Saturday, August 30, 2014

Sonograms of Young Stars

An optical image of the region NGC 2264 containing a cluster of young stars. Astronomers have measured pulsations in the light of some of these stars and used them to infer the physical processes underway before they mature and begin to burn hydrogen fuel.Credit: ESO 

The evolution of a star depends crucially on its initial birth mass and composition, and developments in its early lifetime. These initial properties determine, for example, the production of the chemical elements forged later on in the star's nuclear furnace, and its early angular momentum affects the subsequent distribution of its internal energy. Astronomers are therefore working to understand the physical processes underway in the very earliest stages of a star’s life after a dense clump of interstellar matter has contracted, warmed, and begun the stellar gestation processes.

In the early stages of a star's life, its central temperature and density are not yet high enough to initiate significant nuclear burning; its energy comes from the release of gravitational energy, and it circulates via gas motions. As the internal temperatures rise and the hot gas becomes more transparent, this internal energy begins to redistribute itself through radiation as well through gas motions. Then, as these and related processes compete for primacy, the star starts to vibrate slightly, an effect which can be observed as periodic variations in the star's surface through the luminosity and temperature. In some young stars it is possible to use "asteroseismology" – the measurement of these acoustic patterns - to explore the star’s structure and evolution.

CfA astronomer Dimitar Sasslov has joined with a team of other investigators to study the surface vibrations in thirty-four young stars. Their ages are constrained by their emission spectra or by their being members of stellar clusters whose ages are known to be less than about ten million years, either signaling likely periodic vibrations in the young stars. The team finds a clear relation between seismic observables and evolutionary status, and in particular they are able to infer the internal stellar processes underway and the star’s corresponding evolutionary development. By analogy to a sonogram, this technique of asteroseismology is able to probe the unborn, and offers a new probe of the earliest stages of a star's life.

Reference(s): 
"Echography of Young Stars Reveals Their Evolution," K. Zwintz, L. Fossati, T. Ryabchikova, D. Guenther, C. Aerts, T. G. Barnes, N., Themes, D. Lorenz, C. Cameron, R. Kuschnig, S. Pollack-Drs, E. Moravveji, A. Baglin, J., M. Matthews, A. F. J. Moffat1, E. Poretti, M. Rainer, S. M. Rucinski, D. Sasselov, W. W. Weiss, Science, 345, 550, 2014.



NASA's Spitzer Telescope Witnesses Asteroid Smashup

This artist's concept shows the immediate aftermath of a large asteroid impact around NGC 2547-ID8, a 35-million-year-old sun-like star thought to be forming rocky planets. Image credit: NASA/JPL-Caltech.  Full image and caption

Astronomers were surprised to see these data from NASA's Spitzer Space Telescope in January 2013, showing a huge eruption of dust around a star called NGC 2547-ID8. Image credit: NASA/JPL-Caltech/University of Arizona.   Full image and caption - enlarge image

NASA's Spitzer Space Telescope has spotted an eruption of dust around a young star, possibly the result of a smashup between large asteroids. This type of collision can eventually lead to the formation of planets.

Scientists had been regularly tracking the star, called NGC 2547-ID8, when it surged with a huge amount of fresh dust between August 2012 and January 2013. 

"We think two big asteroids crashed into each other, creating a huge cloud of grains the size of very fine sand, which are now smashing themselves into smithereens and slowly leaking away from the star," said lead author and graduate student Huan Meng of the University of Arizona, Tucson. 

While dusty aftermaths of suspected asteroid collisions have been observed by Spitzer before, this is the first time scientists have collected data before and after a planetary system smashup. The viewing offers a glimpse into the violent process of making rocky planets like ours. 

Rocky planets begin life as dusty material circling around young stars. The material clumps together to form asteroids that ram into each other. Although the asteroids often are destroyed, some grow over time and transform into proto-planets. After about 100 million years, the objects mature into full-grown, terrestrial planets. Our moon is thought to have formed from a giant impact between proto-Earth and a Mars-size object. 

In the new study, Spitzer set its heat-seeking infrared eyes on the dusty star NGC 2547-ID8, which is about 35 million years old and lies 1,200 light-years away in the Vela constellation. Previous observations had already recorded variations in the amount of dust around the star, hinting at possible ongoing asteroid collisions. In hope of witnessing an even larger impact, which is a key step in the birth of a terrestrial planet, the astronomers turned to Spitzer to observe the star regularly. Beginning in May 2012, the telescope began watching the star, sometimes daily. 

A dramatic change in the star came during a time when Spitzer had to point away from NGC 2547-ID8 because our sun was in the way. When Spitzer started observing the star again five months later, the team was shocked by the data they received. 

"We not only witnessed what appears to be the wreckage of a huge smashup, but have been able to track how it is changing -- the signal is fading as the cloud destroys itself by grinding its grains down so they escape from the star," said Kate Su of the University of Arizona and co-author on the study. "Spitzer is the best telescope for monitoring stars regularly and precisely for small changes in infrared light over months and even years." 

A very thick cloud of dusty debris now orbits the star in the zone where rocky planets form. As the scientists observe the star system, the infrared signal from this cloud varies based on what is visible from Earth. For example, when the elongated cloud is facing us, more of its surface area is exposed and the signal is greater. When the head or the tail of the cloud is in view, less infrared light is observed. By studying the infrared oscillations, the team is gathering first-of-its-kind data on the detailed process and outcome of collisions that create rocky planets like Earth.

"We are watching rocky planet formation happen right in front of us," said George Rieke, a University of Arizona co-author of the new study. "This is a unique chance to study this process in near real-time."

The team is continuing to keep an eye on the star with Spitzer. They will see how long the elevated dust levels persist, which will help them calculate how often such events happen around this and other stars. And they might see another smashup while Spitzer looks on.

The results of this study are posted online Thursday in the journal Science.

NASA's Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

For more information about Spitzer, visit: http://www.nasa.gov/spitzer
 
Whitney Clavin
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-4673
whitney.clavin@jpl.nasa.gov

Felicia Chou
NASA Headquarters, Washington
202-358-0257
felicia.chou@nasa.gov

Friday, August 29, 2014

INTEGRAL catches dead star exploding in a blaze of glory


Supernova explosion (annotated)
Astronomers studying SN2014J, a Type Ia supernova discovered in January 2014, have found proof that this type of supernova is caused by a white dwarf star reigniting and exploding.

This finding was made by using ESA’s Integral observatory to detect gamma rays from the radioactive elements created during the explosion.

This sequence shows some of the steps leading up to and following the explosion.

A white dwarf, a star that contain up to 1.4 times the mass of the Sun squeezed into a volume about the same size as the Earth, leeches matter from a companion star (image 1).  The Integral measurements suggest that a belt of gas from the companion star builds up around the equator of the white dwarf (image 2). This belt detonates (image 3) and triggers the internal explosion that becomes the supernova (image 4). Material from the explosion expands (image 5) and eventually becomes transparent to gamma rays (image 6). Copyright: ESA/ATG medialab. Hi-Res Image

 
Supernova SN2014J in nearby galaxy M82
In January 2014, a supernova was discovered in the nearby galaxy M82. At a distance of about 11.5 million light-years from Earth, SN2014J as it is known, is the closest of its type to be detected in decades. 

