18 Temmuz 2009 Cumartesi
The discovery of sharp resonance states in exotic, proton emitting, nuclei.
The discovery of sharp resonance states in exotic, proton emitting, nuclei.Little is known about the structure of unstable, proton emitting, nuclei.These nuclei can be produced during stellar explosions in the cosmos, andlive only for a very small fraction of second before they disintegrateinto more stable products. Nowadays, they are also produced and detectedin experiments made in large modern nuclear physics laboratories that areequipped with radioactive ion beams. Such elusive nuclei rapidly decay byemitting one (and sometimes two) protons, and for this reason areindicated as nuclei that lie outside of the so-called proton drip line.A recent publication (1) reports results of an experiment at the GSIlaboratory in Darmstdtat, Germany that showed the existence of excitedunstable states in two such nuclei, Fluorine-15 and Neon-16. Contrary tothe ground level characterized by a broad resonance with short life, theseexcited states have half-lives sufficiently long that they can beidentified as sharp resonances. Of note is that they have excitationenergy of several MeV, establishing that such particle unstable systemscan have an observable set of levels just as do the many known, particlestable nuclei.The existence of such narrow resonances in particle unstable nuclei, andin Fluorine-15 in particular, was predicted (2) three years ago by amethod of calculation put forward by nuclear theoreticians of the INFN,sez. di Padova, in collaboration with colleagues from Australia, Canadaand South Africa. To implement the method (an algebraic solution ofsystems of coupled equations for the problem of nuclear scattering andreactions) expertise in high performance computing was employed. Theexistence of sharp resonances in the spectra of radioactive, andspecifically of proton (and, may-be, of neutron) emitting nuclei, opensnew and interesting perspectives on the way the nuclei, that we observe atpresent in our Universe, have been formed.(1) Physical Review C (Rapid Communication) 79, 061301 (2009).(2) Physical Review Letters, 96, 072502 (2006).
17 Temmuz 2009 Cuma
Supernova may be in a new class
Supernova may be in a new class
Oddball stellar explosion doesn’t match known outbursts
By Ron Cowen
July 18th, 2009; Vol.176 #2 (p. 9)
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Just in time for July 4, astronomers say they have found a new type of stellar firecracker.
Stars that die an explosive death generally fall into two categories: young, massive stars that collapse under their own weight and hurl their outer layers into space, and older, sunlike stars that undergo a thermonuclear explosion. But the stellar explosion recorded in January 2005 and known as SN 2005E doesn’t fit either class, according to a new analysis reported online June 11 at arXiv.org.
The explosion ejected only a small amount of material — the equivalent of 0.3 solar masses — and erupted in the halo of an isolated galaxy, a region devoid of any star formation. These findings suggest that the explosion, or supernova, did not arise from the collapse of a massive star, report study coauthors Hagai-Binyamin Perets and Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, and their colleagues. A massive star would have cast off much more material and would have erupted in a star-forming region. Since stellar heavyweights are so short-lived, they can’t move far from their birth site.
On the other hand, the researchers note, the explosion’s dimness and the abundance of elements forged in the eruption indicate it was not a typical thermonuclear explosion. Spectra show that the debris from the outburst contains five to 10 times more calcium than observed in any other known stellar explosion and probably contains a high abundance of radioactive titanium-44.
“In my experience, there’s lots of strange supernovas out there … but it really does look like this one might be something different,” comments theorist Andrew MacFadyen of New York University.
The authors of the paper declined to be interviewed because they had submitted the report to Nature. In their article, they report that the erupting oddball matches a model in which a compact star called a white dwarf nabs a thick layer of helium from a companion star. The star would then undergo a thermonuclear explosion that would destroy the helium but leave the rest of the white dwarf intact. By contrast, in a common type of supernova known as a type 1a supernova, a white dwarf made up mostly of carbon and oxygen blows itself to smithereens after stealing matter from a companion.
Perets, Gal-Yam and their collaborators report that SN 2005E resembles a few other peculiar supernova, notably an explosion found last year and known as SN 2008ha.
“Both of these objects have very low luminosity, low velocity [of debris] and strong calcium lines,” says Rober Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Kirshner, along with some of the collaborators on the SN 2005e study, is a coauthor of a study on SN 2008ha set to appear in an upcoming issue of The Astronomical Journal.
The conclusions of both papers suggest a weak thermonuclear explosion, although the study of SN 2005E is more far-reaching, Kirshner says. “My guess is that the same interpretation would probably work for both,” he says.
