Category: Astronomy

Artist’s impression of the stellar object MWC 297. Image credit: ESO Click to enlarge
Using the newly installed AMBER instrument on ESO’s Very Large Telescope Interferometer, which combines the light from two or three 8.2-m Unit Telescopes thereby amounting to observe with a telescope of 40 to 90 metres in diameter, two international teams of astronomers observed with unprecedented detail the environment of two stars. One is a young, still-forming star and the new results provide useful information on the conditions leading to the creation of planets. The other is on the contrary a star entering the latest stages of its life. The astronomers found, in both cases, evidence for a surrounding disc.

A first group of astronomers, led by Fabien Malbet from the Laboratoire d’Astrophysique de Grenoble, France, studied the young 10-solar mass stellar object MWC 297, which is still in the very early stage of its life.

“This scientific breakthrough opens the doors to an especially detailed scrutiny of the very close environment of young stars and will bring us invaluable knowledge on how planets form”, says Malbet.

It is amazing to see the amount of details the astronomers could achieve while observing an object located more than 800 light-years away and hidden by a large amount of gas and dust. They found the object to be surrounded by a proto-planetary disc extending to about the size of our Solar System, but truncated in his inner part until about half the distance between the Earth and the Sun. Moreover, the scientists found the object to be surrounded by an outflowing wind, the velocity of which increased by a factor 9, from about 70 km/s near the disc to 600 km/s in the polar regions.

“The reason why the inner part of the disc should be truncated is not clear”, adds Malbet. “This raises new questions on the physics of the environment of intermediate mass young stars.”

The astronomers now plan to perform observations with AMBER with three telescopes to measure departure from symmetry of the material around MWC 297.

Another international team of astronomers [5] has just done this kind of observations to study the surroundings of a star entering the last stages of its life. In a world premiere, they combined with AMBER the light of three 8.2-m Unit Telescopes of the VLT, gaining unsurpassed knowledge on a B[e] supergiant, a star that is more luminous than our Sun by more than a factor 10,000. This supergiant star is located ten times further away than MCW 297 at more than 8,000 light-years.

The astronomers made the observations to investigate the crucial questions concerning the origin, geometry, and physical structure of the envelope surrounding the star.

These unique observations have allowed the scientists to see structures on scale as small as 1.8 thousandths of an arcsecond – that is the same as distinguishing between the headlights of a car from about 230,000 km away, or slightly less than 2/3 of the distance from the Earth to the Moon!

Armando Domiciano de Souza, from the MPI f??bf?r Radioastronomie in Bonn (Germany) and his colleagues made also use of the MIDI instrument on the VLTI [6], using two Unit Telescopes. Using their full dataset, they found the circumstellar envelope around the supergiant to be non-spherical, most probably because the star is also surrounded by an equatorial disc made of hot dust and a strong polar wind.

“These observations are really opening the doors for a new era of understanding of these complex and intriguing objects”, says Domiciano de Souza.

“Such results could be achieved only due to the spectral resolution as well as spatial resolution that AMBER offers. There isn’t any similar instrument in the world,” concludes Fabien Malbet, who is also the AMBER Project Scientist.

Original Source: ESO News Release

CFHT Observatory. Image credit: CFHT Click to enlarge
The genius of Albert Einstein, who added a “cosmological constant” to his equation for the expansion of the universe but later retracted it, may be vindicated by new research.

The enigmatic dark energy that drives the accelerating expansion of the universe behaves just like Einstein’s famed cosmological constant, according to the Supernova Legacy Survey (SNLS), an international team of researchers in France and Canada that collaborated with large telescope observers at Oxford, Caltech and Berkeley. Their observations reveal that the dark energy behaves like Einstein’s cosmological constant to a precision of 10 per cent.

“The significance is huge,” said Professor Ray Carlberg of the Department of Astronomy and Astrophysics at U of T. “Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands, and as far as we can see, it doesn’t.” The results will be published in an upcoming issue of the journal Astronomy & Astrophysics.

“The Supernova Legacy Survey is arguably the world leader in our quest to understand the nature of dark energy,” said study co-author Chris Pritchet, a professor of physics and astronomy at the University of Victoria in British Columbia, Canada.

The researchers made their discovery using an innovative, 340-million pixel camera called MegaCam, built by the Canada-France-Hawaii Telescope and the French atomic energy agency, Commissariat ? l’?nergie Atomique. “Because of its wide field of view ? you can fit four moons in an image ? it allows us to measure simultaneously, and very precisely, several supernovae, which are rare events,” said Pierre Astier, one of the scientists with the Centre National de la Recherche Scientifique (CNRS) in France.

