RCW 49

Credit: NASA/JPL-Caltech/E. Churchwell (University of Wisconsin)

New Image Shows the Power of Visual Remix

The same way a visible-light photographer can choose to shoot black and white instead of color, astronomers using NASA's Spitzer Space Telescope have their choice of what colors to use or not use in their images, as shown in an image of star-forming region RCW 49 released today.

The new picture is an alternate view of a dusty stellar nursery located 13,700 light-years away in the southern constellation Centaurus. Spitzer released its original version of the image in 2004. That image combined information from four different wavelengths of infrared light, but the new image uses only two.

The typical human eye perceives three different colors of visible light -- red, green, and blue -- with cones on the retina. All the colors we see are made up of some combination of these three colors. Every color of light has a different wavelength, and many wavelengths fall outside the visible spectrum. Infrared light is basically wavelengths of light that vibrate at colors below the red part of the spectrum, colors we can't see.

Spitzer's two imaging cameras effectively see a total of seven different wavelengths, or channels, of infrared light. That would be the equivalent of the eye being sensitive to seven colors instead of just three. The challenge for Spitzer imaging scientists is to present all these different channels using colors that we can see. When astronomers select which channels to include in their false-color composites, each wavelength is assigned a different visible-light color. Therefore, the same observations can be used to make very different images.

Spitzer's two shortest wavelengths, 3.6 microns and 4.5 microns, were mapped as cyan and red respectively for the new image. At 4.5 microns, hot hydrogen gas glows very brightly, much like a neon light glowing in visible light. The two-channel image therefore emphasizes hot hydrogen gas, which shows up as the red regions, in addition to the more than 2,200 stars and organic molecules visible in both the two-channel and four-channel images.

As well as enabling exciting new science discoveries, two-channel images like this one are particularly interesting to scientists who wish to understand how Spitzer will perform after its liquid helium coolant runs out in 2009. At this time the telescope will become too warm to observe at longer wavelengths, but will continue to operate in these channels.

Object name: RCW 49
Object type: Star Formation
Position (J2000): RA: 10h 24m 14.60s Dec: -57° 46' 58.00"
Instrument: IRAC
Wavelength: 3.6 Micron (Cyan), 4.5 Micron (Red)

Fox Fur, a Unicorn, and a Christmas Tree

Credit & Copyright: R Jay Gabany

Clouds of glowing hydrogen gas fill this colorful skyscape in the faint but fanciful constellation Monoceros, the Unicorn. A star forming region cataloged as NGC 2264, the complex jumble of cosmic gas and dust is about 2,700 light-years distant and mixes reddish emission nebulae excited by energetic light from newborn stars with dark interstellar dust clouds.

Where the otherwise obscuring dust clouds lie close to the hot, young stars they also reflect starlight, forming blue reflection nebulae. The wide mosaic spans about 3/4 degree or nearly 1.5 full moons, covering 40 light-years at the distance of NGC 2264.

Its cast of cosmic characters includes the the Fox Fur Nebula, whose convoluted pelt lies at the upper left, bright variable star S Mon immersed in the blue-tinted haze just below the Fox Fur, and the Cone Nebula at the far right.

Of course, the stars of NGC 2264 are also known as the Christmas Tree star cluster. The triangular tree shape traced by the stars appears sideways here, with its apex at the Cone Nebula and its broader base centered near S Mon.

APOD

Tarantula Nebula - 30 Doradus

Credit: NASA/CXC/Penn State/L.Townsley, et al.

Drama In The Heart Of The Tarantula

Found in the nearby Large Magellanic Cloud, 30 Doradus is one of the largest massive star forming regions close to the Milky Way. Enormous stars in 30 Doradus, also known as the Tarantula Nebula, are producing intense radiation and searing winds of multimillion-degree gas that carve out gigantic bubbles in the surrounding cooler gas and dust. Other massive stars have raced through their evolution and exploded catastrophically as supernovae, expanding these bubbles into X-ray-brightened superbubbles. They leave behind pulsars as beacons of their former lives and expanding supernova remnants that trigger the collapse of giant clouds of dust and gas to form new generations of stars.

At the center of 30 Doradus lies the star cluster R136 at the intersection of three of these superbubbles. However, with ages only between one and two million years old, the stars in R136 are too young to be source of the supernovae that brighten the superbubbles in X-rays. Instead, R136 is most likely just the latest cluster to form in 30 Doradus. It may be as massive as it is because these superbubbles have combined to concentrate their gas in this region and thus triggered intense star formation there.

30 Doradus is about 160,000 light years from Earth in the southern constellation of Dorado. It spans 800 light years across and is incredibly bright in many wavelengths. If it were at the distance of the Orion Nebula (1,300 light years), 30 Doradus would span the area of 60 full Moons and its optical light would be bright enough to cast shadows at night on the Earth. This latest X-ray image of 30 Doradus represents almost 114,000 seconds, or 31 hours, of Chandra observing time - three times longer than previously recorded. In this image, red represents the lower range of X-rays that Chandra detects, the medium range is green, while the highest-energy X-rays are blue.

Fast Facts for Tarantula Nebula:
Scale: Image is 24 arcmin across .
Category: Normal Stars & Star Clusters
Coordinates: (J2000) RA 05h 38m 42.9s | Dec -69° 06' 3"
Constellation: Dorado Observation Dates: 09/21/1999 - 01/30/2006
Observation Time: 32 hours
Obs. IDs: 22, 5906, 7263, 7264, 62520
Color Code: Red (0.5-0.7 keV); Green (0.7-1.1 keV); Blue (1.1-2.0 keV)
Instrument: ACIS
Distance Estimate: About 160,000 light years

Annotated Chandra Image of 30 Doradus

Astronomers Find Most Distant Water in the Universe

Credit: Milde Science Communication,

STScI, CFHT, J.-C. Cuillandre, Coelum.

About this image: The spectrum -- a radio "fingerprint" that revealed radio emission from water masers in the distant quasar MG J0414+0534. The background image is an infrared image of the quasar, made with the Hubble Space Telescope. The quasar appears broken up into four components by a foreground galaxy (diffuse object in the center), acting as a gravitational lens and strengthening the signal by a factor of 35. The inset with the galaxy M87 shows how the quasar might be seen from nearby.

Astronomers have found the most distant water yet seen in the Universe, in a galaxy more than 11 billion light-years from Earth. Previously, the most distant water had been seen in a galaxy less than 7 billion light-years from Earth.

Using the giant, 100-meter-diameter radio telescope in Effelsberg, Germany, and the National Science Foundation's Very Large Array (VLA) in New Mexico, the scientists detected a telltale radio "fingerprint" of water molecules in the distant galaxy.

The soggy galaxy, dubbed MG J0414+0534, harbors a quasar -- a supermassive black hole powering bright emission -- at its core. In the region near the core, the water molecules are acting as masers, the radio equivalent of lasers, to amplify radio waves at a specific frequency.

The astronomers say their discovery indicates that such giant water masers were more common in the early Universe than they are today. MG J0414+0534 is seen as it was when the Universe was roughly one-sixth of its current age.

At the galaxy's great distance, even the strengthening of the radio waves done by the masers would not by itself have made them strong enough to detect with the radio telescopes. However, the scientists got help from nature in the form of another galaxy, nearly 8 billion light-years away, located directly in the line of sight from MG J0414+0534 to Earth. That foreground galaxy's gravity served as a lens to further brighten the more-distant galaxy and make the emission from the water molecules visible to the radio telescopes.

"We were only able to discover this distant water with the help of the gravitational lens," said Violette Impellizzeri, an astronomer with the Max-Planck Institute for Radioastronomy (MPIfR) in Bonn, Germany. "This cosmic telescope reduced the amount of time needed to detect the water by a factor of about 1,000," she added.

