star. However, due to its considerable distance – 7,500 light-years – details of the star itself were beyond observation.

This star is known to be surrounded by the Homunculus Nebula, two mushroom-shaped clouds ejected by the star, each of which is hundreds of times larger than our solar system.

Now, for the first time, infrared interferometry with the VINCI instrument on ESO’s Very Large Telescope Interferometer (VLTI) enabled an international team of astronomers [1] to zoom-in on the inner part of its stellar wind. For Roy van Boekel, leader of the team, these results indicate that “the wind of Eta Carinae turns out to be extremely elongated and the star itself is highly unstable because of its fast rotation.”

A monster in the southern sky
Eta Carinae, the most luminous star known in our Galaxy, is by all standards a real monster: it is 100 times more massive than our Sun and 5 million times as luminous. This star has now entered the final stage of its life and is highly unstable. It undergoes giant outbursts from time to time; one of the most recent happened in 1841 and created the beautiful bipolar nebula known as the Homunculus Nebula (see ESO PR Photo 32a/03). At that time, and despite the comparatively large distance – 7,500 light-years – Eta Carinae briefly became the second brightest star in the night sky, surpassed only by Sirius.

Eta Carinae is so big that, if placed in our solar system, it would extend beyond the orbit of Jupiter. This large size, though, is somewhat arbitrary. Its outer layers are continually being blown into space by radiation pressure – the impact of photons on atoms of gas. Many stars, including our Sun, lose mass because of such “stellar winds”, but in the case of Eta Carinae, the resulting mass loss is enormous (about 500 Earth-masses a year) and it is difficult to define the border between the outer layers of the star and the surrounding stellar wind region.

Now, VINCI and NAOS-CONICA, two infrared-sensitive instuments on ESO’s Very Large Telescope (VLT) at the Paranal Observatory (Chile), have probed the shape of the stellar wind region for the first time. Looking down into the stellar wind as far as possible, the astronomers could infer some of the structure of this enigmatic object.

The astronomer team [1] first used the NAOS-CONICA adaptive optics camera [2], attached to the 8.2-m VLT YEPUN telescope, to image the hazy surroundings of Eta Carinae, with a spatial resolution comparable to the size of the solar system, cf. PR Photo 32a/03.

This image shows that the central region of the Homunculus nebula is dominated by an object that is seen as a point-like light source with many luminous “blobs” in the immediate vicinity.

Towards the limit
In order to obtain an even sharper view, the astronomers then turned to interferometry. This technique combines two or more telescopes to achieve an angular resolution [3] equal to that of a telescope as large as the separation of the individual telescopes (cf. ESO PR 06/01 and ESO PR 23/01).

For the study of the rather bright star Eta Carinae the full power of the 8.2-m VLT telescopes is not required. The astronomers thus used VINCI, the VLT INterferometer Commissioning Instrument [4], together with two 35-cm siderostat test telescopes that served to obtain “First Light” with the VLT Interferometer in March 2001 (see ESO PR 06/01).

The siderostats were placed at selected positions on the VLT Observing Platform at the top of Paranal to provide different configurations and a maximum baseline of 62 meters. During several nights, the two small telescopes were pointed towards Eta Carinae and the two light beams were directed towards a common focus in the VINCI test instrument in the centrally located VLT Interferometric Laboratory. It was then possible to measure the angular size of the star (as seen in the sky) in different directions.

Pushing the spatial resolution of this configuration to the limit, the astronomers succeeded in resolving the shape of the outer layer of Eta Carinae. They were able to provide spatial information on a scale of 0.005 arcsec, that is about 11 AU (1650 million km) at the distance of Eta Carinae, corresponding to the full size of the orbit of Jupiter.

Scaled down to terrestial dimensions, this achievement compares to making the distinction between an egg and a billiard ball at a distance of 2,000 kilometers.

A most unusual shape
The VLTI observations brought the astronomers a surprise. They indicate that the wind around Eta Carinae is amazingly elongated: one axis is one-and-a-half times longer than the other! Moreover, the longer axis is found to be aligned with the direction in which the much larger mushroom-shaped clouds (seen on less sharp images) were ejected.