“Improved observations of distant supernovae are the most immediate way in which we can learn more about the mysterious dark energy,” adds Richard Ellis, a professor of astronomy at the California Institute of Technology. “This study is a very big step forward in quantity and quality.”

Study co-author Saul Perlmutter, a physics professor at the University of California, Berkeley, says the findings kick off a dramatic new generation of cosmology work using supernovae. “The data is more beautiful than we could have imagined 10 years ago ? a real tribute to the instrument builders, the analysis teams and the large scientific vision of the Canadian and French science communities.”

The SNLS is a collaborative international effort that uses images from the Canada-France-Hawaii Telescope, a 3.6-metre telescope atop Mauna Kea, a dormant Hawaiian volcano. The current results are based on about 20 nights of data, the first of over nearly 200 nights of observing time for this project. The researchers identify the few dozen bright pixels in the 340 million captured by MegaCam to find distant supernovae, then acquire their spectra using some of the largest telescopes on earth?the Frederick C. Gillett Gemini North Telescope on Mauna Kea, the Gemini South Telescope on the Cerro Pach?n mountain in the Chilean Andes, the European Southern Observatory Very Large Telescopes (VLT) at the Paranal Observatory in Atacama, Chile, and the Keck telescopes on Mauna Kea. The SNLS is one component of a massive 500-night program of imaging being undertaken as the CFHT Legacy Survey.

“Only the world’s largest optical telescopes ? those from eight to 10 metres in diameter ? are capable of studying distant supernovae in detail by examining the spectrum,” said Isobel Hook, an astronomer in the Department of Astrophysics at Oxford University.

The current paper is based on about one-tenth of the imaging data that will be obtained by the end of the survey. Future results are expected to double or even triple the precision of these findings and conclusively solve several remaining mysteries about the nature of dark energy.

The research was funded by the Canada-France-Hawaii Telescope, the Commissariat ? l’?nergie Atomique (CEA), Centre National de la Recherche Scientifique, Institut National des Sciences de l’Univers du CNRS, the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada’s Herzberg Institute of Astrophysics, the Gemini Observatory, the Particle Physics and Astronomy Research Council, the W. M. Keck Observatory and the European Southern Observatory.

Original Source: U of T News Release

Subhorizon halos. Image credit: Don Davis. Click to enlarge
Thanksgiving is the biggest travel holiday of the year in the United States. Millions of people board airplanes and fly long hours to visit friends and family.

Do you dread the trip? Think of it as a sky watching opportunity. There are some things you can see only through the window of an airplane. Atmospheric optics expert Les Cowley lists a few of his favorites:

“Both sides of the aircraft have their own sights,” says Cowley. “On the side opposite the sun, the main thing to look for is the glory. Clouds below the aircraft are required. They are the canvas on which the glory is ‘painted.'”

“Look toward the antisolar point, the place in the clouds directly opposite the sun,” he instructs. “There, if the aircraft is low enough, you will find the shadow of the plane. Surrounding the shadow is the glory–a bright white glow surrounded by one or more shimmering rings of color.”

“These rings are formed when light is scattered backwards by individual water droplets in the cloud. The more uniform the size of the cloud droplets, the more rings you will see. They swell and contract as you travel over clouds with smaller or larger droplets.”

No clouds beneath you?

“In that case,” says Cowley, “another optical effect might be visible, especially over arid regions or pine forests. This is the opposition effect, a bright patch of light moving along the ground below you. The brightening, which is always directly opposite the sun, marks the point where the shadows of objects, like trees or soil granules, are hidden beneath those objects. The area consequently looks brighter, and slightly more yellow, than the surroundings.” (Click here to view an image of the opposition effect, photographed by Eva Seidenfaden flying over Uzbekstan.)

Turning to the sunward side of the aircraft…

“That is the realm of ice halos,” says Cowley. Ice halos are rings and arcs of light caused by ice crystals in high clouds. “They are often rainbow-colored,” he notes, “but they are not rainbows.”

From the ground you look up to see these halos. From an airplane you look down.

“You might be able to see subhorizon halos invisible from low ground,” says Cowley. “The brightest, sometimes blindingly bright, is the subsun. This is a direct reflection of the sun from millions of flat plate-shaped ice crystals floating in the clouds beneath you and acting together as a giant mirror. As the aircraft moves the subsun drifts along the clouds, sometimes growing, sometimes contracting, sometimes wobbling as crystals with different tilts are sampled. Sometimes a column of light will extend upward from the subsun toward the real sun–this is a lower sun pillar.”