This composite Hubble image shows the supernova in visible light, obtained on 31 January with Hubble’s Wide Field Camera 3, superimposed on a mosaic of the entire galaxy taken in 2006 with Hubble’s Advanced Camera for Surveys. Copyright: NASA, ESA, A. Goobar (Stockholm University), and the Hubble Heritage Team (STScI/AURA). 
Hi-Res Image

Astronomers using ESA’s INTEGRAL gamma-ray observatory have demonstrated beyond doubt that dead stars known as white dwarfs can reignite and explode as supernovae. The finding came after the unique signature of gamma rays from the radioactive elements created in one of these explosions was captured for the first time.

The explosions in question are known as Type Ia supernovae, long suspected to be the result of a white dwarf star blowing up because of a disruptive interaction with a companion star. However, astronomers have lacked definitive evidence that a white dwarf was involved until now. The ‘smoking gun’ in this case was evidence for radioactive nuclei being created by fusion during the thermonuclear explosion of the white dwarf star.

“INTEGRAL has all the capabilities to detect the signature of this fusion, but we had to wait for more than ten years for a once-in-a-lifetime opportunity to catch a nearby supernova,” says Eugene Churazov, from the Space Research Institute (IKI) in Moscow, Russia and the Max Planck Institute for Astrophysics,in Garching, Germany.

Although Type Ia supernovae are expected to occur frequently across the Universe they are rare occurrences in any one galaxy, with typical rates of one every few hundred years. 

INTEGRAL’s chance came on 21 January 2014, when students at the University College London’s teaching observatory at Mill Hill, UK detected a type Ia supernova, later named SN2014J, in the nearby galaxy M82.

According to the theory of such explosions, the carbon and oxygen found in a white dwarf should be fused into radioactive nickel during the explosion. This nickel should then quickly decay into radioactive cobalt, which would itself subsequently decay, on a somewhat longer timescale, into stable iron. 

Because of its proximity – at a distance of about 11.5 million light-years from Earth, SN2014J is the closest of its type to be detected in decades – INTEGRAL stood a good chance of seeing the gamma rays produced by the decay. Within one week of the initial discovery, an observing plan to use INTEGRAL had been drawn-up and approved.  

Using INTEGRAL to study the aftermath of the supernova explosion, scientists looked for the signature of cobalt decay – and they found it, in exactly the quantities that the models predicted. 

“The consistency of the spectra, obtained by INTEGRAL 50 days after the explosion, with that expected from cobalt decay in the expanding debris of the white dwarf was excellent,” says Churazov, who is lead author of a paper describing this study and reported in the journal Nature

With that confirmation in hand, other astronomers could begin to look into the details of the process. In particular, how the white dwarf is detonated in the first place. 

White dwarfs are inert stars that contain up to 1.4 times the mass of the Sun squeezed into a volume about the same size as the Earth. Being inert, they can’t simply blow themselves up. Instead, astronomers believe that they leech matter from a companion star, which builds up on the surface until a critical total mass is reached. At that point, the pressure in the heart of the white dwarf triggers a catastrophic thermonuclear detonation.

Early INTEGRAL observations of SN2014J tell a somewhat different story, and have been the focus of a separate study, reported online in Science Express by Roland Diehl from the Max Planck Institute for Extraterrestrial Physics, Germany, and colleagues.

Diehl and his colleagues detected gamma rays from the decay of radioactive nickel just 15 days after the explosion. This was unexpected, because during the early phase of a Type Ia supernova, the explosion debris is thought to be so dense that the gamma rays from the nickel decay should be trapped inside.

“We were puzzled by this surprising signal, and some from the group even thought it must be wrong,” says Diehl. “We had long and ultimately very fruitful discussions about what might explain these data.”

A careful examination of the theory showed that the signal would have been hidden only if the explosion had begun in the heart of the white dwarf. Instead, Diehl and colleagues think that what they are seeing is evidence for a belt of gas from the companion star that must have built up around the equator of the white dwarf. This outer layer detonated, forming the observed nickel and then triggering the internal explosion that became the supernova.

“Regardless of the fine details of how these supernovae are triggered, INTEGRAL has proved beyond doubt that a white dwarf is involved in these stellar cataclysms,” says Erik Kuulkers, ESA’s INTEGRAL Project Scientist. “This clearly demonstrates that even after almost twelve years in operation, INTEGRAL is still playing a crucial role in unraveling some of the mysteries of the high-energy Universe.”  

Notes for editors
 
56Co gamma-ray emission lines from the type Ia supernova SN2014J” by E. Churazov et al., is published in the 28 August 2014 issue of Nature; DOI: 10.1038/nature13672 

“Early 56Ni decay γ rays from SN2014J suggest an unusual explosion” by R. Diehl et al., appeared online in Science Express on 31 July 2014; DOI: 10.1126/science.1254738

Some of the observations of SN2014J were obtained as part of an INTEGRAL Target of Opportunity programme led by Principal Investigator Jordi Isern (ICE-CSIC/IEEC, Spain). The INTEGRAL Project Scientist, Erik Kuulkers, made additional observing time available, on request of the INTEGRAL supernova community, to maximise the scientific return. This was supplemented by a contribution from the Russian guaranteed time on the recommendation of the Russian INTEGRAL Advisory Committee. 

Type Ia supernovae are particularly important because they are used to gauge distances across much of the visible Universe. In the 1990s, their study led to the discovery of the cosmic acceleration that is now thought to be powered by a mysterious form of energy called ‘dark energy’. The Nobel Prize for Physics in 2011 was awarded to Saul Perlmutter, Adam Riess, and Brian Schmidt for their role in the discovery of dark energy. 

The International Gamma-ray Astrophysics Laboratory (INTEGRAL) was launched on 17 October 2002. It is an ESA project with the instruments and a science data centre funded by ESA Member States (especially the Principal Investigator countries: Denmark, France, Germany, Italy, Spain, Switzerland), and with the participation of Russia and the USA. The mission is dedicated to the fine spectroscopy (E/∆E = 500) and fine imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray (4-35 keV) and optical (V-band, 550 nm) wavelengths.

For further information, please contact:
 
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Phone: +31 71 565 6799
Mobile: +31 61 594 3 954
Email:
markus.bauer@esa.int

Eugene Churazov
Space Research Institute (IKI), Moscow, Russia
Phone: +7-495-3333377
Email: churazov@hea.iki.rssi.ru
and Max Planck Institute for Astrophysics, Germany
Phone: +49-89-30000-2219
Email:
churazov@mpa-garching.mpg.de

Roland Diehl
Max Planck Institute for Extraterrestrial Physics, Germany
Phone: +49-89-30000-3850
Email:
rodmpe.mpg.de

Erik Kuulkers
INTEGRAL Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Phone: +34-91-8131-358
Email:
Erik.Kuulkers@sciops.esa.int

Source: ESA  

Light and dark

Credit: ESA/Hubble & NASA

This new NASA/ESA Hubble Space Telescope image shows a variety of intriguing cosmic phenomena.