Spanning a scale from 10 to 20-30,000 AU, the star itself and the Homunculus Nebula are thus closely aligned in space.

VINCI was able to detect the boundary where the stellar wind from Eta Carinae becomes so dense that it is no longer transparent. Apparently, this stellar wind is much stronger in the direction of the long axis than of the short axis.

According to mainstream theories, stars lose most mass around their equator. This is because this is where the stellar wind gets “lifting” assistance from the centrifugal force caused by the star’s rotation. However, if this were so in the case of Eta Carinae, the axis of rotation (through the star’s poles) would then be perpendicular to both mushroom-shaped clouds. But it is virtually impossible that the mushroom clouds are positioned like spokes in a wheel, relative to the rotating star. The matter ejected in 1841 would then have been stretched into a ring or torus.

For Roy van Boekel, “the current overall picture only makes sense if the stellar wind of Eta Carinae is elongated in the direction of its poles. This is a surprising reversal of the usual situation, where stars (and planets) are flattened at the poles due to the centrifugal force.
The next supernova?

Such an exotic shape for Eta Carinae-type stars was predicted by theoreticians. The main assumption is that the star itself, which is located deep inside its stellar wind, is flattened at the poles for the usual reason. However, as the polar areas of this central zone are then closer to the centre where nuclear fusion processes take place, they will be hotter. Consequently, the radiation pressure in the polar directions will be higher and the outer layers above the polar regions of the central zone will get more “puffed up” than the outer layers at the equator.

Assuming this model is correct, the rotation of Eta Carinae can be calculated. It turns out that it should spin at over 90 percent of the maximum speed possible (before break-up).

Eta Carinae has experienced large outbursts other than the one in 1841, most recently around 1890. Whether another outburst will happen again in the near future is unknown, but it is certain that this unstable giant star will not settle down.

At the present, it is losing so much mass so rapidly that nothing will be left of it after less than 100,000 years. More likely, though, Eta Carinae will destroy itself long before that in a supernova blast that could possibly become visible in the daytime sky with the naked eye. This may happen “soon” on the astronomical time-scale, perhaps already within the next 10-20,000 years.

Original Source: ESO News Release

Seeing a Star’s Final Moments

Image credit: Hubble

Although stars can burn for billions of years, their final stages can take a relatively short period of time. In many cases, it only takes a few hundred thousand years for dying stars to slough off their outer layers to create the familiar planetary nebula. Since they happen so quickly, they’re relatively rare to find, but astronomers think they’ve got a candidate with a relatively nearby star called V Hydra. The star is in its final stages, and jets of material have just begun emanating from it.

It takes only a few hundred to a thousand years for a dying Sun-like star, many billions of years old, to transform into a dazzling, glowing cloud called a planetary nebula. This relative blink in a long lifetime means that a Sun-like star’s final moments – the crucial phase when its planetary nebula takes shape – have, until now, gone undetected.

In research reported in the Nov. 20 issue of Nature, astronomers led by Dr. Raghvendra Sahai of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., have caught one such dying star in the act. This nearby star, called V Hydrae, has been captured by the Space Telescope Imaging Spectrograph onboard NASA’s Hubble Space Telescope in the last stages of its demise, just as material has begun to shoot away from it in a high-speed jet outflow.

While previous studies have indicated the role of jet outflows in shaping planetary nebulae, the new findings represent the first time these jets have been directly detected.

“The discovery of a newly launched jet outflow is likely to have a significant impact on our understanding of this short-lived stage of stellar evolution and will open a window onto the ultimate fate of our Sun,” said Sahai.

Other institutions contributing to this paper include: University of California, Los Angeles; Princeton University, Princeton, New Jersey; Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts; and Valdosta State University, Valdosta, Georgia.

Low-mass stars like the Sun typically survive around ten billion years before their hydrogen fuel begins to run out and they start to die. Over the next ten to hundred thousand years, the stars slowly eject nearly half of their mass in expanding, spherical winds. Then – in a poorly understood phase lasting just 100 to 1,000 years – the stars evolve into a stunning array of geometrically shaped glowing clouds called planetary nebulae.