“Sunrise and sunset from high altitudes are special,” Cowley adds. “The speed of the aircraft can make them faster or slower than usual. Furthermore, the sun is extra-flattened because its light is refracted almost twice the normal amount by its passage into the dense lower atmosphere and then out again to you. On a night flight, you might catch the moonrise; its distortions and flattening are greater for the same reason.”

“And if none of these things are visible on your particular flight, ignore fellow passengers and crane your head to see some of the sky above you. It is dark and a deep violet blue–darker than you will ever see on the ground. A large part of Earth’s atmosphere is beneath and there are far fewer molecules to scatter the sun’s light and turn the sky blue. You are not far from space.”

Happy Thanksgiving!

Original Source: NASA News Release

A slice through a 3-D simulation of a turbulent clump of molecular hydrogen. Image credit: Mark Krumholz. Click to enlarge
Astrophysicists at the University of California, Berkeley, and Lawrence Livermore National Laboratory (LLNL) have exploded one of two competing theories about how stars form inside immense clouds of interstellar gas.

That model, which is less than 10 years old and is championed by some British astronomers, predicts that interstellar hydrogen clouds develop clumps in which several small cores – the seeds of future stars – form. These cores, less than a light year across, collapse under their own gravity and compete for gas in the surrounding clump, often gaining 10 to 100 times their original mass from the clump.

The alternative model, often termed the “gravitational collapse and fragmentation” theory, also presumes that clouds develop clumps in which proto-stellar cores form. But in this theory, the cores are large and, though they may fragment into smaller pieces to form binary or multiple star systems, contain nearly all the mass they ever will.

“In competitive accretion, the cores are seeds that grow to become stars; in our picture, the cores turn into the stars,” explained Chris McKee, professor of physics and of astronomy at UC Berkeley. “The observations to date, which focus primarily on regions of low-mass star formation, like the sun, are consistent with our model and inconsistent with theirs.”

“Competitive accretion is the big theory of star formation in Europe, and we now think it’s a dead theory,” added Richard Klein, an adjunct professor of astronomy at UC Berkeley and a researcher at LLNL.

Mark R. Krumholz, now a post-doctoral fellow at Princeton University, McKee and Klein report their findings in the Nov. 17 issue of Nature.

Both theories try to explain how stars form in cold clouds of molecular hydrogen, perhaps 100 light years across and containing 100,000 times the mass of our sun. Such clouds have been photographed in brilliant color by the Hubble and Spitzer space telescopes, yet the dynamics of a cloud’s collapse into one or many stars is far from clear. A theory of star formation is critical to understanding how galaxies and clusters of galaxies form, McKee said.

“Star formation is a very rich problem, involving questions such as how stars like the sun formed, why a very large number of stars are in binary star systems, and how stars ten to a hundred times the mass of the sun form,” he said. “The more massive stars are important because, when they explode in a supernova, they produce most of the heavy elements we see in the material around us.”

The competitive accretion model was hatched in the late 1990s in response to problems with the gravitational collapse model, which seemed to have trouble explaining how large stars form. In particular, the theory couldn’t explain why the intense radiation from a large protostar doesn’t just blow off the star’s outer layers and prevent it from growing larger, even though astronomers have discovered stars that are 100 times the mass of the sun.

While theorists, among them McKee, Klein and Krumholz, have advanced the gravitational collapse theory farther toward explaining this problem, the competitive accretion theory has come increasingly into conflict with observations. For example, the accretion theory predicts that brown dwarfs, which are failed stars, are thrown out of clumps and lose their encircling disks of gas and dust. In the past year, however, numerous brown dwarfs have been found with planetary disks.

“Competitive accretion theorists have ignored these observations,” Klein said. “The ultimate test of any theory is how well it agrees with observation, and here the gravitational collapse theory appears to be the clear winner.”

The model used by Krumholz, McKee and Klein is a supercomputer simulation of the complicated dynamics of gas inside a swirling, turbulent cloud of molecular hydrogen as it accretes onto a star. Theirs is the first study of the effects of turbulence on the rate at which a star accretes matter as it moves through a gas cloud, and it demolishes the “competitive accretion” theory.

Employing 256 parallel processors at the San Diego Supercomputer Center at UC San Diego, they ran their model for nearly two weeks to show that it accurately represented star formation dynamics.

“For six months, we worked on very, very detailed, high-resolution simulations to develop that theory,” Klein said. “Then, having that theory in hand, we applied it to star forming regions with the properties that one could glean from a star forming region.”