The astronomers first detected the water signal with the Effelsberg telescope. They then turned to the VLA's sharper imaging capability to confirm that it was indeed coming from the distant galaxy. The gravitational lens produces not one, but four images of MG J0414+0534 as seen from Earth. Using the VLA, the scientists found the specific frequency attributable to the water masers in the two brightest of the four lensed images. The other two lensed images, they said, are too faint for detecting the water signal.

The radio frequency emitted by the water molecules was Doppler shifted by the expansion of the Universe from 22.2 GHz to 6.1 GHz.

Water masers have been found in numerous galaxies at closer distances. Typically, they are thought to arise in disks of molecules closely orbiting a supermassive black hole at the galaxy's core. The amplified radio emission is more often observed when the orbiting disk is seen nearly edge-on. However, the astronomers said MG J0414+0534 is oriented with the disk almost face-on as seen from Earth.

"This may mean that the water molecules in the masers we're seeing are not in the disk, but in the superfast jets of material being ejected by the gravitational power of the black hole," explained John McKean, also of MPIfR.

Impellizzeri and McKean worked with Alan Roy, Christian Henkel, and Andreas Brunthaler, also of the Max-Planck Institute; Paola Castangia of the Max-Planck Institute and the INAF Astronomical Observatory of Cagliari in Italy; and Olaf Wucknitz of the Argelander Institute for Astronomy in Bonn, Germany. The scientists reported their results in the December 18 issue of the scientific journal Nature.

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

Contact:
Dave Finley, Public Information Officer
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Abell 85 - Dark Energy Found Stifling Growth in Universe

Credit: X-ray (NASA/CXC/SAO/A.Vikhlinin et al.);
Optical (SDSS);
Illustration (MPE/V.Springel)

The composite image on the left is of the galaxy cluster Abell 85, located about 740 million light years from Earth. The purple emission is multi-million degree gas detected in X-rays by NASA's Chandra X-ray Observatory and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. This galaxy cluster is one of 86 observed by Chandra to trace how dark energy has stifled the growth of these massive structures over the last 7 billion years. Galaxy clusters are the largest collapsed objects in the Universe and are ideal for studying the properties of dark energy, the mysterious form of repulsive gravity that is driving the accelerated expansion of the Universe.

The illustration on the right shows snapshots from a simulation by Volker Springel, representing the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now). This shows how the Universe has evolved from a smooth state to one containing a vast amount of structure. Gas is shown in these snapshots, where the yellow regions are stars and the brightest structures are galaxies and galaxy clusters. The growth of these structures was initially driven only by the attractive force of gravity, but then later there was competition with the repulsive force of dark energy.

Understanding the nature of dark energy is one of the biggest problems in science. Possibilities include the cosmological constant, equivalent to the energy of empty space, a modification in general relativity on the largest scales, or a more general physical field. To help decide between these options, Chandra was used to study the increase in mass of galaxy clusters with time over the last 7 billion years. The results are remarkably consistent with those from previous results that measure the expansion of the Universe using distance measurements, revealing that general relativity works as expected on large scales. The cluster work, in combination with other studies, also provides the strongest evidence to date that dark energy is the cosmological constant, or that 'nothing weighs something'.

Fast Facts for Abell 85:
Scale: Left panel is 42 arcmin across.
Category: Groups & Clusters of Galaxies
Coordinates: (J2000) RA 00h 41m 37.8s | Dec -09º 20' 33''
Constellation: Cetus
Observation Date: Aug 19, 2000
Observation Time: 11 hours Obs. ID: 904
Color Code: X-ray (Violet); Optical (Red, Yellow, Blue)
Instrument: ACIS
Distance Estimate: 740 million light years

A Sparkling Spray of Stars

ESO PR Photo 48/08
NGC 2264 and the Christmas Tree cluster
Credit: ESO

The festive season has arrived for astronomers at the European Southern Observatory (ESO) in the form of this dramatic new image. It shows the swirling gas around the region known as NGC 2264 — an area of sky that includes the sparkling blue baubles of the Christmas Tree star cluster.

NGC 2264 lies about 2600 light-years from Earth in the obscure constellation of Monoceros, the Unicorn, not far from the more familiar figure of Orion, the Hunter. The image shows a region of space about 30 light-years across.

William Herschel discovered this fascinating object during his great sky surveys in the late 18th century. He first noticed the bright cluster in January 1784 and the brightest part of the visually more elusive smudge of the glowing gas clouds at Christmas nearly two years later. The cluster is very bright and can easily be seen with binoculars. With a small telescope (whose lenses will turn the view upside down) the stars resemble the glittering lights on a Christmas tree. The dazzling star at the top is even bright enough to be seen with the unaided eye. It is a massive multiple star system that only emerged from the dust and gas a few million years ago.

As well as the cluster there are many interesting and curious structures in the gas and dust. At the bottom of the frame, the dark triangular feature is the evocative Cone Nebula, a region of molecular gas flooded by the harsh light of the brightest cluster members. The region to the right of the brightest star has a curious, fur-like texture that has led to the name Fox Fur Nebula.

Much of the image appears red because the huge gas clouds are glowing under the intense ultra-violet light coming from the energetic hot young stars. The stars themselves appear blue as they are hotter, younger and more massive than our own Sun. Some of this blue light is scattered by dust, as can be seen occurring in the upper part of the image.

This intriguing region is an ideal laboratory for studying how stars form. The entire area shown here is just a small part of a vast cloud of molecular gas that is in the process of forming the next generation of stars. Besides the feast of objects in this picture there are many interesting objects hidden behind the murk of the nebulosity. In the region between the tip of the Cone Nebula and the brightest star at the top of the picture there are several stellar birthing grounds where young stars are forming. There is even evidence of the intense stellar winds from these youthful embryos blasting out from the hidden stars in the making.

This picture of NGC 2264, including the Christmas Tree Cluster, was created from images taken with the Wide Field Imager (WFI), a specialised astronomical camera attached to the 2.2-metre Max-Planck Society/ESO telescope at the La Silla observatory in Chile. Located nearly 2400 m above sea level, in the mountains of the Atacama Desert, ESO's La Silla enjoys some of the clearest and darkest skies on the whole planet, making the site ideally suited for studying the farthest depths of the Universe. To make this image, the WFI stared at the cluster for more than ten hours through a series of specialist filters to build up a full colour image of the billowing clouds of fluorescing hydrogen gas.

ESO La Silla - Paranal - ELT Press Officer
Dr. Henri Boffin
+49 89 3200 6222
offin@eso.org

ESO Press Officer in Chile
Valentina Rodriguez
+56 2 463 3123
vrodrigu@eso.org

Solar Flare Surprise

Solar flares are the most powerful explosions in the solar system. Packing a punch equal to a hundred million hydrogen bombs, they obliterate everything in their immediate vicinity. Not a single atom should remain intact.

At least that's how it's supposed to work.

"We've detected a stream of perfectly intact hydrogen atoms shooting out of an X-class solar flare," says Richard Mewaldt of Caltech. "What a surprise! These atoms could be telling us something new about what happens inside flares."

The X9-class solar flare of Dec. 5, 2006, observed by the Solar X-Ray Imager aboard NOAA's GOES-13 satellite.

The event occurred on Dec. 5, 2006. A large sunspot rounded the sun's eastern limb and with little warning it exploded. On the "Richter scale" of flares, which ranks X1 as a big event, the blast registered X9, making it one of the strongest flares of the past 30 years.