Just how these extraordinary “star-clouds” are shaped has remained unclear, though Sahai, in several previous papers, put forth a new hypothesis. Based on results from a recent Hubble Space Telescope imaging survey of young planetary nebulae, he proposed that two-sided, or bipolar, high-speed jet-like outflows are the primary means of shaping these objects. The latest study will allow Sahai and his colleagues to test this hypothesis with direct data for the first time.

“Now, in the case of V Hydrae, we can observe the evolution of the jet outflow in real-time,” said Sahai, who together with his colleagues will study the star with the Hubble Space Telescope for three more years.

The new findings also suggest what may be driving the jet outflows. Past models of dying stars predict that accretion discs – swirling rings of matter encircling stars – may trigger jet outflows. The V Hydrae data support the presence of an accretion disc surrounding, not V Hydrae itself, but a companion object around the star. This companion is likely to be another star or even a giant planet too dim to be detected. The authors have also found evidence for an outlying large dense disc in V Hydrae, which could enable the formation of the accretion disc around the companion.

Further support in favor of a companion-driven jet outflow comes from the scientists’ observation that the jet fires in bursts: because the companion orbits the star in a periodic fashion, the accretion disc around it is expected to produce regular spurts of material rather than a steady stream.

The Space Telescope Imaging Spectrograph is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The California Institute of Technology, Pasadena manages JPL for NASA.

Original Source: NASA/JPL News Release

Galactic Wind Connects Galaxies

Image credit: Hubble

Astronomers have known for nearly a century that galaxies are distinct islands of stars, floating apart from each other in space. But it turns out that galaxies are more connected than previously believed because of large-scale “galactic winds” which blow off of galaxies and interact with each other. Researchers from the University of Maryland studied galactic winds in both visible and X-ray light around 10 galaxies, and found that they can often fill an area larger than the galaxy itself. This wind is thought to come from stars and actively feeding black holes.

It was the 17th Century English preacher and poet John Donne who wrote the immortal lines “No man is an island, entire of itself; every man is a piece of the continent, a part of the main.”

Today, astronomers have determined we also do not live in an “island Universe” – that is, a Universe in which the vast agglomerations of gas and stars known as galaxies are wholly independent of the influence of neighboring galaxies and their surrounding environment. Sylvain Veilleux, an astronomer at the University of Maryland, and his colleagues have found important new evidence to support the connectedness of galaxies in the form of unexpectedly large-scale “galactic winds” blowing off of galaxies, altering their surroundings out to distances much farther than previously thought. Galactic winds are the streams of charged particles that blow off of galaxies.

“We are seeing that these galactic winds are blowing off of galaxies on a very large scale,” said Veilleux. “We have detected these winds in both visible light and X-ray light on scales that are sometimes much larger than the galaxies themselves.” The findings are published in the November 2003 issue of the Astronomical Journal, Vol. 126 No. 5 (http://www.journals.uchicago.edu/AJ/journal/issues/v126n5/203224/203224.html). Veilleux’s colleagues in this study were David S. Rupke, a graduate student in physics at the University of Maryland, Patrick L. Shopbell of the California Institute of Technology, Jonathan Bland-Hawthorn of the Anglo-Australian Observatory in Australia, and Gerald N. Cecil of the University of North Carolina at Chapel Hill.

Based on data from the Chandra X-ray Observatory, the Anglo-Australian Observatory located near Coonabarabran in Australia, and the William Herschel Telescope on La Palma in the Canary Islands, Veilleux said these findings have important consequences for the evolution of galaxies and their environment. Veilleux and his colleagues examined the galactic winds surrounding 10 galaxies. Located between 20 and 900 million light years from Earth, the galaxies are in different galaxy clusters and none are in our Milky Way Galaxy’s Local Group cluster. But Veilleux, who is presently on sabbatical at the California Institute of Technology, believes the findings hold for the Milky Way’s galactic wind as well. Galactic winds result from two sources: stars and actively feeding (accreting) giant black holes lurking at the centers of most galaxies. In the first case, Veilleux said, the winds are primarily produced by a combination of the stellar winds blowing off massive stars during their youth and by the titanic explosions known as supernovae that mark their death. Winds produced by these stars are referred to as “starburst-driven.” Starbursts are periods during which large numbers of massive stars are created. These, periods of star creation, in turn, produce strong stellar winds. These massive stars eventually die as supernova. In the second case, he said, enormous (supermassive) and active black holes lurking in the hearts of their host galaxies generate galactic winds. “An ‘active’ black hole is one that is accreting or pulling in a significant amount of the material that is available to it,” Veilleux said. “Such black holes are called ‘active galactic nuclei’ or AGN and the winds they produce are referred to as AGN-driven.”