The models, which also were run on supercomputers at Lawrence Berkeley National Laboratory and LLNL, showed that turbulence in the core and surrounding clump would prevent accretion from adding much mass to a protostar.

“We have shown that, because of turbulence, a star cannot efficiently accrete much more mass from the surrounding clump,” Klein said. “In our theory, once a core collapses and fragments, that star basically has all the mass it is ever going to have. If it was born in a low-mass core, it will end up being a low-mass star. If it’s born in a high mass core, it may become a high-mass star.”

McKee noted that the researchers’ supercomputer simulation indicates competitive accretion may work well for small clouds with very little turbulence, but these rarely, if ever, occur and have not been observed to date. Real star formation regions have much more turbulence than assumed in the accretion model, and the turbulence does not quickly decay, as that model presumes. Some unknown processes, perhaps matter flowing out of protostars, keep the gases roiled up so that the core does not collapse quickly.

“Turbulence opposes gravity; without it, a molecular cloud would collapse far more rapidly than observed,” Klein said. “Both theories assume turbulence is there. The key is (that) there are processes going on as stars begin to form that keep turbulence alive and prevent it from decaying. The competitive accretion model doesn’t have any way to put this into the calculations, which means they’re not modeling real star forming regions.”

Klein, McKee and Krumholz continue to refine their model to explain how radiation from large protostars escapes without blowing away all the infalling gas. For example, they have shown that some of the radiation can escape through cavities created by the jets observed to come out the poles of many stars in formation. Many predictions of the theory may be answered by new and larger telescopes now under construction, in particular the sensitive, high-resolution ALMA telescope being constructed in Chile by a consortium of United States, European and Japanese astronomers, McKee said.

The work was supported by the National Aeronautics and Space Administration, the National Science Foundation and the Department of Energy.

Original Source: UC Berkeley News Release

Artist’s impression of a ‘supergiant fast X-ray transient’. Image credit: ESA Click to enlarge
ESA’s Integral gamma-ray observatory has discovered a new, highly populated class of X-ray fast “transient” binary stars, undetected in previous observations.

With this discovery, Integral confirms how much it is contributing to revealing a whole hidden Universe.

The new class of double star systems is characterised by a very compact object that produces highly energetic, recurrent and fast-growing X-ray outbursts, and a very luminous “supergiant” companion.

The compact object can be an accreting body such as a black hole, a neutron star or a pulsar. Scientists have called such class of objects “supergiant fast X-ray transients”. “Transients” are systems which display periods of enhanced X-ray emission.

Before the launch of Integral, only a dozen X-ray binary stars containing supergiants had been detected. Actually, scientists thought that such high-mass X-ray systems were very rare, assuming that only a few of them would exist at once since stars in supergiant phase have a very short lifetime.

However, Integral’s data combined with other X-ray satellite observations indicate that transient supergiant X-ray binary systems are probably much more abundant in our Galaxy than previously thought.

In particular, Integral is showing that such “supergiant fast X-ray transients”, characterised by fast outbursts and supergiant companions, form a wide class that lies hidden throughout the Galaxy.

Due to the transitory nature, in most cases these systems were not detected by other observatories because they lacked the combination of sensitivity, continuous coverage and wide field of view of Integral.

They show short outbursts with very fast rising times – reaching the peak of the flare in only a few tens of minutes – and typically lasting a few hours only. This makes the main difference with most other observed transient X-ray binary systems, which display longer outbursts, lasting typically a few weeks up to months.

In the latter case, the long duration of the outburst is consistent with a “viscous” mass exchange between the star and an accreting compact object.

In “supergiant fast X-ray transients”, associated with highly luminous supergiant stars, the short duration of the outburst seems to point to a different and peculiar mass exchange mechanism between the two bodies.

This may have something to do with the way the strong radiative winds, typical of highly massive stars, feed the compact object with stellar material.

Scientists are now thinking about the reasons for such short outbursts. It could be due to the supergiant donor ejecting material in a non-continuous way. For example, a clumpy and intrinsically variable nature of a supergiant”s radiative winds may give rise to sudden episodes of increased accretion rate, leading to the fast X-ray flares.

Alternatively, the flow of material transported by the wind may become, for reasons not very well understood, very turbulent and irregular when falling into the enormous gravitational potential of the compact object.

“In any case, we are pretty confident that the fast outbursts are associated to the mass transfer mode from the supergiant star to the compact object,” says Ignacio Negueruela, lead author of the results, from the University of Alicante, Spain.

“We believe that the short outbursts cannot be related to the nature of the compact companion, as we observed fast outbursts in cases where the compact objects were very different – black holes, slow X-ray pulsars or fast X-ray pulsars.”