NASA managers braced themselves. Such a ferocious blast usually produces a blizzard of high-energy particles dangerous to both satellites and astronauts. Indeed, moments after the explosion, radio emissions from a shock wave in the sun's atmosphere signaled that a swarm of particles was on its way.

An hour later they arrived. But they were not the particles researchers expected.

NASA's twin STEREO spacecraft made the discovery: "It was a burst of hydrogen atoms," says Mewaldt. "No other elements were present, not even helium (the sun's second most abundant atomic species). Pure hydrogen streamed past the spacecraft for a full 90 minutes."

Next came more than 30 minutes of quiet. The burst subsided and STEREO's particle counters returned to low levels. The event seemed to be over when a second wave of particles enveloped the spacecraft. These were the "broken atoms" that flares are supposed to produce—protons and heavier ions such as helium, oxygen and iron. "Better late than never," he says.
STEREO particle counts on Dec. 5, 2006. The vertical axis measures the angle to the sun. Note how the initial hydrogen burst arrived from a narrow angle while the ions that followed swarmed in from all directions. The "swarming action" is a result of deflections by the sun's magnetic field--a force not felt by the neutral hydrogen.

At first, this unprecedented sequence of events baffled scientists, but now Mewaldt and colleagues believe they're getting to the bottom of the mystery.

First, how did the hydrogen atoms resist destruction?

"They didn't," says Mewaldt. "We believe they began their journey to Earth in pieces, as protons and electrons. Before they escaped the sun’s atmosphere, however, some of the protons recaptured an electron, forming intact hydrogen atoms. The atoms left the sun in a fast, straight shot before they could be broken apart again." (For experts: The team believes the electrons were recaptured by some combination of radiative recombination and charge exchange.)

Second, what delayed the ions?
"Simple," says Mewaldt. "Ions are electrically charged and they feel the sun's magnetic field. Solar magnetism deflects ions and slows their progress to Earth. Hydrogen atoms, on the other hand, are electrically neutral. They can shoot straight out of the sun without magnetic interference."

Imagine two runners dashing for the finish line. One (the ion) is forced to run in a zig-zag pattern with zigs and zags as wide as the orbit of Mars. The other (the hydrogen atom) runs in a straight line. Who's going to win?

"The hydrogen atoms reached Earth two hours before the ions," says Mewaldt.

Mewaldt believes that all strong flares might emit hydrogen bursts, but they simply haven't been noticed before. He's looking forward to more X-flares now that the two STEREO spacecraft are widely separated on nearly opposite sides of the Sun. (In 2006 they were still together near Earth.) STEREO-A and –B may be able to triangulate future bursts and pinpoint the source of the hydrogen. This would allow the team to test their ideas about the surprising phenomenon.

"All we need now," he says, "is some solar activity."

For more information about this research, look for the article "STEREO Observations of Energetic Neutral Atoms during the 5 December 2006 Solar Flare" by R. A. Mewaldt et al, in a future issue of the Astrophysical Journal Letters.

Author:Dr.Tony Phillips - james.a.phillips@earthlink.net
Credit: Science@NASA

Astronomers Dissect a Supermassive Black Hole with Natural Magnifying Glassesvv

ESO PR Photo 47a/08

The Einstein Cross

Credit: ESO/F. Courbin et al.

About this image: The Einstein Cross and the galaxy that causes this 'cosmic mirage', as seen with the FORS instrument on ESO's Very Large Telescope. This cross-shaped configuration consists of four images of a single very distant source. The multiple images are a result of gravitational lensing by a foreground galaxy, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light source in the Einstein Cross is a quasar approximately ten billion light-years away, whereas the foreground lensing galaxy is ten times closer. The light from the quasar is bent in its path and magnified by the gravitational field of the lensing galaxy.

ESO PR Photo 47b/08

The Einstein Cross

Credit: ESO/F. Courbin et al.

About this image: Close-up of the Einstein Cross, as observed with the SINFONI instrument on ESO's Very Large Telescope. SINFONI makes use of the adaptive optics technique and so, allows astronomers to overcome the blurring effect of the atmosphere, thereby providing very sharp images. The central blob is the nucleus of the lensing galaxy, surrounded by the four mirage images of the distant quasar.

ESO PR Video 47/08v (see videos here)

Macro and microlensing

This animation shows the principle of macro- and microlensing. In "macrolensing", a galaxy plays the role of a cosmic magnifying glass or a natural telescope, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light from a distant quasar is bent in its path and magnified by the gravitational field of the lensing galaxy. This proves very useful in astronomy as it allows us to observe distant objects that would otherwise be too faint to explore using currently available telescopes. In addition to macrolensing by the galaxy, stars in the lensing galaxy act as secondary lenses to produce an additional magnification. This secondary magnification is based on the same principle as macrolensing, but on a smaller scale, and since stars are much smaller than galaxies, is known as "microlensing". As the stars are moving in the lensing galaxy, the microlensing magnification also changes with time. From Earth, the brightness of the quasar images (four in the case of the Einstein Cross) flickers around a mean value, due to microlensing.

Credit: ESO

Combining a double natural "magnifying glass" with the power of ESO's Very Large Telescope, astronomers have scrutinised the inner parts of the disc around a supermassive black hole 10 billion light-years away. They were able to study the disc with a level of detail a thousand times better than that of the best telescopes in the world, providing the first observational confirmation of the prevalent theoretical models of such discs.

The team of astronomers from Europe and the US studied the "Einstein Cross", a famous cosmic mirage. This cross-shaped configuration consists of four images of a single very distant source. The multiple images are a result of gravitational lensing by a foreground galaxy, an effect that was predicted by Albert Einstein as a consequence of his theory of general relativity. The light source in the Einstein Cross is a quasar approximately ten billion light-years away, whereas the foreground lensing galaxy is ten times closer. The light from the quasar is bent in its path and magnified by the gravitational field of the lensing galaxy.

This magnification effect, known as "macrolensing", in which a galaxy plays the role of a cosmic magnifying glass or a natural telescope, proves very useful in astronomy as it allows us to observe distant objects that would otherwise be too faint to explore using currently available telescopes. "The combination of this natural magnification with the use of a big telescope provides us with the sharpest details ever obtained," explains Frédéric Courbin, leader of the programme studying the Einstein Cross with ESO's Very Large Telescope.

In addition to macrolensing by the galaxy, stars in the lensing galaxy act as secondary lenses to produce an additional magnification. This secondary magnification is based on the same principle as macrolensing, but on a smaller scale, and since stars are much smaller than galaxies, is known as "microlensing". As the stars are moving in the lensing galaxy, the microlensing magnification also changes with time. From Earth, the brightness of the quasar images (four in the case of the Einstein Cross) flickers around a mean value, due to microlensing. The size of the area magnified by the moving stars is a few light-days, i.e., comparable in size to the quasar accretion disc.

The microlensing affects various emission regions of the disc in different ways, with smaller regions being more magnified. Because differently sized regions have different colours (or temperatures), the net effect of the microlensing is to produce colour variations in the quasar images, in addition to the brightness variations. By observing these variations in detail for several years, astronomers can measure how matter and energy are distributed about the supermassive black hole that lurks inside the quasar. Astronomers observed the Einstein Cross three times a month over a period of three years using ESO's Very Large Telescope (VLT), monitoring all the brightness and colour changes of the four images.

"Thanks to this unique dataset, we could show that the most energetic radiation is emitted in the central light-day away from the supermassive black hole and, more importantly, that the energy decreases with distance to the black hole almost exactly in the way predicted by theory," says Alexander Eigenbrod, who completed the analysis of the data.