Surrounded by bright stars, towards the upper middle of the frame we see a small young stellar object (YSO) known as SSTC2D J033038.2+303212. Located in the constellation of Perseus, this star is in the early stages of its life and is still forming into a fully grown star. In this view from Hubble’s Advanced Camera for Surveys (ACS) it appears to have a murky chimney of material emanating outwards and downwards, framed by bright bursts of gas flowing from the star itself. This fledgling star is actually surrounded by a bright disc of material swirling around it as it forms — a disc that we see edge-on from our perspective.

However, this small bright speck is dwarfed by its cosmic neighbour towards the bottom of the frame, a clump of bright, wispy gas swirling around as it appears to spew dark material out into space. The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63, most notably the emission-line star LkHA 326, and its very near neighbour LZK 18.

These stars are lighting up the surrounding gas and sculpting it into the wispy shape seen in this image. However, the most dramatic part of the image seems to be a dark stream of smoke piling outwards from [B77] 63 and its stars — a dark nebula called Dobashi 4173. Dark nebulae are incredibly dense clouds of pitch-dark material that obscure the patches of sky behind them, seemingly creating great rips and eerily empty chunks of sky. The stars speckled on top of this extreme blackness actually lie between us and Dobashi 4173.

Link


Source: ESA/Hubble - Space Telescope

Thursday, August 28, 2014

NASA Telescopes Help Uncover Early Construction Phase Of Giant Galaxy Distant Galaxy in GOODS-North

Artist's View of a Dense Galaxy Core Forming 
This illustration reveals the celestial fireworks deep inside the crowded core of a developing galaxy, as seen from a hypothetical planetary system. The sky is ablaze with the glow from nebulae, fledgling star clusters, and stars exploding as supernovae. The rapidly forming core may eventually become the heart of a mammoth galaxy similar to one of the giant elliptical galaxies seen today.  

Artist's Illustration Credit: NASA, ESA, and Z. Levay and G. Bacon (STScI). 

Science Credit: NASA, ESA, E. Nelson and P. van Dokkum (Yale University), M. Franx (Leiden Observatory), G. Brammer (STScI), I. Momcheva (Yale University), N. Schreiber (Max Planck Institute for Extraterrestrial Physics), E. da Cunha (Max Planck Institute for Astronomy), L. Tacconi (Max Planck Institute for Extraterrestrial Physics), R. Bezanson (Steward Observatory/University of Arizona), A. Kirkpatrick (University of Massachusetts), J. Leja (Yale University), H.-W. Rix (Max Planck Institute for Astronomy), R. Skelton (South African Astronomical Observatory), A. van der Wel (Max Planck Institute for Astronomy), K. Whitaker (Goddard Space Flight Center), and S. Wuyts (Max Planck Institute for Extraterrestrial Physics). Release Images

Distant Galaxy in GOODS-North (GOODS-N -77)
The core of an emerging galaxy is ablaze with newly formed stars in this never-before-seen view of the early construction phase of an elliptical galaxy. Astronomers spotted the glowing core in this Hubble Space Telescope image from the Great Observatories Origins Deep Survey (GOODS). The arrow in the enlarged inset image points to the growing galaxy's bright, compact core. The galaxy is seen as it appeared 11 billion years ago, just 3 billion years after the Big Bang. 

 Although only a fraction of the size of the Milky Way, the tiny powerhouse galaxy already contains about twice as many stars as our galaxy, all crammed into a region only 6,000 light-years across. The Milky Way is about 100,000 light-years across. Astronomers think the newly formed galaxy will continue to grow, possibly becoming similar to the giant elliptical galaxies seen today. This barely visible galaxy may be representative of a much larger population of similar objects that are obscured by dust.

The image combines observations taken in near-infrared light with the Wide Field Camera 3 and exposures made in visible light with the Advanced Camera for Surveys. Credit: NASA, ESA, and G. Illingworth (University of California, Santa Cruz), and the GOODS teamT

Astronomers have for the first time gotten a glimpse of the earliest stages of massive galaxy construction. The building site, dubbed "Sparky," is a developing galaxy containing a dense core that is blazing with the light of millions of newborn stars which are forming at a ferocious rate. The discovery was made possible through combining observations from NASA's Hubble and Spitzer space telescopes, the European Space Agency's Herschel Space Observatory, and the W.M. Keck Observatory in Hawaii.

Because the infant galaxy is so far away, it is seen as it appeared 11 billion years ago, just 3 billion years after the birth of the universe in the big bang. Astronomers think the compact galaxy will continue to grow, possibly becoming a giant elliptical galaxy, a gas-deficient assemblage of ancient stars theorized to develop from the inside out, with a compact core marking its beginnings.

"We really hadn't seen a formation process that could create things that are this dense," explained Erica Nelson of Yale University in New Haven, Connecticut, lead author of the science paper announcing the results. "We suspect that this core-formation process is a phenomenon unique to the early universe because the early universe, as a whole, was more compact. Today, the universe is so diffuse that it cannot create such objects anymore."

The research team's paper appears in the August 27 issue of the journal Nature.

Although only a fraction of the size of the Milky Way, the tiny powerhouse galaxy already contains about twice as many stars as our galaxy, all crammed into a region only 6,000 light-years across. The Milky Way is about 100,000 light-years across. This barely visible galaxy may be representative of a much larger population of similar objects that are obscured by dust.

"They're very extreme environments," Nelson said. "It's like a medieval cauldron forging stars. There's a lot of turbulence, and it's bubbling. If you were in there, the night sky would be bright with young stars, and there would be a lot of dust, gas, and remnants of exploding stars. To actually see this happening is fascinating."

Alongside determining the galaxy's size from the Hubble images, the team dug into archival far-infrared images from the Spitzer and Herschel telescopes. The analysis allowed them to see how fast the young galaxy is churning out stars. Sparky is producing roughly 300 stars per year. By comparison, the Milky Way produces roughly 10 stars per year.

Astronomers believe that this frenzied star formation occurred because the galactic center is forming deep inside a gravitational well of dark matter, an invisible form of matter that makes up the scaffolding upon which galaxies formed in the early universe. A torrent of gas is flowing into this well at the galaxy's core, sparking waves of star birth.

The sheer amount of gas and dust within an extreme star-forming region like this may explain why these compact galaxies have eluded astronomers until now. Bursts of star formation create dust, which builds up within the forming galaxy and can block some starlight. Sparky was only barely visible, and it required the infrared capabilities of Hubble's Wide Field Camera 3, Spitzer, and Herschel to reveal the developing galaxy.

The observations indicate that the galaxy had been furiously making stars for more than a billion years (at the time the light we now observe began its long journey). But the galaxy didn't keep up this frenetic pace for very long, the researchers suggested. Eventually, the galaxy probably stopped forming stars in the packed core. Smaller galaxies then might have merged with the growing galaxy, making it expand outward in size over the next 10 billion years, possibly becoming similar to one of the mammoth, sedate elliptical galaxies seen today.