Studying sources such as “supergiant fast X-ray transients”, and understanding the reasons for their behaviour, is very important to increase our knowledge of accretion processes of compact stellar objects. Furthermore, it is providing valuable insight into the evolution paths that lead to the formation of high-mass X-ray binary systems.

Original Source: ESA Portal

Star forming region NGC 1333. Image credit: Spitzer. Click to enlarge.
Located 1,000 light-years from Earth in the constellation Perseus, a reflection nebula called NGC 1333 epitomizes the beautiful chaos of a dense group of stars being born. Most of the visible light from the young stars in this region is obscured by the dense, dusty cloud in which they formed. With NASA’s Spitzer Space Telescope, scientists can detect the infrared light from these objects. This allows a look through the dust to gain a more detailed understanding of how stars like our sun begin their lives.

The young stars in NGC 1333 do not form a single cluster, but are split between two sub-groups. One group is to the north near the nebula shown as red in the image. The other group is south, where the features shown in yellow and green abound in the densest part of the natal gas cloud. With the sharp infrared eyes of Spitzer, scientists can detect and characterize the warm and dusty disks of material that surround forming stars. By looking for differences in the disk properties between the two subgroups, they hope to find hints of the star- and planet-formation history of this region.

The knotty yellow-green features located in the lower portion of the image are glowing shock fronts where jets of material, spewed from extremely young embryonic stars, are plowing into the cold, dense gas nearby. The sheer number of separate jets that appear in this region is unprecedented. This leads scientists to believe that by stirring up the cold gas, the jets may contribute to the eventual dispersal of the gas cloud, preventing more stars from forming in NGC 1333.

In contrast, the upper portion of the image is dominated by the infrared light from warm dust, shown as red.

Original Source: Spitzer News Release

W5 star forming region in Cassiopeia. Image credit: NASA/JPL/Spitzer. Click to enlarge.
A new image from NASA’s Spitzer Space Telescope reveals billowing mountains of dust ablaze with the fires of stellar youth.

Captured by Spitzer’s infrared eyes, the majestic image resembles the iconic “Pillars of Creation” picture taken of the Eagle Nebula in visible light by NASA’s Hubble Space Telescope in 1995. Both views feature star-forming clouds of cool gas and dust that have been sculpted into pillars by radiation and winds from hot, massive stars.

The Spitzer image, which can be found at http://www.spitzer.caltech.edu/Media, shows the eastern edge of a region known as W5, in the Cassiopeia constellation 7,000 light-years away. This region is dominated by a single massive star, whose location outside the pictured area is “pointed out” by the finger-like pillars. The pillars themselves are colossal, together resembling a mountain range. They are more than 10 times the size of those in the Eagle Nebula.

The largest of the pillars observed by Spitzer entombs hundreds of never-before-seen embryonic stars, and the second largest contains dozens.

“We believe that the star clusters lighting up the tips of the pillars are essentially the offspring of the region’s single, massive star,” said Dr. Lori Allen, lead investigator of the new observations, from the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. “It appears that radiation and winds from the massive star triggered new stars to form.”

Spitzer was able to see the stars forming inside the pillars thanks to its infrared vision. Visible-light images of this same region show dark towers outlined by halos of light. The stars inside are cloaked by walls of dust. But infrared light coming from these stars can escape through the dust, providing astronomers with a new view.

“With Spitzer, we can not only see the stars in the pillars, but we can estimate their age and study how they formed,” said Dr. Joseph Hora, a co-investigator, also from the Harvard-Smithsonian Center for Astrophysics.

The W5 region and the Eagle Nebula are referred to as high-mass star-forming regions. They start out as thick and turbulent clouds of gas and dust that later give birth to families of stars, some of which are more than 10 times more massive than the sun. Radiation and winds from the massive stars subsequently blast the cloudy material outward, so that only the densest pillar-shaped clumps of material remain. The process is akin to the formation of desert mesas, which are made up of dense rock that resisted water and wind erosion.

According to theories of triggered star formation, the pillars eventually become dense enough to spur the birth of a second generation of stars. Those stars, in turn, might also trigger successive generations. Astronomers do not know if the sun, which formed about five billion years ago, was originally a member of this type of extended stellar family.

Allen and her colleagues believe they have found evidence for triggered star formation in the new Spitzer image. Though it is possible the clusters of stars in the pillars are siblings of the single massive star, the astronomers say the stars are more likely its children.