The use of the macro- and microlensing, coupled with the giant eye of the VLT, enabled astronomers to probe regions on scales as small as a millionth of an arcsecond. This corresponds to the size of a one euro coin seen at a distance of five million kilometres, i.e., about 13 times the distance to the Moon! "This is 1000 times better than can be achieved using normal techniques with any existing telescope," adds Courbin.

Measuring the way the temperature is distributed around the central black hole is a unique achievement. Various theories exist for the formation and fuelling of quasars, each of which predicts a different profile. So far, no direct and model-independent observation has allowed scientists to validate or invalidate any of these existing theories, particularly for the central regions of the quasar. "This is the first accurate and direct measurement of the size of a quasar accretion disc with wavelength (colour), independent of any model," concludes team member Georges Meylan.

More information:

Eigenbrod, A., Courbin, F., Sluse, D., Meylan, G. & Agol, E. 2008, Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 ≡ the Einstein Cross. I. Spectrophotometric monitoring with the VLT, Astronomy & Astrophysics, 480, 647

Eigenbrod, A., Courbin, F., Meylan, G., Agol, E., Anguita, T., Schmidt, R. W. & Wambsganss, J. 2008, Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 ≡ the Einstein Cross. II. Energy profile of the accretion disk, Astronomy & Astrophysics, 490, 933

The team is composed of Frédéric Courbin, Alexander Eigenbrod, and Georges Meylan (Ecole Polytechnique Fédérale de Lausanne, Switzerland), Dominique Sluse, Robert Schmidt, Timo Anguita, and Joachim Wambsganss (Astronomisches Rechen-Institut, Heidelberg, Germany), and Eric Agol (University of Washington, Seattle, USA).

Read more on this story in the associated page.

Contacts:
Frédéric Courbin, Alexander Eigenbrod, Georges Meylan
EPFL (Ecole Polytechnique Fédérale de Lausanne)
Switzerland
Phone: +41 22 379 24 18, +41 22 379 24 21, +41 22 379 24 25
E-mail: frederic.courbin@epfl.ch, alexander.eigenbrod@epfl.ch, georges.meylan@epfl.ch

Joachim Wambsganss
Astronomisches Rechen-Institut, Heidelberg, Germany
Phone: +49 6221 54 1800
E-mail: jkw@ari.uni-heidelberg.de

Eric Agol
University of Washington, Seattle, USA
Phone: +1 206 543 71 06
E-mail: agol@astro.washington.edu

ESO La Silla - Paranal - ELT Press Officer: Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

Astronomers Find the Two Dimmest Stellar Bulbs

ssc2008-22a: Not-So-Bright Bulbs
Credit: NASA/JPL-Caltech

It's a tie! The new record-holder for dimmest known star-like object in the universe goes to twin "failed" stars, or brown dwarfs, each of which shines feebly with only one millionth the light of our sun.

Previously, astronomers thought the pair of dim bulbs was just one typical, faint brown dwarf with no record-smashing titles. But when NASA's Spitzer Space Telescope observed the brown dwarf with its heat-seeking infrared vision, it was able to accurately measure the object's extreme faintness and low temperature for the first time. What's more, the Spitzer data revealed the brown dwarf is, in fact, twins.

"Both of these objects are the first to break the barrier of one millionth the total light-emitting power of the sun," said Adam Burgasser of the Massachusetts Institute of Technology, Cambridge. Burgasser is lead author of a new paper about the discovery appearing in the Astrophysical Journal Letters.

Brown dwarfs are the misfits of the cosmos. They are compact balls of gas floating freely in space, but they are too cool and lightweight to be stars, and too warm and massive to be planets. The name "brown dwarf" comes from the fact that these small, star-like bodies change color over time as they cool, and thus have no definitive color. In reality, most brown dwarfs would appear reddish if they could be seen with the naked eye. Their feeble light output also means they are hard to find. The first brown dwarf wasn't discovered until 1995. While hundreds are known today, astronomers say there are many more in space still waiting to be discovered.

The newfound dim duo of brown dwarfs, while notable for their exceptional faintness, will probably not be remembered for their name. They are called 2MASS J09393548-2448279 after the Two Micron All-Sky Survey, or "2MASS," the mission partially funded by NASA that first detected the object in 1999.

Astronomers recently used Spitzer's ultrasensitive infrared vision to learn more about the object, which was still thought to be a solo brown dwarf. These data revealed a warm atmospheric temperature of 565 to 635 Kelvin (560 to 680 degrees Fahrenheit). While this is hundreds of degrees hotter than Jupiter, it's still downright cold as far as stars go. In fact, 2MASS J09393548-2448279, or 2M 0939 for short, is one of the coldest star-like bodies measured so far.

To calculate the object's brightness, the researchers had to first determine its distance from Earth. After three years of precise measurements with the Anglo-Australian Observatory in Australia, they concluded that 2M 0939 is the fifth-closest known brown dwarf to us, 17 light-years away toward the constellation Antlia. This distance, together with Spitzer's measurements, told the astronomers the object was both cool and extremely dim.

But something was puzzling. The brightness of the object was twice what would be expected for a brown dwarf with its particular temperature. The solution? The object must have twice the surface area. In other words, it's twins, with each body shining only half as bright, and each with a mass of 30 to 40 times that of Jupiter. Both bodies are one million times fainter than the sun in total light, and at least one billion times fainter in visible light alone.

"These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of," said Burgasser. "And like low-energy fluorescent light bulbs, they emit most of their light in a narrow range of wavelengths, in this case in the infrared."

According to the authors, there are even dimmer brown dwarfs scattered throughout the universe, most too faint to see with current sky surveys. NASA's upcoming Wide-Field Infrared Survey Explorer mission will scan the entire sky at infrared wavelengths, and is expected to uncover hundreds of these inconspicuous characters.

"The holy grail in the study of brown dwarfs is to find out how low you can go in terms of temperature, mass and brightness," said Davy Kirkpatrick, a co-author of the paper at NASA's Infrared Processing and Analysis Center at the California Institute of Technology, Pasadena. "This will tell us more about how brown dwarfs form and evolve."

Other authors of this paper are Chris Tinney of the University of New South Wales, Australia; Michael C. Cushing of the University of Hawaii, Manoa; Didier Saumon of the Los Alamos National Laboratory, NM; Mark S. Marley, NASA Ames Research Center, Moffett Field, Calif.; and Clara S. Bennett of the Massachusetts Institute of Technology.

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory
whitney.clavin@jpl.nasa.gov

Unprecedented 16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole

ESO PR Photo 46/08
Centre of the Milky Way
Credit: ESO/S. Gillessen et al.

In a 16-year long study, using several of ESO's flagship telescopes, a team of German astronomers has produced the most detailed view ever of the surroundings of the monster lurking at our Galaxy's heart — a supermassive black hole. The research has unravelled the hidden secrets of this tumultuous region by mapping the orbits of almost 30 stars, a five-fold increase over previous studies. One of the stars has now completed a full orbit around the black hole.

By watching the motions of 28 stars orbiting the Milky Way's most central region with admirable patience and amazing precision, astronomers have been able to study the supermassive black hole lurking there. It is known as "Sagittarius A*" (pronounced "Sagittarius A star"). The new research marks the first time that the orbits of so many of these central stars have been calculated precisely and reveals information about the enigmatic formation of these stars — and about the black hole to which they are bound.

"The centre of the Galaxy is a unique laboratory where we can study the fundamental processes of strong gravity, stellar dynamics and star formation that are of great relevance to all other galactic nuclei, with a level of detail that will never be possible beyond our Galaxy," explains Reinhard Genzel, leader of the team from the Max-Planck-Institute for Extraterrestrial Physics in Garching near Munich.