The Milky Way’s central black hole is an inactive or dormant black hole simply because there isn’t much material in its vicinity available for it to accrete. Measuring the Galactic Wind Veilleux said astronomers are able to detect galactic winds because of the energy emitted when particles that make up the wind collide with other particles. “We can detect these galactic winds because collisions among the charged particles create electromagnetic energy emissions in the form of X rays, visible light and radio waves,” he explained. “These emissions are not uniform in the regions around the galaxies. Rather, they are clumpy, being most notable in the regions where hot gas in the wind collides with colder material from the galaxies themselves or from the intergalactic medium.” The result is filaments of emissions surrounding galaxies in irregular bubble-shaped regions out to at least 65,000 light years from the galaxy centers. Veilleux and his colleagues compared existing Chandra X-ray data with new ground-based observations obtained with a special tunable filter on the Anglo-Australian telescope, which permitted the detection of optical emission down to unprecedented brightness levels. They found the clumpy filaments correlated quite well. This, they say, indicates that galactic winds are indeed influencing the surrounding inter-galactic environment out to previously unknown distances. A Role in the Evolution Galaxies? “What we found is that these winds have a very large zone of influence and probably a strong impact not only on the host galaxy but also on scales in excess of 65,000 light years, possibly well out into the intergalactic medium,” Veilleux said.

Veilleux said the findings mean any comprehensive understanding of long-term galaxy evolution must take into account the flow of gaseous material out of, and back into, the galaxy.

“Galactic winds move at between about 300 and 3000 kilometers per second and if they don’t have enough speed to escape the gravitational pull of the galaxy entirely, it means the material in them would rain back down on the galactic halo and even the disk,” he said. Veilleux explained that such a return “rain” would contribute to the re-enrichment of the host galaxy itself and in this way the more massive galaxies would be able to keep their heavier metals (the sort forged by massive stars during their lives and deaths in supernovae). “The whole issue of the flow of warm gas back into galaxies is very important to understanding the rate at which new stars form.” As for the implications to the Milky Way, Veilleux said the findings for these far away galaxies suggest our Galaxy has its own galactic wind that is creating large-scale bubbles of material around it. Previous findings for the Milky Way have shown direct evidence for a galactic-scale wind at a variety of wavelengths. It is unclear if the Milky Way’s wind is interacting with the nearby Sagittarius dwarf galaxy, which astronomers have discovered is being assimilated into our galaxy through tidal (gravitational) forces. However, Veilleux’s findings have established that galaxies do indeed interact with their surroundings in important ways. “As a result of findings such as these, we now know the closed box or ‘island Universe’ view is not true,” he said.

Original Source: University of Maryland

ESO Watches Burst Afterglow for Five Weeks

Image credit: ESO

Gamma-ray bursts are some of the largest explosions in the Universe; one can generate more energy in a few seconds than the Sun creates in 10 billion years. It’s believed they’re caused when a super-massive star collapses, called a hypernova. Astronomers from the European Southern Observatory tracked the afterglow of a recent burst by using a technique called polarimetry, which lets them track the shape of the explosion. If it was a spherical explosion, the light would have random polarity, but they found that gas is flowing out in jets which are widening over time.

“Gamma-ray bursts (GRBs)” are certainly amongst the most dramatic events known in astrophysics. These short flashes of energetic gamma-rays, first detected in the late 1960’s by military satellites, last from less than one second to several minutes.

GRBs have been found to be situated at extremely large (“cosmological”) distances. The energy released in a few seconds during such an event is larger than that of the Sun during its entire lifetime of more than 10,000 million years. The GRBs are indeed the most powerful events since the Big Bang known in the Universe, cf. ESO PR 08/99 and ESO PR 20/00.