Luis Chavarria is also a member of the investigating team at the Harvard-Smithsonian Center for Astrophysics. This research was originally led by Dr. Lynne Deutsch of the Center for Astrophysics, who passed away April 2, 2004.

For graphics and more information about Spitzer, visit , http://www.spitzer.caltech.edu/spitzer/ . To view or download Hubble’s Pillars of Creation image, visit http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/44/image/a . For more information about NASA and agency programs on the Web, visit http://www.nasa.gov/home/ .

The image is also available in a NASA TV video file that airs beginning at 9 a.m. Eastern time. NASA TV’s Public, Education and Media channels are available on an MPEG-2 digital C-band signal accessed via satellite AMC-6, at 72 degrees west longitude, transponder 17C, 4040 MHz, vertical polarization. In Alaska and Hawaii, they’re on AMC-7 at 137 degrees west longitude, transponder 18C, at 4060 MHz, horizontal polarization. A Digital Video Broadcast compliant Integrated Receiver Decoder is required for reception. For digital downlink information for each NASA TV channel and access to NASA TV’s Public Channel on the Web, visit http://www.nasa.gov/ntv .

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech. NASA’s Goddard Space Flight Center, Greenbelt, Md., built Spitzer’s infrared array camera, which took the observations. The instrument’s principal investigator is Dr. Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics.

Original Source: NASA/JPL/Spizer News Release

Artist illustration of a star ejected from the Large Magellanic Cloud. Image credit: ESO. Click to enlarge.
Observations with Kueyen, one of the 8.2m telescopes composing the ESO Very Large Telescope (VLT), have led to the discovery of a short-lived massive star that is moving at a very high speed through the outer halo of the Milky Way galaxy and into intergalactic space. This finding could provide evidence for a previously unknown massive black hole in the heart of the Milky Way’s closest neighbour, the Large Magellanic Cloud.

The star, named HE 0437-5439, was discovered by the Hamburg/ESO sky survey [1] , a project aimed at detecting quasars but which discovered many faint blue stars as well. Scientists [2] at the Dr. Remeis-Sternwarte (University of Erlangen-Nürnberg, Germany) and the Centre for Astrophysics Research (University of Hertfordshire, UK) found what is likely to be a hot massive main-sequence star, far out in the halo.

This came as a great surprise. Massive stars have lifetimes of only some tens or hundreds of million years, short lived for astronomical standards, but the halo does not usually host stars as young as this. In fact, it contains the oldest stars in the Milky Way that are more than ten billion years old. Massive stars are usually found in or near star forming regions in the Galactic disc such as the famous Orion nebula: HE 0437-5439 is indeed similar to the trapezium stars that make the Orion nebula shine.

Data were obtained with the ESO VLT and its high resolution UVES spectrograph. This allowed the chemical composition to be measured which turned out to be similar to that of the Sun, confirming that HE0437-5439 is a young star. Its mass is eight times larger than that of the Sun and the star is only 30 million years old. It is almost 200,000 light years away from us in the direction of the Doradus Constellation (“the Swordfish”).

Even more exciting was the fact that the data indicated the star to be receding at a velocity of 723 km/s, or 2.6 million kilometres per hour. HE0437-5439 moves so fast that the gravitational attraction of the Milky Way is too small to keep it bound to the Galaxy. Hence the hyper-velocity star will escape into intergalactic space.

As the star is moving so fast, it must have been born far away from its present position and accelerated to where we observe it today. What accelerated the star to such a high speed? Calculations carried out already in the late 1980s showed that a so-called massive black hole (SMBH), i.e. a black hole a million times as massive as the Sun, or larger, could provide the enormous acceleration. If a binary star approaches the SMBH, one star falls towards the SMBH while its companion is ejected. The Galactic Centre of the Milky Way hosts such a black hole of about 2.5 million solar masses, and this might have accelerated HE0437-5439.

But the necessary travel time was found to be more than three times the age of the star. Hence the star is too young to have travelled all the way from the Galactic centre to its present location. Either the star is older than it appears or it was born and accelerated elsewhere.

A different clue to the origin of HE0457-5439 comes from its position in the sky. HE0437-5439 is 16 degrees away from the Large Magellanic Cloud (LMC), one of the nearest neighbouring galaxies to the Milky Way. This galaxy lies at a distance of 156,000 light years. HE0457-5439 is even more distant than the LMC and is much closer to the LMC than to the galaxy. The astronomers showed that the star could have reached its present position within its lifetime if it were ejected from the centre of the LMC. This, in turn, would provide evidence for the existence of a SMBH in the LMC.