The interstellar dust that fills the Galaxy blocks our direct view of the Milky Way's central region in visible light. So astronomers used infrared wavelengths that can penetrate the dust to probe the region. While this is a technological challenge, it is well worth the effort. "The Galactic Centre harbours the closest supermassive black hole known. Hence, it is the best place to study black holes in detail," argues the study's first author, Stefan Gillessen.

The team used the central stars as "test particles" by watching how they move around Sagittarius A*. Just as leaves caught in a wintry gust reveal a complex web of air currents, so does tracking the central stars show the nexus of forces at work at the Galactic Centre. These observations can then be used to infer important properties of the black hole itself, such as its mass and distance. The new study also showed that at least 95% of the mass sensed by the stars has to be in the black hole. There is thus little room left for other dark matter.

"Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt," says Genzel. The observations also allow astronomers to pinpoint our distance to the centre of the Galaxy with great precision, which is now measured to be 27 000 light-years.

To build this unparalleled picture of the Milky Way's heart and calculate the orbits of the individual stars the team had to study the stars there for many years. These latest groundbreaking results therefore represent 16 years of dedicated work, which started with observations made in 1992 with the SHARP camera attached to ESO's 3.5-metre New Technology Telescope located at the La Silla observatory in Chile. More observations have subsequently been made since 2002 using two instruments mounted on ESO's 8.2 m Very Large Telescope (VLT). A total of roughly 50 nights of observing time with ESO telescopes, over the 16 years, has been used to complete this incredible set of observations.

The new work improved the accuracy by which the astronomers can measure the positions of the stars by a factor of six compared to previous studies. The final precision is 300 microarcseconds, equivalent at seeing a one euro coin from a distance of roughly 10 000 km.

For the first time the number of known stellar orbits is now large enough to look for common properties among them. "The stars in the innermost region are in random orbits, like a swarm of bees," says Gillessen. "However, further out, six of the 28 stars orbit the black hole in a disc. In this respect the new study has also confirmed explicitly earlier work in which the disc had been found, but only in a statistical sense. Ordered motion outside the central light-month, randomly oriented orbits inside – that's how the dynamics of the young stars in the Galactic Centre are best described."

One particular star, known as S2, orbits the Milky Way's centre so fast that it completed one full revolution within the 16-year period of the study. Observing one complete orbit of S2 has been a crucial contribution to the high accuracy reached and to understanding this region. Yet the mystery still remains as to how these young stars came to be in the orbits they are observed to be in today. They are much too young to have migrated far, but it seems even more improbable that they formed in their current orbits where the tidal forces of the black hole act. Excitingly, future observations are already being planned to test several theoretical models that try to solve this riddle.

"ESO still has much to look forward to," says Genzel. "For future studies in the immediate vicinity of the black hole, we need higher angular resolution than is presently possible." According to Frank Eisenhauer, principal investigator of the next generation instrument GRAVITY, ESO will soon be able to obtain that much needed resolution. "The next major advance will be to combine the light from the four 8.2-metre VLT unit telescopes – a technique known as interferometry. This will improve the accuracy of the observations by a factor 10 to 100 over what is currently possible. This combination has the potential to directly test Einstein's general relativity in the presently unexplored region close to a black hole."

Notes for editors:

These observations are the culmination of 16 years of a large monitoring campaign, begun in 1992 at ESO's New Technology Telescope with SHARP. It was then pursued at ESO's Very Large Telescope with the NACO and SINFONI instruments. These two instruments rely on the use of adaptive optics, which allows astronomers to remove the blurring effect of the atmosphere. As the centre of the Milky Way is very crowded, it is necessary to observe it with the finest resolution possible, hence, the need for adaptive optics.

Only radio signals, infrared light and X-rays can reach us from the Galactic Centre. While radio observations show mostly gas and X-ray observatories are sensitive to high energy processes, the infrared allows these stars to be observed.

First results obtained in the course of this campaign can be found in ESO 17/02, 26/03 and 21/04.

More information:
S. Gillessen et al., Monitoring stellar orbits around the Massive Black Hole in the Galactic Center, 2008, Astrophysical Journal, in press. Link to the article.
The team is composed of Stefan Gillessen, Frank Eisenhauer, Sascha Trippe, Reinhard Genzel, Thomas Ott (MPE, Garching, Germany), Tal Alexander (Weizmann Institute of Science, Israel), and Fabrice Martins (GRAAL-CNRS, University of Montpellier, France).
Reinhard Genzel was awarded the prestigious Shaw Prize in Astronomy for 2008 for this research (see ESO 18/08).

Contacts:
Stefan Gillessen, Reinhard Genzel, Frank Eisenhauer
Max-Planck-Institute for Extraterrestrial Physics
Garching, Germany
Phone: +49 89 30000 3839, +49 89 30000 3281, +49 89 30000 3563
E-mail: ste (at) mpe.mpg.de, genzel (at) mpe.mpg.de, eisenhau (at) mpe.mpg.de

Hubble Finds Carbon Dioxide on an Extrasolar Planet

Credit: ESA, NASA, M. Kornmesser (ESA/Hubble), and STScI

NASA's Hubble Space Telescope has discovered carbon dioxide in the atmosphere of a planet orbiting another star. This is an important step along the trail of finding the chemical biotracers of extraterrestrial life as we know it.

The Jupiter-sized planet, called HD 189733b, is too hot for life. But the Hubble observations are a proof-of-concept demonstration that the basic chemistry for life can be measured on planets orbiting other stars. Organic compounds can also be a by-product of life processes, and their detection on an Earth-like planet may someday provide the first evidence of life beyond Earth.

Previous observations of HD 189733b by Hubble and the Spitzer Space Telescope found water vapor. Earlier this year, Hubble astronomers reported that they found methane in the planet's atmosphere.

"This is exciting because Hubble is allowing us to see molecules that probe the conditions, chemistry, and composition of atmospheres on other planets," says Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Thanks to Hubble we're entering an era where we are rapidly going to expand the number of molecules we know about on other planets."

Swain used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to study infrared light emitted from the planet, which lies 63 light-years away. Gases in the planet's atmosphere absorb certain wavelengths of light from the planet's hot glowing interior. Swain identified not only carbon dioxide, but also carbon monoxide. The molecules leave their own unique spectral fingerprint on the radiation from the planet that reaches Earth. This is the first time a near-infrared emission spectrum has been obtained for an exoplanet.

"The carbon dioxide is kind of the main focus of the excitement, because that is a molecule that under the right circumstances could have a connection to biological activity as it does on Earth," Swain says. "The very fact that we're able to detect it, and estimate its abundance, is significant for the long-term effort of characterizing planets both to find out what they're made of and to find out if they could be a possible host for life."

This type of observation is best done for planets with orbits tilted edge-on to Earth. They routinely pass in front of and then behind their parent stars, phenomena known as eclipses. The planet HD 189733b passes behind its companion star once every 2.2 days. This allows an opportunity to subtract the light of the star alone (when the planet is blocked) from that of the star and planet together prior to eclipse, thus isolating the emission of the planet alone and making possible a chemical analysis of its "day-side" atmosphere.

In this way, Swain explains that he's using the eclipse of the planet behind the star to probe the planet's day side, which contains the hottest portions of its atmosphere. "We're starting to find the molecules and to figure out how many of them there are to see the changes between the day side and the night side," Swain says.

This successful demonstration of looking at near-infrared light emitted from a planet is very encouraging for astronomers planning to use NASA's James Webb Space Telescope when it is launched in 2013. These biomarkers are best seen at near-infrared wavelengths.

Astronomers look forward to using Webb to spectroscopically look for biomarkers on a terrestrial planet the size of Earth, or a "super-Earth" several times our planet's mass. "The Webb telescope should be able to make much more sensitive measurements of these primary and secondary eclipse events," Swain says.