Because both SN 2005E and SN 2008ha are so faint, telescopes may have failed to detect other similar explosions, comments MacFadyen. Supernovas are known to seed galaxies with an assortment of heavy elements. If the number of explosions in the new class is large enough, they may be an important contributor to this process. It’s a well-known story how supernovas produce these elements, “but there’s always room for adding new players to the team,” says MacFadyen
Oddball stellar explosion doesn’t match known outbursts
By Ron Cowen
July 18th, 2009; Vol.176 #2 (p. 9)
Text Size
Just in time for July 4, astronomers say they have found a new type of stellar firecracker.
Stars that die an explosive death generally fall into two categories: young, massive stars that collapse under their own weight and hurl their outer layers into space, and older, sunlike stars that undergo a thermonuclear explosion. But the stellar explosion recorded in January 2005 and known as SN 2005E doesn’t fit either class, according to a new analysis reported online June 11 at arXiv.org.
The explosion ejected only a small amount of material — the equivalent of 0.3 solar masses — and erupted in the halo of an isolated galaxy, a region devoid of any star formation. These findings suggest that the explosion, or supernova, did not arise from the collapse of a massive star, report study coauthors Hagai-Binyamin Perets and Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, and their colleagues. A massive star would have cast off much more material and would have erupted in a star-forming region. Since stellar heavyweights are so short-lived, they can’t move far from their birth site.
On the other hand, the researchers note, the explosion’s dimness and the abundance of elements forged in the eruption indicate it was not a typical thermonuclear explosion. Spectra show that the debris from the outburst contains five to 10 times more calcium than observed in any other known stellar explosion and probably contains a high abundance of radioactive titanium-44.
“In my experience, there’s lots of strange supernovas out there … but it really does look like this one might be something different,” comments theorist Andrew MacFadyen of New York University.
The authors of the paper declined to be interviewed because they had submitted the report to Nature. In their article, they report that the erupting oddball matches a model in which a compact star called a white dwarf nabs a thick layer of helium from a companion star. The star would then undergo a thermonuclear explosion that would destroy the helium but leave the rest of the white dwarf intact. By contrast, in a common type of supernova known as a type 1a supernova, a white dwarf made up mostly of carbon and oxygen blows itself to smithereens after stealing matter from a companion.
Perets, Gal-Yam and their collaborators report that SN 2005E resembles a few other peculiar supernova, notably an explosion found last year and known as SN 2008ha.
“Both of these objects have very low luminosity, low velocity [of debris] and strong calcium lines,” says Rober Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Kirshner, along with some of the collaborators on the SN 2005e study, is a coauthor of a study on SN 2008ha set to appear in an upcoming issue of The Astronomical Journal.
The conclusions of both papers suggest a weak thermonuclear explosion, although the study of SN 2005E is more far-reaching, Kirshner says. “My guess is that the same interpretation would probably work for both,” he says.
Because both SN 2005E and SN 2008ha are so faint, telescopes may have failed to detect other similar explosions, comments MacFadyen. Supernovas are known to seed galaxies with an assortment of heavy elements. If the number of explosions in the new class is large enough, they may be an important contributor to this process. It’s a well-known story how supernovas produce these elements, “but there’s always room for adding new players to the team,” says MacFadyen
Pairing off in the early universe
Pairing off in the early universe
Simulations reveal that some of the first stars had partners
By Ron Cowen
August 1st, 2009; Vol.176 #3 (p. 7)
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EnlargeStellar companionA simulation of a star-forming region about 200 million years after the Big Bang shows two embryonic stars (yellow), each about five times the mass of the sun and separated by 800 times the Earth-sun distance. The computer model indicates that the two embryos will draw closer and form a binary star system, with each member as massive as 100 suns.Copyright Science/AAAS
It’s usually nice to have a companion. And in the lonely, dark expanse of the early universe, even some of the first stars had soul mates, new simulations reveal.
Previous studies had indicated that the first stars were extraordinarily massive — at least 100 times as heavy as the sun — but were also loners (SN: 6/8/2002, p. 362). Now, more detailed modeling, including a careful consideration of how atomic and molecular hydrogen interact at low densities, reveals that at least 5 percent and perhaps as many as half of these heavyweights were gravitationally bound to similar-mass companions, says Tom Abel of Stanford University He and his colleagues, Matthew Turk of Stanford and Brian O’Shea of Michigan State University in East Lansing, report their findings online July 9 in Science.
Pairs of massive stars are intriguing, notes Abel, because each star will probably collapse into a black hole. The coalescence of the two black holes would be a key source of gravitational waves, ripples in space-time predicted by Einstein’s theory of general relativity but never directly detected.