Another explanation would require the star to be the result of the merging of two stars, belonging to so-called blue stragglers class of stars, which are older than standard evolution models predict them to be. Indeed, its age could then be as much as the lifetime of a 4 solar mass star which is more than 6 times the lifetime of an 8 solar mass star.

The astronomers propose two additional observations to distinguish between the two options. The abundance of certain elements in stars belonging to the LMC is only half that of the Sun. A more precise measurement with UVES would indicate whether the star has a metal abundance appropriate to LMC stars or not. The second is to measure how much the star moves in the transverse direction on the sky, using astrometric measurements.

The research presented here is detailed in a paper to be published in Astrophysical Journal Letters.

Notes
[1]: The Hamburg/ESO sky survey is a collaborative project of the Hamburger Sternwarte and ESO to provide spectral information for half of the southern sky using photographic plates taken with the now retired ESO-Schmidt telescope. These plates were digitized at Hamburger Sternwarte.

[2]: The astronomers are Heinz Edelmann (Dr. Remeis-Sternwarte of the University of Erlangen-Nürnberg, Germany, now at University of Texas, Austin, USA), Ralf Napiwotzki (Centre for Astrophysics Research, University of Hertfordshire, UK), Uli Heber (Dr. Remeis-Sternwarte of the University of Erlangen-Nürnberg, Germany), Norbert Christlieb and Dieter Reimers (Hamburger Sternwarte, Germany).

Original Source: ESO News Release

Robert’s Quartet. Image credit: ESO. Click to enlarge.
ESO PR Photo 34a/05 shows in amazing details a group of galaxies known as Robert’s Quartet [1]. The image is based on data collected with the FORS2 multi-mode instrument on ESO’s Very Large Telescope.

Robert’s Quartet is a family of four very different galaxies, located at a distance of about 160 million light-years, close to the centre of the southern constellation of the Phoenix. Its members are NGC 87, NGC 88, NGC 89 and NGC 92, discovered by John Herschel in the 1830s. NGC 87 (upper right) is an irregular galaxy similar to the satellites of our Milky Way, the Magellanic Clouds. NGC 88 (centre) is a spiral galaxy with an external diffuse envelope, most probably composed of gas. NGC 89 (lower middle) is another spiral galaxy with two large spiral arms. The largest member of the system, NGC 92 (left), is a spiral Sa galaxy with an unusual appearance. One of its arms, about 100,000 light-years long, has been distorted by interactions and contains a large quantity of dust.

The quartet is one of the finest examples of compact groups of galaxies. Because such groups contain four to eight galaxies in a very small region, they are excellent laboratories for the study of galaxy interactions and their effects, in particular on the formation of stars.

Using another set of VLT data also obtained with FORS2, astronomers [2] were able to study the properties of regions of active star formation (“HII regions” [3]) in the sister members of Robert’s Quartet. They found more than 200 of such regions in NGC 92, with a size between 500 and 1,500 light-years. For NGC 87, they detected 56 HII regions, while the two other galaxies appear to have far fewer of them. For NGC 88, however, they found two plume-like features, while NGC 89 presents a ring of enhanced stellar activity. The system is thus clearly showing increased star formation activity, most probably as the result of the interaction between its members. The sisters clearly belong to a perturbed family.

The quartet has a total visual magnitude of almost 13, i.e. it is about 600 times fainter than the faintest object that can be seen with the unaided eye. The brightest member of the group has a magnitude of about 14. On the sky, the four galaxies are all within a circle of radius of 1.6 arcmin, corresponding to about 75,000 light-years.

Notes
[1]: The group of galaxies was known as a Compact Group since 1977 by J.A. Rose, under the designation Rose 34. Robert’s Quartet is also known under the less poetic name of AM 0018-485 from the Catalogue of Southern Peculiar Galaxies and Associations, compiled in 1987 by astronomers Halton “Chip” Arp and Barry Madore. But who is Robert then? As discovered by Australian amateur astronomer Mike Kerr, Arp and Madore named Robert’s Quartet after Robert Freedman who generated many of the updated positions of galaxies in the catalogue. The astronomers clearly had a very good sense of humour as the catalogue also contains a system of galaxies called Wendy (ESO 147- 8; for Wendy Freedman) and another called the Conjugal galaxy (ESO 384- 53)!