Swain next plans to search for molecules in the atmospheres of other exoplanets, as well as trying to increase the number of molecules detected in exoplanet atmospheres. He also plans to use molecules to study changes that may be present in exoplanet atmospheres to learn something about the weather on these distant worlds.

CONTACT
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Mark Swain
Jet Propulsion Laboratory, Pasadena, Calif.
818-455-2396
mark.swain@jpl.nasa.gov

Rivers of Gas Flow Around Stars in New Space Image

ssc2008-21a: Celestial Sea of Stars (1)

Credit: NASA/JPL-Caltech/Univ. of Wisc. 

A new image from NASA's Spitzer Space Telescope shows a turbulent star-forming region, where rivers of gas and stellar winds are eroding thickets of dusty material.

The picture provides some of the best examples yet of the ripples of gas, or bow shocks, that can form around stars in choppy cosmic waters.

"The stars are like rocks in a rushing river," said Matt Povich of the University of Wisconsin, Madison. "Powerful winds from the most massive stars at the center of the cloud produce a large flow of expanding gas. This gas then piles up with dust in front of winds from other massive stars that are pushing back against the flow." Povich is lead author of a paper describing the new findings in the Dec. 10 issue of the Astrophysical Journal.

Spitzer's new infrared view of the stormy region, called M17, or the Swan nebula, can be found online in the Spitzer Image Gallery. The Swan is located about 6,000 light-years away in the constellation Sagittarius.

Dominating the center of the Swan is a group of massive stars, some exceeding 40 times the mass of our sun. These central stars are 100,000 to one million times as bright as the sun, and roar with radiation and fierce winds made of charged particles that speed along at up to 7.2 million kilometers per hour (4.5 million miles per hour). Both the wind and radiation carve out a deep cavity at the center of the picture -- an ongoing process thought to trigger the birth of new stars.

The growth of this cavity pushes gas up against winds from other massive stars, causing "smiley-faced" bow shocks -- three of which can be seen in the new picture. The direction of the bow shocks tells researchers exactly which way the "wind is blowing."

"The bow shocks are like interstellar weather vanes, indicating the direction of the stellar winds in the nebula," said Povich.

Povich and his colleagues also used Spitzer to take an infrared picture of a star-forming region called RCW 49. Both photographs are described in the same Astrophysical Journal paper, and both provide the first examples of multiple bow shocks around the massive stars of star-forming regions.

Spitzer was able to spot the bow shocks because its infrared eyes can pierce intervening dust, and because it can photograph large swaths of sky quickly.

Ultimately, the new observations will help researchers understand how solar systems like our own are able to form and persist in the rough, celestial seas of space.

"The gas being lit up in these star-forming regions looks very wispy and fragile, but looks can be deceiving," said co-author Robert Benjamin of the University of Wisconsin, Whitewater. "These bow shocks serve as a reminder that stars aren't born in quiet nurseries but in violent regions buffeted by winds more powerful than anything we see on Earth."

Other authors include Barbara A. Whitney of the Space Science Institute, Boulder, Colo.; Brian L. Babler, Marilyn R. Meade and Ed Churchwell of the University of Wisconsin, Madison; and Remy Indebetouw of the University of Virginia, Charlottesville.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., 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, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared array camera was built by NASA's Goddard Space Flight Center, Greenbelt, Md. The instrument's principal investigator is Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics.

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory
whitney.clavin@jpl.nasa.gov

About the Image:

(1) NASA's Spitzer Space Telescope has captured a new, infrared view of the choppy star-making cloud called M17, also known as Omega Nebula or the Swan nebula.

The cloud, located about 6,000 light-years away in the constellation Sagittarius, is dominated by a central group of massive stars -- the most massive stars in the region (see yellow circle). These central stars give off intense flows of expanding gas, which rush like rivers against dense piles of material, carving out the deep pocket at center of the picture. Winds from the region's other massive stars push back against these oncoming rivers, creating bow shocks like those that pile up in front of speeding boats.

Three of these bow shocks are labeled in the magnified inset. They are composed of compressed gas in addition to dust that glows at infrared wavelengths Spitzer can see. The smiley-shaped bow shocks curve away from the stellar winds of the central massive stars.

This picture was taken with Spitzer's infrared array camera. It is a four-color composite, in which light with a wavelength of 3.6 microns is blue; 4.5-micron light is green; 5.8-micron light is orange; and 8-micron light is red. Dust is red, hot gas is green and white is where gas and dust intermingle. Foreground and background stars appear scattered through the image.

About the Object:
Object name: Omega Nebula, Swan Nebula, M 17, NGC 6618
Object type: Star Formation
Position (J2000): RA: 18h 20m 26.00s Dec: -16° 10' 0.00"
Distance: 5,200 Light Years
Constellation: Sagittarius
Instrument: IRAC

Students Discover Unique Planet

ESO PR Photo 45a/08

Artist's impression of the planet OGLE-TR-L9b. Circling its host star in about 2.5 days, it lies at only three percent of the Earth-Sun distance from its star, making the planet very hot with a bloated roiling atmosphere. The star itself is the hottest star with a planet ever discovered.
Credit: ESO/H. Zodet

The students, Meta de Hoon, Remco van der Burg, and Francis Vuijsje

Three undergraduate students, from Leiden University in the Netherlands, have discovered an extrasolar planet. The extraordinary find, which turned up during their research project, is about five times as massive as Jupiter. This is also the first planet discovered orbiting a fast-rotating hot star.

The students were testing a method of investigating the light fluctuations of thousands of stars in the OGLE database in an automated way. The brightness of one of the stars was found to decrease for two hours every 2.5 days by about one percent. Follow-up observations, taken with ESO's Very Large Telescope in Chile, confirmed that this phenomenon is caused by a planet passing in front of the star, blocking part of the starlight at regular intervals.

According to Ignas Snellen, supervisor of the research project, the discovery was a complete surprise. "The project was actually meant to teach the students how to develop search algorithms. But they did so well that there was time to test their algorithm on a so far unexplored database. At some point they came into my office and showed me this light curve. I was completely taken aback!"

The students, Meta de Hoon, Remco van der Burg, and Francis Vuijsje, are very enthusiastic. "It is exciting not just to find a planet, but to find one as unusual as this one; it turns out to be the first planet discovered around a fast rotating star, and it's also the hottest star found with a planet," says Meta. "The computer needed more than a thousand hours to do all the calculations," continues Remco.

The planet is given the prosaic name OGLE2-TR-L9b. "But amongst ourselves we call it ReMeFra-1, after Remco, Meta, and myself," says Francis.

The planet was discovered by looking at the brightness variations of about 15 700 stars, which had been observed by the OGLE survey once or twice per night for about four years between 1997 and 2000. Because the data had been made public, they were a good test case for the students' algorithm, who showed that for one of stars observed, OGLE-TR-L9, the variations could be due to a transit — the passage of a planet in front of its star. The team then used the GROND instrument on the 2.2 m telescope at ESO's La Silla Observatory to follow up the observations and find out more about the star and the planet.

"But to make sure it was a planet and not a brown dwarf or a small star that was causing the brightness variations, we needed to resort to spectroscopy, and for this, we were glad we could use ESO's Very Large Telescope," says Snellen.

The planet, which is about five times as massive as Jupiter, circles its host star in about 2.5 days. It lies at only three percent of the Earth-Sun distance from its star, making it very hot and much larger than normal planets.

The spectroscopy also showed that the star is pretty hot — almost 7000 degrees, or 1200 degrees hotter than the Sun. It is the hottest star with a planet ever discovered, and it is rotating very fast. The radial velocity method — that was used to discover most extrasolar planets known — is less efficient on stars with these characteristics. "This makes this discovery even more interesting," concludes Snellen.