"I think our discovery settles the question of whether this mode of building galaxies actually happened or not," said team member Pieter van Dokkum of Yale University. "The question now is, how often did this occur? We suspect there are other galaxies like this that are even fainter in near-infrared wavelengths. We think they'll be brighter at longer wavelengths, and so it will really be up to future infrared telescopes such as NASA's James Webb Space Telescope to find more of these objects."

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514

dweaver@stsci.edu / villard@stsci.edu

Erica Nelson / Pieter van Dokkum
Yale University
New Haven, Connecticut
203-432-0573

erica.nelson@yale.edu / pieter.vandokkum@yale.edu

Source: HubbleSite 


Evidence for Supernovas Near Earth

Once every 50 years, more or less, a massive star explodes somewhere in the Milky Way.  The resulting blast is terrifyingly powerful, pumping out more energy in a split second than the sun emits in a million years.  At its peak, a supernova can outshine the entire Milky Way. 

It seems obvious that you wouldn't want a supernova exploding near Earth.  Yet there is growing evidence that one did—actually, more than one. About 10 million years ago, a nearby cluster of supernovas went off like popcorn.  We know because the explosions blew an enormous bubble in the interstellar medium, and we're inside it.

A new ScienceCast video examines evidence that our solar system is inside a bubble of hot gas created by supernova explosions. 
Play it

A diagram of the local Galactic neighborhood including the Sun and the Local Bubble
Illustration Credit & Copyright: Linda Huff (American Scientist), Priscilla Frisch (U. Chicago

Astronomers call it "the Local Bubble." It is peanut-shaped, about 300 light years long, and filled with almost nothing. Gas inside the bubble is very thin (0.001 atoms per cubic centimeter) and very hot (roughly a million degrees)—a sharp departure from ordinary interstellar material. 

The Local Bubble was discovered gradually in the 1970s and 1980s. Optical and radio astronomers looked carefully for interstellar gas in our part of the galaxy, but couldn't find much in Earth's neighborhood. 

Meanwhile, x-ray astronomers were getting their first look at the sky using sounding rockets and orbiting satellites, which revealed a million-degree x-ray glow coming from all directions.  It all added up to Earth being inside a bubble of hot gas blown by exploding stars.    

However, not all researchers agreed.

"Within the last decade, some scientists have been challenging the [supernova] interpretation, suggesting that much or all of the soft X-ray diffuse background is instead a result of charge exchange," says F. Scott Porter of the Goddard Space Flight Center.

"Charge exchange": Basically, it happens when the electrically-charged solar wind comes into contact with a neutral gas. The solar wind can steal electrons from the neutral gas, resulting in an X-ray glow that looks a lot like the glow from an old supernova. Charge exchange has been observed many times in comets.

So, is the X-ray glow that fills the sky a sign of peaceful "charge exchange" in the solar system or evidence of terrifying explosions in the distant past?

To find out, an international team researchers including Porter and led by physics professor Massimiliano Galeazzi at the University of Miami in Coral Gables, developed an X-ray detector that could distinguish between the two possibilities.  The device was named DXL, for Diffuse X-ray emission from the Local Galaxy.

On Dec. 12, 2012, DXL launched from White Sands Missile Range in New Mexico atop a NASA Black Brant IX sounding rocket, reaching a peak altitude of 160 miles and spending five minutes above Earth's atmosphere.  That was all the time they needed to measure the amount of "charge exchange" X-rays inside the solar system.

The results, published online in the journal Nature on July 27, indicate that only about 40 percent of the soft X-ray background originates within the solar system.  The rest must come from a Local Bubble of hot gas, the relic of ancient supernovas outside the solar system.

Obviously, those supernovas were not close enough to exterminate life on Earth—but they were close enough to wrap our solar system in a bubble of hot gas that persists millions of years later.

"This is a significant discovery,' said Galeazzi.  "[It] affects our understanding of the area of the galaxy close to the sun, and can, therefore, be used as a foundation for future models of the galaxy structure." 

Galeazzi and collaborators are already planning the next flight of DXL, which will include additional instruments to better characterize the emission. The launch is currently planned for December 2015.


More information:

How did DXL distinguish between X-rays from charge exhange in the solar system vs. X-rays from hot gas in the Local Bubble?

Answer: Basically, there is a stream of interstellar helium atoms that flows through the solar system.  You can read about it here. Every year in December, Earth passes through the "helium focusing cone," a region where this neutral helium is concentrated by the gravitational influence of the sun.  The researchers figured the helium focusing cone was probably the strongest source of charge exchange x-rays in the solar system.  Using the sounding rocket, they measured the X-ray glow of the helium and found that it could not account for all of the X-rays astronomers had been seeing.  There must be a Local Bubble of hot gas to account for the difference.


 Production editor: Dr. Tony Phillips | Credit: Science@NASA
 
 
 

Wednesday, August 27, 2014

Hunting for Gravity Waves

An artist's impression of a binary star system containing a pulsar (the smaller object seen with jets of light) and one white dwarf star. The result of their mutual orbit generates gravitational waves, shown schematically as the ripples in space. Astronomers have recently detected a pair of white dwarf stars that orbit in about 20 minutes and that can be used as a calibration source for gravity wave instruments.Credit: Luis Calçada/European Southern Observatory

Einstein's general theory of relativity predicts that accelerating masses should radiate gravity waves in a roughly similar way that accelerating electrical charges radiate electromagnetic (light) waves. One notable difference is that the gravitational force is intrinsically about trillion trillion trillion times weaker than the electromagnetic force, and so gravity waves are phenomenally weak. In fact, none has ever been measured in the laboratory, although their presence has been reliably inferred from the decaying orbital energy of binary stars as they radiate these waves into space. Astronomers expect that new generations of gravity wave detectors being built for space will be able not only to test relativity in new regimes, but also measure many important astrophysical phenomenon that are otherwise mysterious, from the motion, growth, and evolution of black holes to binary star evolution, and even details of the early universe. 

The primary space gravity-wave mission planned is called eLISA ("evolved Laser Interferometer Space Antenna"). It is not scheduled for launch for another decade or more, but a proof-of-concept mission is scheduled for launch in 2015. One of the many issues facing the new gravity-wave instruments is their accurate testing and calibration. White-dwarf binary stars are expected to play this role. When a star like our Sun gets to be old, in another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of its mass remaining it will shrink to a fraction of its radius and become a white dwarf star. White dwarfs are common, either isolated or in a multi-star system of some kind, the most famous one being the companion to the brightest star in the sky, Sirius. 

Scientists estimate that there are about one hundred million detectable white dwarf stars in binary systems in our galaxy. As they orbit around each other they should emit considerable gravitational radiation. Today, however, only eight confirmed candidates are known. CfA astronomers Warren Brown and Scott Kenyon and four colleagues used the MMT and Gemini telescopes to identify a new member of this select group, a pair of white dwarf stars that orbit each other in only about 20 minutes (recall that the Earth’s orbital period is one year!), making it the second fastest known system. A third star appears to be near them and may be physically related; further observations are needed to resolve this possibility, but if it is part of the system, then the predicted gravity waves from the new triple source will make it one of the strongest prospective emitters of gravity waves.