[2]: The astronomers are S. Temporin (University of Innsbruck, Austria), S. Ciroi and P. Rafanelli (University of Padova, Italy), A. Iovino (INAF-Brera Astronomical Observatory, Italy), E. Pompei (ESO), and M. Radovich (INAF-Capodimonte Astronomical Observatory, Italy). (The article describing this result is available in PDF format at http://www.ast.cam.ac.uk/%7Esb2004/posters/files/Temporin.pdf)

[3]: The radiation of young hot stars embedded in an interstellar cloud is able to heat the surrounding gas, resulting in the apparition of an emission nebula that shines mostly in the light of ionized hydrogen (H) atoms. Such nebulae are therefore often referred to as “HII regions”. The well-known Orion Nebula is an outstanding example of that type of nebula.

Original Source: ESO News Release

Cloudshine in L1448. Image credit: CfA. Click to enlarge.
Hubble’s iconic images include many shots of cosmic clouds of gas and dust called nebulae. For example, the famous “Pillars of Creation” mark the birthplace of new stars within the Eagle Nebula. Yet despite their beauty, visible-light images show only the nebulae surfaces. Baby stars may hide beneath, invisible even to Hubble’s powerful gaze.

Harvard astronomers have pioneered a new way to peer below the surface using near-infrared light that is invisible to the human eye. The resulting images are both beautiful and scientifically valuable because they can be used to map the structure of interstellar matter.

“We can now see the structure of gigantic star-forming regions over vast distances with a resolution 50 times better than before,” said Alyssa Goodman of the Harvard-Smithsonian Center for Astrophysics (CfA). “This technique will revolutionize the way we map stellar birthplaces.”

While Hubble’s NICMOS instrument and NASA’s Spitzer Space Telescope also use infrared light to study nebular interiors, ground-based images at near-infrared wavelengths provide an unparalleled combination of wide-field coverage and high resolution.

“Images like these will give astronomers new insight into what those giant complexes of gas and dust really look like,” added Jonathan Foster, a graduate student at Harvard University and the paper’s first author.

The researchers took long-exposure photographs of a star-forming region in the constellation Perseus and were surprised to see something they had never seen before. Just as earthly clouds shine orange at night as they reflect light from streetlights below, they discovered that clouds in outer space show a similar effect. In space, otherwise “dark” clouds of dust and gas are illuminated by faint starlight washing over them.

Goodman and Foster dubbed the new celestial phenomenon “cloudshine.” Their long-exposure, near-infrared images uncovered the faintly shining billows of material. Recent advances in infrared detectors, combined with longer than usual imaging times, led to the discovery.

“Other astronomers have seen hints of cloudshine in their images, but our new photographs are the most spectacular evidence of cloudshine to date,” said Goodman.

Reflection nebulae such as the wisps surrounding the Pleiades star cluster have been observed for decades. Importantly, the Pleiades and other famous “nebulae” are illuminated from within, by the stars associated with them, as a cloud is when fireworks explode inside of it. Cloudshine is the result of the illumination of otherwise “dark” clouds from “without,” by the faint, and nearly uniform, ambient light produced by the sum of all the stars outside the cloud. Simple modeling in Foster & Goodman’s paper demonstrates that there is enough of this faint ambient light to illuminate the clouds at the levels observed.

The cloudshine images were obtained as part of the COMPLETE survey (Coordinated Molecular Probe Line Extinction Thermal Emission) of star-forming regions. COMPLETE involves making wide-field, high-resolution studies of three nearby star-forming regions. COMPLETE will allow for detailed analysis and understanding of the physics of star formation on scales ranging from one-hundredth of a light-year up to 30 light-years.

A companion paper by astronomer Paolo Padoan (UC San Diego) and colleagues describes theoretical modeling of the cloudshine effect in turbulent clouds of gas. They showed that the near-infrared “color” of a nebula correlates to the nebula’s density, and can therefore be used to map its structure.

“By using cloudshine, astronomers can study star-forming regions at a very small scale,” said Padoan. “We will be able to learn much more about the physics of star formation.”

Foster and Goodman anticipate gathering many additional images of cloudshine as the COMPLETE survey continues.

“We can cover wide areas of the sky at high resolution relatively quickly,” said Foster. “We expect that this will become the best technique for mapping the density of `dark’ clouds with very high resolution.”

Foster and Goodman’s paper reporting the cloudshine observations has been submitted for publication to The Astrophysical Journal Letters and is available online at http://arxiv.org/abs/astro-ph/0510624.

A paper on the theory of cloudshine by Padoan, Mika Juvela and Veli-Matti Pelkonen (University of Helsinki) also has been submitted for publication to The Astrophysical Journal Letters and is available online at http://arxiv.org/abs/astro-ph/0510600.

Foster and Goodman’s work, and the COMPLETE Survey, are supported by the National Science Foundation, NASA, and Harvard University.

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

Original Source: CfA News Release