More information:

Snellen I. et al. 2008, OGLE2-TR-L9b: An exoplanet transiting a fast-rotating F3 star, Astronomy and Astrophysics, in press.
The team is composed of I.A.G. Snellen, M.D.J. de Hoon, R.F.J. van der Burg, F.N. Vuijsje (Leiden Observatory, The Netherlands), J. Koppenhoefer (Observatory of Munich, Germany), S. Dreizler (Georg-August University Göttingen, Germany), J. Greiner, T. Krühler, R.P. Saglia (Max-Planck Institute for Extraterrestrial Physics, Garching, Germany), and T.O. Husser (South African Astronomical Observatory).

Contacts:
Ignas Snellen
Leiden Observatory, The Netherlands
Phone: +31 71 52 75 838
Mobile: +31 63 00 31 983
E-mail: snellen (at) strw.leidenuniv.nl

Meta de Hoon, Remco van der Burg, and Francis Vuijsje
Leiden Observatory, The Netherlands
Mobile: +31 65 24 36 179
E-mail: meta.dehoon (at) gmail.com, vdburg (at) strw.leidenuniv.nl, vuijsje (at) strw.leidenuniv.nl

ESO Press Officer: Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

Omega Centauri — the glittering giant of the southern skies

ESO PR Photo 44/08

The globular cluster Omega Centauri — with as many as ten million stars — is seen in all its splendour in this image captured with the WFI camera from ESO's La Silla Observatory. The image shows only the central part of the cluster — about the size of the full moon on the sky (half a degree). North is up, East is to the left. This colour image is a composite of B, V and I filtered images. Note that because WFI is equipped with a mosaic detector, there are two small gaps in the image which were filled with lower quality data from the Digitized Sky Survey. Can you find them? Credit: ESO/EIS

Omega Centauri is one of the finest jewels of the southern hemisphere night sky, as ESO's latest stunning image beautifully illustrates. Containing millions of stars, this globular cluster is located roughly 17 000 light-years from Earth in the constellation of Centaurus.

Sparkling away at magnitude 3.7 and appearing nearly as large as the full moon on the southern night sky, Omega Centauri is visible with the unaided eye from a clear, dark observing site. Even through a modest amateur telescope, the cluster is revealed as an incredible, densely packed sphere of glittering stars. But astronomers need to use the full power of professional telescopes to uncover the amazing secrets of this beautiful globular cluster.

This new image is based on data collected with the Wide Field Imager (WFI), mounted on the 2.2-metre diameter Max-Planck/ESO telescope, located at ESO's La Silla observatory, high up in the arid mountains of the southern Atacama Desert in Chile. Omega Centauri is about 150 light-years across and is the most massive of all the Milky Way's globular clusters. It is thought to contain some ten million stars!

Omega Centauri has been observed throughout history. Both the great astronomer Ptolemy and later Johann Bayer catalogued the cluster as a star. It was not until much later, in the early 19th century, that an Englishman, the astronomer John Frederick William Herschel (son of the discoverer of Uranus), realised that Omega Centauri was in fact a globular cluster. Globular clusters are some of the oldest groupings of stars to be found in the halos that surround galaxies like our own Milky Way. Omega Centauri itself is thought to be around 12 billion years old.

Recent research into this intriguing celestial giant suggests that there is a medium sized black hole sitting at its centre. Observations made with the Hubble Space Telescope (see heic0809 ) and the Gemini Observatory showed that stars at the cluster's centre were moving around at an unusual rate — the cause, astronomers concluded, was the gravitational effect of a massive black hole with a mass of roughly 40 000 times that of the Sun.

The presence of this black hole is just one of the reasons why some astronomers suspect Omega Centauri to be an imposter. Some believe that it is in fact the heart of a dwarf galaxy that was largely destroyed in an encounter with the Milky Way. Other evidence (see ESO 07/05 and heic0708) points to the several generations of stars present in the cluster — something unexpected in a typical globular cluster, which is thought to contain only stars formed at one time. Whatever the truth, this dazzling celestial object provides professional and amateur astronomers alike with an incredible view on clear dark nights.

ESO Press Officer: Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

Swift Looks to Comets for a Cool View

Swift's Ultraviolet/Optical Telescope (UVOT) captured Comet 73P/Schwassmann-Wachmann 3's fragment C as it passed the famous Ring Nebula (oval, bottom) on May 7, 2006. Swift watched as the crumbling comet left dusty blobs behind. Credit: NASA/Swift/Stefan Immler and Dennis Bodewits - Watch Video

NASA's Swift Gamma-ray Explorer satellite rocketed into space in 2004 on a mission to study some of the highest-energy events in the universe. The spacecraft has detected more than 380 gamma-ray bursts, fleeting flares that likely signal the birth of a black hole in the distant universe. In that time, Swift also has observed 80 exploding stars and studied six comets.

Comets? ... Comets are "dirty snowballs" made of frozen gases mixed with dust. X-rays come from superhot plasmas. What do cold comets have in common with exploding stars or the birth of black holes?

"It was a big surprise in 1996 when the NASA-European ROSAT mission showed that comet Hyakutake was emitting X-rays," says Dennis Bodewits, a NASA Postdoctural Fellow at the Goddard Space Flight Center in Greenbelt, Md. "After that discovery, astronomers searched through ROSAT archives. It turns out that most comets emit X-rays when they come within about three times Earth's distance from the sun."

Bodewits is working with the Swift team at NASA's Goddard Space Flight Center in Greenbelt, Md., to study comets using data from the spacecraft's Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT). "Swift is an excellent platform for studying dynamic processes in comets," he says.

Ultraviolet wavelengths let astronomers identify the chemical composition of the comet's atmosphere, observe the structure of dust emission, and identify the rotation of the comet's icy nucleus. X-rays reveal the structure of the comet's gas and the state of the solar wind, a stream of charged particles that flows from the sun at speeds upwards of 900,000 mph.

This movie combines Swift UVOT observations of Comet 73P/Schwassmann-Wachmann 3's fragment C from May 1 to May 11, 2006. Look for blobs of dust moving down the comet's inner tail (orange and yellow). Credit: NASA/Swift/Stefan Immler and Dennis Bodewits
> Watch video Swift's UVOT captured a striking sequence that shows unresolved blobs of dust trailing from a crumbling comet. In early May 2006, when the largest fragment of Comet 73P/Schwassmann-Wachmann 3 (SW3) passed Earth, Swift monitored its approach.

The piece, known as fragment C, is believed to be the comet's main body, which began splintering in 1995. In 2006, astronomers counted 66 fragments. Telescopes -- including NASA's Hubble and Spitzer -- revealed dust and condensations trailing several pieces. But fragment C showed no unusual changes -- except to Swift's ultraviolet eye. "It's subtle, but Swift caught clouds of dust and perhaps small pieces that no one else was able to," Immler says.

The UVOT also includes an ultraviolet grism, which combines a grating with a prism to separate incoming light by wavelength. "Swift's grism spans the wavelengths where carbon-bearing molecules and the hydroxyl molecule are most active. This gives us a unique view into the types and quantities of gas a comet produces, and that gives us clues about the origin of comets and the solar system," Bodewits explains. In fact, with the failure of the Hubble Space Telescope's ultraviolet spectrograph in 2004, Swift is currently the only space observatory covering this wavelength range.

As a comet's surface warms near the sun, the ices turn to gas and form a tenuous atmosphere, or coma, measuring hundreds of thousands of miles across. The solar wind pushes this gas back to form a comet's glowing gas tail. X-ray emission is a side effect of this interaction.