Reference(s): 

"A New Gravitational Wave Verification Source," Mukremin Kilic, Warren R. Brown, A. Gianninas, J. J. Hermes, Carlos Allende Prieto, and S. J. Kenyon, MNRASL 444, L1, 2014.


Eta Carinae: Our Neighboring Superstars

 Eta Carinae
Credit  NASA/CXC/GSFC/K.Hamaguchi, et al.
JPEG (184.9 kb) - Large JPEG (1.7 MB) - Tiff (2.6 MB)  - More Images

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Tour of Eta Carinae

animation


The Eta Carinae star system does not lack for superlatives. Not only does it contain one of the biggest and brightest stars in our galaxy, weighing at least 90 times the mass of the Sun, it is also extremely volatile and is expected to have at least one supernova explosion in the future.

As one of the first objects observed by NASA's Chandra X-ray Observatory after its launch some 15 years ago, this double star system continues to reveal new clues about its nature through the X-rays it generates.

Astronomers reported extremely volatile behavior from Eta Carinae in the 19th century, when it became very bright for two decades, outshining nearly every star in the entire sky. This event became known as the "Great Eruption." Data from modern telescopes reveal that Eta Carinae threw off about ten times the Sun's mass during that time. Surprisingly, the star survived this tumultuous expulsion of material, adding "extremely hardy" to its list of attributes.

Today, astronomers are trying to learn more about the two stars in the Eta Carinae system and how they interact with each other. The heavier of the two stars is quickly losing mass through wind streaming away from its surface at over a million miles per hour. While not the giant purge of the Great Eruption, this star is still losing mass at a very high rate that will add up to the Sun's mass in about a millennium.

X-ray, Optical & Infrared Images of Eta Carinae
This multi-panel image shows Eta Carinae in the same field of view using three different telescopes. From left to right, the images are from the Chandra X-ray Observatory, the Hubble Space Telescope in optical light, with the ground-based 2MASS survey in infrared. The X-ray image reveals an outer horseshoe-shaped ring, a hot inner core, and a hot central source. These structures glow in X-rays because they have been heated multi-million degrees by shock waves. The optical image shows two giant bubbles expanding away from the center of the system at over a million miles per hour. The infrared data reveal that Eta Carinae is one of the most luminous systems in the Milky Way. Eta Carinae is shrouded in a rapidly expanding cloud of dust that absorbs radiation from the central star and re-radiates in in the infrared. Credit: Optical: NASA/STScI, Near-Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF

Though smaller than its partner, the companion star in Eta Carinae is also massive, weighing in at about 30 times the mass of the Sun. It is losing matter at a rate that is about a hundred times lower than its partner, but still a prodigious weight loss compared to most other stars. The companion star beats the bigger star in wind speed, with its wind clocking in almost ten times faster.

When these two speedy and powerful winds collide, they form a bow shock - similar to the sonic boom from a supersonic airplane - that then heats the gas between the stars. The temperature of the gas reaches about ten million degrees, producing X-rays that Chandra detects.

The Chandra image of Eta Carinae shows low energy X-rays in red, medium energy X-rays in green, and high energy X-rays in blue. Most of the emission comes from low and high energy X-rays. The blue point source is generated by the colliding winds, and the diffuse blue emission is produced when the material that was purged during the Great Eruption reflects these X-rays. The low energy X-rays further out show where the winds from the two stars, or perhaps material from the Great Eruption, are striking surrounding material. This surrounding material might consist of gas that was ejected before the Great Eruption.

An interesting feature of the Eta Carinae system is that the two stars travel around each other along highly elliptical paths during their five-and-a-half-year long orbit. Depending on where each star is on its oval-shaped trajectory, the distance between the two stars changes by a factor of twenty. These oval-shaped trajectories give astronomers a chance to study what happens to the winds from these stars when they collide at different distances from one another.

Throughout most of the system's orbit, the X-rays are stronger at the apex, the region where the winds collide head-on. However, when the two stars are at their closest during their orbit (a point that astronomers call "periastron"), the X-ray emission dips unexpectedly.

To understand the cause of this dip, astronomers observed Eta Carinae with Chandra at periastron in early 2009. The results provided the first detailed picture of X-ray emission from the colliding winds in Eta Carinae. The study suggests that part of the reason for the dip at periastron is that X-rays from the apex are blocked by the dense wind from the more massive star in Eta Carinae, or perhaps by the surface of the star itself.

Another factor responsible for the X-ray dip is that the shock wave appears to be disrupted near periastron, possibly because of faster cooling of the gas due to increased density, and/or a decrease in the strength of the companion star's wind because of extra ultraviolet radiation from the massive star reaching it. Researchers are hoping that Chandra observations of the latest periastron in August 2014 will help them determine the true explanation.

These results were published in the April 1, 2014 issue of The Astrophysical Journal and are available online. The first author of the paper is Kenji Hamaguchi of Goddard Space Flight Center in Greenbelt, MD, and his co-authors are Michael Corcoran of Goddard Space Flight Center (GSFC); Christopher Russell of University of Delaware in Newark; A. Pollock from the European Space Agency in Madrid, Spain; Theodore Gull, Mairan Teodoro, and Thomas I. Madura from GSFC; Augusto Damineli from Universidade de Sao Paulo in Sao Paulo, Brazil and Julian Pittard from the University of Leeds in the UK.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington, DC. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.


Fast Facts for Eta Carinae:

Release Date: August 26, 2014
Scale: Image is about 2 arcmin across (about 4.6 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 10h 45m 04s | Dec -59° 41' 03"
Constellation: Carina
Observation Date: 8 pointings between Sep 1999 and Feb 2009
Observation Time: 30 hours (1 day 6 hours).
Obs. ID: 50, 1249, 4455, 9933-9937
Instrument: ACIS
References: Hamaguchi, K. et al, 2014, ApJ 784, 125; arXiv:1401.5870
Color Code: X-ray (Red, Green, Blue)
Distance Estimate: About 7,500 light years


Tuesday, August 26, 2014

Best View Yet of Merging Galaxies in Distant Universe

PR Image eso1426a
Merging galaxies in the distant Universe through a gravitational magnifying glass
 
How gravitational lensing acts like a magnifying glass
Wide-field view of the sky around the gravitationally lensed galaxy merger H-ATLAS J142935.3-002836
 
Merging galaxies in the distant Universe through a gravitational magnifying glass

 

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Videos

Zooming in on a gravitationally lensed galaxy merger in the distant Universe
Zooming in on a gravitationally lensed galaxy merger in the distant Universe

Artist's impression of gravitational lensing of a distant merger
Artist's impression of gravitational lensing of a distant merger


ALMA applies methods of Sherlock Holmes

Using the Atacama Large Millimeter/submillimeter Array (ALMA), and many other telescopes on the ground and in space, an international team of astronomers has obtained the best view yet of a collision that took place between two galaxies when the Universe was only half its current age. They enlisted the help of a galaxy-sized magnifying glass to reveal otherwise invisible detail. These new studies of the galaxy H-ATLAS J142935.3-002836 have shown that this complex and distant object looks like the well-known local galaxy collision, the Antennae Galaxies.