During the past years circumstantial evidence has mounted that GRBs signal the collapse of extremely massive stars, the so-called hypernovae. This was finally demonstrated some months ago when astronomers, using the FORS instrument on ESO’s Very Large Telescope (VLT), documented in unprecedented detail the changes in the spectrum of the light source (“the optical afterglow”) of the gamma-ray burst GRB 030329 (cf. ESO PR 16/03). A conclusive and direct link between cosmological gamma-ray bursts and explosions of very massive stars was provided on this occasion.

Gamma-Ray Burst GRB 030329 was discovered on March 29, 2003 by NASA’s High Energy Transient Explorer spacecraft. Follow-up observations with the UVES spectrograph at the 8.2-m VLT KUEYEN telescope at the Paranal Observatory (Chile) showed the burst to have a redshift of 0.1685 [1]. This corresponds to a distance of about 2,650 million light-years, making GRB 030329 the second-nearest long-duration GRB ever detected. The proximity of GRB 030329 resulted in very bright afterglow emission, permitting the most extensive follow-up observations of any afterglow to date.

A team of astronomers [2] led by Jochen Greiner of the Max-Planck-Institut f?r extraterrestrische Physik (Germany) decided to make use of this unique opportunity to study the polarisation properties of the afterglow of GRB 030329 as it developed after the explosion.

Hypernovae, the source of GRBs, are indeed so far away that they can only be seen as unresolved points of light. To probe their spatial structure, astronomers have thus to rely on a trick: polarimetry (see ESO PR 23/03).

Polarimetry works as follows: light is composed of electromagnetic waves which oscillate in certain directions (planes). Reflection or scattering of light favours certain orientations of the electric and magnetic fields over others. This is why polarising sunglasses can filter out the glint of sunlight reflecting off a pond.

The radiation in a gamma-ray burst is generated in an ordered magnetic field, as so-called synchrotron radiation [3]. If the hypernova is spherically symmetric, all orientations of the electromagnetic waves will be present equally and will average out, so there will be no net polarisation. If, however, the gas is not ejected symmetrically, but into a jet, a slight net polarisation will be imprinted on the light. This net polarisation will change with time since the opening angle of the jet widens with time, and we see a different fraction of the emission cone.

Studying the polarisation properties of the afterglow of a gamma-ray burst thus allows to gain knowledge about the underlying spatial structures and the strength and orientation of the magnetic field in the region where the radiation is generated. “And doing this over a long period of time, as the afterglow fades and evolves, provides us with a unique diagnostic tool for gamma-ray burst studies”, says Jochen Greiner.

Although previous single measurements of the polarisation of GRB’s optical afterglow exist, no detailed study has ever been done of the evolution of polarisation with time. This is indeed a very demanding task, only possible with an extremely stable instrument on the largest telescope… and a sufficient bright optical afterglow.

As soon as GRB 030329 was detected, the team of astronomers therefore turned to the powerful multi-mode FORS1 instrument on the VLT ANTU telescope. They obtained 31 polarimetric observations over a period of 38 days, enabling them to measure, for the first time, the changes of the polarisation of an optical gamma-ray burst afterglow with time. This unique set of observational data documents the physical changes in the remote object in unsurpassed detail.

Their data show the presence of polarisation at the level of 0.3 to 2.5 % throughout the 38-day period with significant variability in strength and orientation on timescales down to hours. This particular behaviour has not been predicted by any of the major theories.

Unfortunately, the very complex light curve of this GRB afterglow, in itself not understood, prevents a straightforward application of existing polarisation models. “It turns out that deriving the direction of the jet and the magnetic field structure is not as simple as we thought originally”, notes Olaf Reimer, another member of the team. “But the rapid changes of the polarisation properties, even during smooth phases of the afterglow light curve, provide a challenge to afterglow theory”.

“Possibly”, adds Jochen Greiner, “the overall low level of polarisation indicates that the strength of the magnetic field in the parallel and perpendicular directions do not differ by more than 10%, thus suggesting a field strongly coupled with the moving material. This is different from the large-scale field which is left-over from the exploding star and which is thought to produce the high-level of polarisation in the gamma-rays.”

Original Source: ESO News Release