The X-rays arise through a process called charge exchange. Fast-moving ions in the solar wind snatch electrons from uncharged atoms in the comet's atmosphere. The solar-wind ions give off X-rays as the relocated electrons settle into their new home. Because the interaction occurs over such a broad region, the total power output of these emissions can reach one billion watts.

Charge exchange may play important roles in any objects where hot, expanding gas collides with cooler gas. One example: Young stars interacting with the gas and planets that might surround them. Comets provide excellent laboratories to explore these interactions.

When Comet 17P/Holmes underwent a surprising outburst in October 2007, Bodewits tasked both Swift and NASA's Chandra X-ray Observatory to observe it. "The comet was too bright to observe with the UVOT. We were afraid we'd damage the instrument," Bodewits says. "Despite this, we're still not sure whether we detected Holmes with the XRT or Chandra."

At the time of the outburst, Holmes was about 19 degrees above the ecliptic, the plane in which the planets orbit the sun. At that elevation, the comet was probably experiencing a cooler, steadier flow from the solar wind. "The source of this cooler flow wasn't hot enough to produce the ions Holmes needed to make X-rays," Bodewits notes.

Four years ago today, Swift captured its first x-rays. The radiation came from Cygnus X-1, one of sky's strongest sources at these energies. The system, located within our galaxy, contains a hot, blue-giant star orbited by a black hole.

"Swift has operated two years longer than we had hoped," says Neil Gehrels, the mission's lead scientist at NASA's Goddard Space Flight Center. "And while gamma-ray bursts and stellar explosions are the satellite's bread and butter, it's clear that Swift has a lot to contribute to other areas of astronomy."

Francis Reddy
NASA's Goddard Space Flight Center

Super Explosion in 16th Century Caught by Subaru in 21st Century

Reference Figure (MPIA): Wide Field Image of Tycho's Supernova Remnant. Image is a color composite of Mid-Infrared by Spitzer Space Telescope (red), and X-ray (blue: high-energy X-ray, green: middle energy, yellow: low-energy) by Chandra X-Ray Observatory on Near-Infrared by Calar Alto 3.5m Telescope. The remnant is approximately 25 ly in diameter.

Figure1: (a) Optical R-band images of the Tycho’s supernova light echo taken by Calar Alto 2.2m telescope (black means bright). The rectangle shown in a indicates the location of a previous light-echo detection in 2006. The vector towards Tycho’s supernova remnant is indicated (arrow). (b) R-band images taken by FOCAS on Subaru Telescope. The optical spectrum was obtained at the position of the brightness peak marked for reference (red cross).

Figure 2: The view of the light echoes from Tycho’s supernova. The optical light arrived at Earth in 1572 (sky blue arrow). Optical light was scattered by dust cloud around the supernova arrived in 2008 (yellow arrows). Since the emitting regions were apparently shifted from 23 August 2008 to September 24, the optical lights were confirmed as light echoes.

Figure 3: The spectrum of Tycho’s supernova which was obtained by FOCAS on Subaru Telescope (the horizontal axis is the rest wavelength and vertical axis is flux). Black solid lines show the spectrum of Tycho’s SN1572. Comparison of the spectrum of Tycho’s supernova (black solid lines) with templates of subluminous, normal and overluminous type Ia supernova (upper: overluminous (blue), middle: normal (orange), bottom: subluminous (red)). The agreement between the black and orange lines indicates that the Tycho’s supernova belongs to the majority class of normal Type Ia.

When a person looks up into the nighttime sky, they see stars, planets, and galaxies across a sea of darkness. The movements of planets and seasonal variations to the constellations have been relatively the same for thousands of years. What if the sky changed overnight and a new star brighter than anything else appeared? Would it be noticed if it happened in the 16th century?

A few months ago astronomers at the Subaru Telescope went back in time and observed light from a “new star” that originally was seen on 11 November 1572 by astronomer Tycho Brahe and others. What Brahe observed as a bright star in the constellation Cassiopeia, outshining even Venus, was actually a rare supernova event where the violent death of a star sends out an extremely bright outburst of energy. He studied the brightness and color of the “new star” until March 1572 when it faded from view. The remains of this milestone event are seen today as Tycho’s supernova remnant (see Reference Figure).

A team of international astronomers recently completed a study at Subaru that focused on ‘light echos’ from Tycho’s supernova to determine its origin and exact type, and relate that information to what we see from its remnant today. A ‘light echo’ is light from the original supernova event that bounces off dust particles in surrounding interstellar clouds and reaches Earth many years after the direct light passes by; in this case, 436 years ago. This same team used similar methods to uncover the origin of supernova remnant Cassiopeia A in 2007. Lead project astronomer at Subaru, Dr. Tomonori Usuda, said “using light echoes in supernova remnants is time-travelling in a way, in that it allows us to go back hundreds of years to observe the first light from a supernova event. We got to relive a significant historical moment and see it as famed astronomer Tycho Brahe did hundreds of years ago. More importantly, we get to see how a supernova in our own galaxy behaves from its origin.”

On 24 September 2008, using the Faint Object Camera and Spectrograph (FOCAS) instrument at Subaru, the light echoes were broken apart into the signatures of atoms (spectra) present when Supernova 1572 exploded, bearing all the information about the nature of the original blast. The results showed clear absorption of once-ionized silicon and absence of the hydrogen H-alpha emission. The findings were very typical of a Type Ia supernova observed at maximum brightness of its outburst.

During the study, the astronomers tested theories of the explosion mechanism and the nature of the supernova progenitor. For Type Ia supernovae, a white dwarf star in a close binary system is the typical source, and as the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst. However, as Type Ia supernovae with luminosity brighter/fainter than standard ones have been reported recently, the understanding of the supernova outburst mechanism has come under debate. In order to explain the diversity of the Type Ia supernovae, the Subaru team studied the outburst mechanisms in detail.

What they discovered is that Supernova 1572 shows indications of an aspherical/nonsymmetrical explosion, which, in turn, puts limits on explosion models for future studies. In addition, follow-up comparisons with template spectra of Type Ia supernovae found outside our Galaxy shows that Tycho's supernova belongs to the majority class of Normal Type Ia, and, as such, is now the first confirmed and precisely classified supernova in our galaxy. This finding is significant because Type Ia supernovae are the primary source of heavy elements in the Universe, and play an important role as cosmological distance indicators, serving as ‘standard candles’ because the level of the luminosity is always the same for this type of supernova.

This observational study at Subaru established how light echoes can be used in a spectroscopic manner to study supernovae outburst that occurred hundreds of years ago. The light echoes, when observed at different position angles from the source, enabled the team to look at the supernova in a three dimensional view. For the future, this 3D aspect will accelerate the study of the outburst mechanism of supernova based on their spatial structure, which, to date, has been impossible with distant supernovae in galaxies outside the Milky Way.

The results of this study appear in the 4 December 2008 issue of the science journal Nature.

Note 1 :
Tycho Supernova Remnant Team Members -
Oliver Krause(Max-Planck-Institut for Astronomy, Heidelberg [MPIA])
Masaomi Tanaka (Institute for the Physics and Mathematics of the Universe, University of Tokyo / Japan Society for the Promotion of Science Research Fellowship)
Tomonori Usuda (Subaru Telescope, National Astronomical Observatory of Japan [NAOJ]),
Takashi Hattori (Subaru Telescope, NAOJ),
Miwa Goto (MPIA),
Stephan Birkmann(MPIA)
Ken’ichi Nomoto (Institute for the Physics and Mathematics of the Universe, University of Tokyo)

Note 2:
Other types of supernova (Type II, Ib, Ic, etc.) are explosions resulting from the death of a massive star.

Subaru Telescope