The famous fictional detective Sherlock Holmes used a magnifying lens to reveal barely visible but important evidence. Astronomers are now combining the power of many telescopes on Earth and in space [1] with a vastly larger form of cosmic lens to study a case of vigorous star formation in the early Universe.

While astronomers are often limited by the power of their telescopes, in some cases our ability to see detail is hugely boosted by natural lenses, created by the Universe,” explains lead author Hugo Messias of the Universidad de Concepción (Chile) and the Centro de Astronomia e Astrofísica da Universidade de Lisboa (Portugal). “Einstein predicted in his theory of general relativity that, given enough mass, light does not travel in a straight line but will be bent in a similar way to light refracted by a normal lens.

These cosmic lenses are created by massive structures like galaxies and galaxy clusters, which deflect the light from objects behind them due to their strong gravity — an effect, called gravitational lensing. The magnifying properties of this effect allow astronomers to study objects which would not be visible otherwise and to directly compare local galaxies with much more remote ones, seen when the Universe was significantly younger.

But for these gravitational lenses to work, the lensing galaxy, and the one far behind it, need to be very precisely aligned.

H-ATLAS J142935.3-002836 (or just H1429-0028 for short) is one of these sources and was found in the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). Although very faint in visible light pictures, it is among the brightest gravitationally lensed objects in the far-infrared regime found so far, even though we are seeing it at a time when the Universe was just half its current age.

Probing this object was at the limit of what is possible, so the international team of astronomers started an extensive follow-up campaign using the most powerful telescopes — both on the ground as well as in space — including the NASA/ESA Hubble Space Telescope, ALMA, the Keck Observatory, the Karl Jansky Very Large Array (JVLA), and others. The different telescopes provided different views, which could be combined to get the best insight yet into the nature of this unusual object.

The Hubble and Keck images revealed a detailed gravitationally-induced ring of light around the foreground galaxy. These high resolution images also showed that the lensing galaxy is an edge-on disc galaxy — similar to our galaxy, the Milky Way — which obscures parts of the background light due to the large dust clouds it contains.

But this obscuration is not a problem for ALMA and the JVLA, since these two facilities observe the sky at longer wavelengths, which are unaffected by dust. Using the combined data the team discovered that the background system was actually an ongoing collision between two galaxies. From this point on, ALMA and the JVLA started to play a key role in further characterising this object.

In particular, ALMA traced carbon monoxide, which allows detailed studies of star formation mechanisms in galaxies. The ALMA observations also allowed the motion of the material in the more distant object to be measured. This was essential to show that the lensed object is indeed an ongoing galactic collision forming hundreds of new stars each year, and that one of the colliding galaxies still shows signs of rotation; an indication that it was a disc galaxy just before this encounter.

The system of these two colliding galaxies resembles an object that is much closer to us: the Antennae Galaxies. This is a spectacular collision between two galaxies, which are believed to have had a disc structure in the past. While the Antennae system is forming stars at a rate of only a few tens of the mass of our Sun each year, H1429-0028 turns more than 400 times the mass of the Sun of gas into new stars each year.

Rob Ivison, ESO’s Director of Science and a co-author of the new study, concludes: “ALMA enabled us to solve this conundrum because it gives us information about the velocity of the gas in the galaxies, which makes it possible to disentangle the various components, revealing the classic signature of a galaxy merger. This beautiful study catches a galaxy merger red handed as it triggers an extreme starburst.”

Notes

[1] Among the armada of instruments that were used to provide evidence to help unravel the mysteries of this case were no fewer than three ESO telescopes — ALMA, APEX and VISTA. The other telescopes and surveys that were brought to bear were: the NASA/ESA Hubble Space Telescope, the Gemini South telescope, the Keck-II telescope, the NASA Spitzer Space Telescope, the Jansky Very Large Array, CARMA, IRAM and SDSS and WISE.

More information 

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

This research was presented in a paper entitled “Herschel-ATLAS and ALMA HATLAS J142935.3-002836, a lensed major merger at redshift 1.027”, by Hugo Messias et al., to appear online on 26 August 2014 in the journal Astronomy & Astrophysics.

The team is composed of Hugo Messias (Universidad de Concepción, Barrio Universitario, Chile; Centro de Astronomia e Astrofísica da Universidade de Lisboa, Portugal), Simon Dye (School of Physics and Astronomy, University of Nottingham, UK), Neil Nagar (Universidad de Concepción, Barrio Universitario, Chile), Gustavo Orellana (Universidad de Concepción, Barrio Universitario, Chile), R. Shane Bussmann (Harvard-Smithsonian Center for Astrophysics, USA), Jae Calanog (Department of Physics & Astronomy, University of California, USA), Helmut Dannerbauer (Universität Wien, Institut für Astrophysik, Austria), Hai Fu (Astronomy Department, California Institute of Technology, USA), Edo Ibar (Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Chile), Andrew Inohara (Department of Physics & Astronomy, University of California, USA), R. J. Ivison (Institute for Astronomy, University of Edinburgh, Royal Observatory, UK; ESO, Garching, Germany), Mattia Negrello (INAF, Osservatorio Astronomico di Padova, Italy), Dominik A. Riechers (Astronomy Department, California Institute of Technology, USA; Department of Astronomy, Cornell University, USA), Yun-Kyeong Sheen (Universidad de Concepción, Barrio Universitario, Chile), Simon Amber (The Open University, Milton Keynes, UK), Mark Birkinshaw (H. H. Wills Physics Laboratory, University of Bristol, UK; Harvard-Smithsonian Center for Astrophysics, USA), Nathan Bourne (School of Physics and Astronomy, University of Nottingham, UK), Dave L. Clements (Astrophysics Group, Imperial College London, UK), Asantha Cooray (Department of Physics & Astronomy, University of California, USA; Astronomy Department, California Institute of Technology, USA), Gianfranco De Zotti (INAF, Osservatorio Astronomico di Padova, Italy), Ricardo Demarco (Universidad de Concepción, Barrio Universitario, Chile), Loretta Dunne (Department of Physics and Astronomy, University of Canterbury, New Zealand; Institute for Astronomy, University of Edinburgh, Royal Observatory, UK), Stephen Eales (School of Physics and Astronomy, Cardiff University,UK), Simone Fleuren (School of Mathematical Sciences, University of London, UK), Roxana E. Lupu (Department of Physics and Astronomy, University of Pennsylvania, USA), Steve J. Maddox (Department of Physics and Astronomy, University of Canterbury, New Zealand; Institute for Astronomy, University of Edinburgh, Royal Observatory, UK), Michał J. Michałowski (Institute for Astronomy, University of Edinburgh, Royal Observatory, UK), Alain Omont (Institut d’Astrophysique de Paris, UPMC Univ. Paris, France), Kate Rowlands (School of Physics & Astronomy, University of St Andrews, UK), Dan Smith (Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, UK), Matt Smith (School of Physics and Astronomy, Cardiff University,UK) and Elisabetta Valiante (School of Physics and Astronomy, Cardiff University, UK).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links


Contacts


Hugo Messias
Universidad de Concepción, Chile / Centro de Astronomia e Astrofísica da Universidade de Lisboa, Portugal
Tel: +351 21 361 67 47/30
Email: hmessias@oal.ul.pt

Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Source: ESO

ALMA Confirms Comets Forge Organic Molecules in Their Dusty Atmospheres

Approximate location of Comet ISON in our Solar System at the time of the ALMA observations. 
Credit: B. Saxton (NRAO/AUI/NSF); NASA/ESA Hubble; M. Cordiner, NASA, et al.

Approximate location of Comet Lemmon in our Solar System at the time of the ALMA observations. 
Credit: B. Saxton (NRAO/AUI/NSF); Gerald Rhemann; M. Cordiner, NASA, et al.

The emission from organic molecules in the atmosphere of comet ISON as observed with ALMA. 
Credit: B. Saxton (NRAO/AUI/NSF); M. Cordiner, NASA, et al.

The emission from organic molecules in the atmosphere of comet Lemmon as observed with ALMA. 
Credit: B. Saxton (NRAO/AUI/NSF); M. Cordiner, NASA, et al.


This rotating 3-D ALMA map shows how HCN molecules are released from the nucleus of comet Lemmon and then spread evenly throughout the atmosphere, or coma. Similar maps revealed that HNC and formaldehyde are produced in the coma, rather than originating from the comet's nucleus. Credit: Visualization by Brian Kent (NRAO/AUI/NSF)

An international team of scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) has made incredible 3D images of the ghostly atmospheres surrounding comets ISON and Lemmon. These new observations provided important insights into how and where comets forge new chemicals, including intriguing organic compounds.

Comets contain some of the oldest and most pristine materials in our Solar System. Understanding their unique chemistry could reveal much about the birth of our planet and the origin of organic compounds that are the building blocks of life. ALMA's high-resolution observations provided a tantalizing 3D perspective of the distribution of the molecules within these two cometary atmospheres, or comas.

“We achieved truly first-of-a-kind mapping of important molecules that help us understand the nature of comets,” said team leader Martin Cordiner, a Catholic University of America astrochemist working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The critical 3D component of the ALMA observations was made by combining high-resolution, two-dimensional images of the comets with high-resolution spectra obtained from three important organic molecules – hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and formaldehyde (H2CO). These spectra were taken at every point in each image. They identified not only the molecules present but also their velocities, which provided the third dimension, indicating the depths of the cometary atmospheres.

The new results revealed that HCN gas flows outward from the nucleus quite evenly in all directions, whereas HNC is concentrated in clumps and jets. ALMA’s exquisite resolution could clearly resolve these clumps moving into different regions of the cometary comas on a day-to-day and even hour-to-hour basis. These distinctive patterns confirm that the HNC and H2CO molecules actually form within the coma and provide new evidence that HNC may be produced by the breakdown of large molecules or organic dust.

"Understanding organic dust is important, because such materials are more resistant to destruction during atmospheric entry, and some could have been delivered intact to the early Earth, thereby fueling the emergence of life,” said Michael Mumma, director of the Goddard Center for Astrobiology and a co-author on the study. "These observations open a new window on this poorly known component of cometary organics."

“So, not only does ALMA let us identify individual molecules in the coma, it also gives us the ability to map their locations with great sensitivity,” said Anthony Remijan, an astronomer with the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, and a study co-author.

The observations, published today in the Astrophysical Journal Letters, were also significant because modest comets like Lemmon and ISON contain relatively low concentrations of these crucial molecules, making them difficult to probe in depth with Earth-based telescopes. The few comprehensive studies of this kind so far have been conducted on extremely bright comets, such as Hale-Bopp. The present results extend them to comets of only moderate brightness.

Comet ISON (formally known as C/2012 S1) was observed with ALMA on November 15-17, 2013, when it was only 75 million kilometers from the Sun (about half the distance of the Earth to the Sun). Comet Lemmon (formally known as C/2012 F6) was observed on June 1-2, 2013, when it was 224 million kilometers from the Sun (about 1.5 times the distance of the Earth to the Sun).

"The high sensitivity achieved in these studies paves the way for observations of perhaps hundreds of the dimmer or more distant comets,” said Goddard’s Stefanie Milam, a study co-author. “The findings suggest that it should also be possible to map more complex molecules that have so far eluded detection in comets.”

This research was funded by the NASA Astrobiology Institute through the Goddard Center for Astrobiology and by NASA’s Planetary Atmospheres and Planetary Astronomy programs.

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

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

Contacts:

Charles E. Blue, Public Information Officer
+1 (434) 296-0314; email: cblue@nrao.edu

Nancy Neal-Jones/Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.
301-286-0039/301-614-5438


Monday, August 25, 2014

A Chemical Signature of First-Generation Very-Massive Stars

A team of astronomers from the National Astronomical Observatory of Japan (NAOJ), the Konan University and the University of Hyogo in Japan, the University of Notre Dame, and New Mexico State University (Note 1) has used the 8.2 m Subaru Telescope's High Dispersion Spectrograph (HDS) to discover a low-mass star, SDSS J0018-0939 (Fig. 1), that exhibits the peculiar chemical abundance ratios associated with the process of creating new atomic nuclei (nucleosynthesis) in a first-generation very-massive star. Until now, no observational evidence has supported numerical simulations of the existence of very-massive stars among the first generation of stars formed after the Big Bang.

Figure 1: An optical image of the star SDSS J0018-0939, obtained by the Sloan Digital Sky Survey. This is a low-mass star with a mass about half that of the Sun; the distance to this star is about 1000 light years; its location in the sky is close to the constellation Cetus. (Credit: SDSS/NAOJ)

The Importance of the Mass of First-Generation Stars


First-generation stars are objects formed in the early Universe (within a few hundred million years after the Big Bang) from gas clouds containing only hydrogen and helium (Fig. 4). First-generation stars are the probable precursors of the formation of the Universe's structure and chemical enrichment; large stellar systems, e.g., galaxies, formed later.

Numerical simulations have made significant progress in understanding the formation of the first stars (Note 2). Recent simulations suggest that a small fraction of very-massive stars with masses exceeding one hundred times that of the Sun could have formed in the early Universe, even though the large majority of first stars formed with masses of ten to a hundred times that of the Sun. Their strong UV radiation and energetic explosions are likely to have had a significant impact on the evolution of stellar systems.

Signatures of First Stars Recorded by Low-Mass Milky-Way Stars


Supernova explosions ejected elements formed by the first massive stars and dispersed them into the gas that formed the next generations of stars (Fig. 5).

Stars with masses slightly less than the Sun's have very long lifetimes, long enough that they are still shining.

The Milky Way contains such low-mass stars with low overall metal content, including the elements produced by the first massive stars. The distinctive chemical abundance patterns of these stars can be used to estimate the masses of the first stars.

Over the past thirty years astronomers have conducted large-scale investigations to find low-mass, metal-poor stars formed in the early Universe. Follow-up spectroscopic studies, which measured their chemical abundances, have identified stars that recorded the abundance patterns associated with the first stars that had several tens of solar masses and produced large amounts of carbon and other light elements (Note 3). However, no previous research of low-mass metal-poor Milky-Way stars has found the signature of supernova explosions of very-massive stars with more than 100 solar masses, which synthesize large amounts of iron but little carbon (Note 4). 

Figure 2: The chemical abundance ratios (with respect to iron) of SDSS J0018-0939 (red circles) compared with model predictions for a supernova explosion of a massive star. The model well-explains the chemical abundance ratios of a comparison star (a similar low-mass star, G39-36; blue triangles), whereas the lighter elements, such as carbon and magnesium, as well as the heavier element cobalt, of SDSS J0018-0939 are not well-reproduced. (Credit: NAOJ)

Discovery of a Low-Mass Star with Unique Chemical Abundance Ratios


The current team of researchers used the High Dispersion Spectrograph mounted on the Subaru Telescope to conduct a high-resolution spectroscopic follow-up of a large sample of low-mass metal-poor stars (Note 5, Fig. 1) and discovered a star, SDSS001820.5-093939.2 (SDSS J0018-0939), that exhibits a very peculiar set of chemical abundance ratios. Whereas the star contains an amount of iron 300 times lower than the Sun's, it is significantly deficient in lighter elements such as carbon and magnesium. The extremely low abundances of elements other than iron indicates that this star formed directly from a hydrogen gas cloud that contained elements dispersed by a first-generation massive star.

Nucleosynthesis models for supernova explosions of massive stars, which successfully reproduce the abundance ratios found in most of the early-generation stars previously known (Fig. 2) do not readily explain the chemical abundance ratios observed in the newly discovered star.

Figure 3: The chemical abundance ratios (with respect to iron) of SDSS J0018-0939 (red circles) compared with model prediction for explosions of very-massive stars. The black line indicates the model of a pair-instability supernova by a star with 300 solar masses, whereas the blue line shows the model of an explosion caused by a core-collapse of a star with 1000 solar masses. The abundance ratios of sodium (Na) and aluminum (Al), which are not well-reproduced by these models, might be produced during the evolution of stars before the explosion, but that is not included in the current model. (Credit: NAOJ)

Rather, explosion models of very-massive stars can explain both the relatively high abundance ratio of iron as well as the low abundances of lighter elements (Fig. 3). This means that this star most likely preserves the elemental abundance ratios produced by a first-generation very-massive star.

Impact of this Study


The discovery of a star that could have recorded the chemical yields of a first- generation very-massive star will stimulate further modeling of the evolution of very-massive stars and the nucleosynthesis processes that occurred during their explosions. If more detailed modeling of the elemental abundance patterns in this star confirms the existence of very-massive stars, this new discovery will help to focus our understanding of the formation of the first stars and the birth of the elements.

The strong UV radiation, energetic explosions, and production of heavy elements from very-massive stars influence subsequent star as well as galaxy formation. If stars with masses up to 1000 solar masses existed, their remnants are probably black holes with several hundred solar masses, which may have formed the "seeds" of super-massive black holes, such as found in the Galactic Center.

Further research to find early generations of low-mass metal-poor stars is necessary to estimate the proportion of very-massive stars among the first stars. If very-massive stars are relatively common, next-generation large telescopes such as the Thirty Meter Telescope (TMT) and the James Webb Space Telescope (JWST) will have the potential to directly detect groups of such first stars in studies of the most distant galaxies (Note 6).

Reference:


The research paper on which this release was based, "A chemical signature of first-generation very-massive stars" by W. Aoki, N. Tominaga, T. C. Beers, S. Honda, Y. S. Lee, is published in Science on August 22, 2014.

Acknowledgements:


This study was supported by:
  • The JSPS Grants-in-Aid for Scientific Research (23224004)
  • Grant PHY 08-22648: Physics Frontiers Center/Joint Institute for Nuclear Astrophysics (JINA), awarded by the U.S. National Science Foundation.

Notes:

  1. Team members are:
    • Wako Aoki (National Astronomical Observatory of Japan)
    • Nozomu Tominaga (Konan University & Kavli IPMU [WPI], University of Tokyo)
    • Timothy C. Beers (University of Notre Dame)
    • Satoshi Honda (University of Hyogo)
    • Young Sun Lee (New Mexico State University)
  2. Numerical simulations usually start with considering initial non-uniform distributions (i.e., inhomogeneities) in the dark matter density of small regions formed after the Big Bang. The larger gravitational forces associated with a high-density region gathers normal (i.e., baryonic) matter, such as primordial gas, and the density of the region increases. First stars probably formed in such regions.
  3. A massive star ends its life by collapse of its central core and a supernova explosion, leaving behind a black hole or a neutron star. The explosion ejects a certain amount of heavy elements, from carbon to iron. Previous observations of early-generation low-mass stars have identified stars exhibiting elements produced by such supernovae.
  4. Discovery of the Most Metal-deficient Star Ever Found: Studying Nucleosynthesis Signatures of the First Stars
    A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36−670839.3
  5. The central temperature of evolved very-massive stars becomes so high that electron-positron pair-creation occurs, resulting in the collapse of the central core, and a nuclear reaction runaway that explodes the star. If the star is more massive than about 300 solar masses, the explosive nuclear reaction is insufficient to prevent the core from collapsing, resulting in the direct formation of a black hole, but some fraction of the material might have still been ejected.
  6. The team conducted chemical abundance measurements based on data obtained with the Subaru Telescope High Dispersion Spectrograph (HDS) for early generations of stars found by the SDSS. SDSS J001820.5-093939.2 (SDSS J0018-0939) is a star showing peculiar abundance ratios among 150 stars studied with the Subaru Telescope. The team used the same instrument to make a more detailed measurement.
  7. Observations of more distant objects allow scientists to study earlier eras of the Universe. Although the next generations of large telescopes may not even be able to distinguish individual first stars, they should be able to observe groups of such stars, if very-massive stars indeed existed.

Figure 4: Artist's rendition of massive, luminous first-generation stars in the Universe which would form a cluster. The most massive ones, which could be over a 100 times more massive than the Sun, exploded and ejected material that included heavy elements, particularly iron. (Credit: NAOJ)

Figure 5: Artist's rendition of new generations of stars. The material that included heavy elements from the first-generation, very-massive stars mixed with hydrogen around the star. New generations of stars formed from the gas clouds that included small amount of heavy elements. SDSS J0018-0939, a low-mass star with a long lifetime, formed as one of these second-generation stars, recording the products of a first-generation very-massive star. (Credit: NAOJ)