A second star’s presence could also enhance the production of distant gamma-ray bursts, flashes of high-energy light that have long-lasting afterglows and provide a window on the early universe. Gamma-ray bursts are produced when a single, massive star that crunches down into a black hole generates powerful jets of particles. A companion star can spin up its partner, and such rapid rotation may help generate the energetic jets, Abel says.
Star formation in the early universe is relatively easy to model because the infant cosmos contained only a few elements — mainly hydrogen and helium gas — which cooled and collapsed to produce stars. But even the simple interactions between atomic and molecular hydrogen hadn‘t been previously studied at low-enough densities, Abel says. He and his colleagues followed the star-forming process about 200 million years after the Big Bang, as gravity condensed gas clouds, to much higher densities than his team could in past simulations, Abel adds.
Only one in five of the team’s simulations produced stellar pairs, and for now the researchers can only provide a rough estimate of the percentage of first stars that would have had partners. He expects that within a year, larger, more comprehensive simulations will pin down the number.
The first generation of stars is not visible with today’s telescopes, but the proposed successor to Hubble, the infrared James Webb Space Telescope, scheduled for launch in 2014, has a good chance of recording large groupings of these earliest of glimmers.
“The simulations make good sense,” says theorist Volker Bromm of the University of Texas at Austin. Bromm says that his own team’s simulations track the evolution of pairs or groups of embryonic stars for an additional 100,000 years beyond that of Turk’s group — a necessity, he says, to determine if the baby stars merge into one big star or remain separate. His team indeed finds that the fledgling stars remain close partners. Bromm says his team plans to post a paper online describing the results later this month.
Simulations reveal that some of the first stars had partners
By Ron Cowen
August 1st, 2009; Vol.176 #3 (p. 7)
Text Size
EnlargeStellar companionA simulation of a star-forming region about 200 million years after the Big Bang shows two embryonic stars (yellow), each about five times the mass of the sun and separated by 800 times the Earth-sun distance. The computer model indicates that the two embryos will draw closer and form a binary star system, with each member as massive as 100 suns.Copyright Science/AAAS
It’s usually nice to have a companion. And in the lonely, dark expanse of the early universe, even some of the first stars had soul mates, new simulations reveal.
Previous studies had indicated that the first stars were extraordinarily massive — at least 100 times as heavy as the sun — but were also loners (SN: 6/8/2002, p. 362). Now, more detailed modeling, including a careful consideration of how atomic and molecular hydrogen interact at low densities, reveals that at least 5 percent and perhaps as many as half of these heavyweights were gravitationally bound to similar-mass companions, says Tom Abel of Stanford University He and his colleagues, Matthew Turk of Stanford and Brian O’Shea of Michigan State University in East Lansing, report their findings online July 9 in Science.
Pairs of massive stars are intriguing, notes Abel, because each star will probably collapse into a black hole. The coalescence of the two black holes would be a key source of gravitational waves, ripples in space-time predicted by Einstein’s theory of general relativity but never directly detected.
A second star’s presence could also enhance the production of distant gamma-ray bursts, flashes of high-energy light that have long-lasting afterglows and provide a window on the early universe. Gamma-ray bursts are produced when a single, massive star that crunches down into a black hole generates powerful jets of particles. A companion star can spin up its partner, and such rapid rotation may help generate the energetic jets, Abel says.
Star formation in the early universe is relatively easy to model because the infant cosmos contained only a few elements — mainly hydrogen and helium gas — which cooled and collapsed to produce stars. But even the simple interactions between atomic and molecular hydrogen hadn‘t been previously studied at low-enough densities, Abel says. He and his colleagues followed the star-forming process about 200 million years after the Big Bang, as gravity condensed gas clouds, to much higher densities than his team could in past simulations, Abel adds.
Only one in five of the team’s simulations produced stellar pairs, and for now the researchers can only provide a rough estimate of the percentage of first stars that would have had partners. He expects that within a year, larger, more comprehensive simulations will pin down the number.
The first generation of stars is not visible with today’s telescopes, but the proposed successor to Hubble, the infrared James Webb Space Telescope, scheduled for launch in 2014, has a good chance of recording large groupings of these earliest of glimmers.
“The simulations make good sense,” says theorist Volker Bromm of the University of Texas at Austin. Bromm says that his own team’s simulations track the evolution of pairs or groups of embryonic stars for an additional 100,000 years beyond that of Turk’s group — a necessity, he says, to determine if the baby stars merge into one big star or remain separate. His team indeed finds that the fledgling stars remain close partners. Bromm says his team plans to post a paper online describing the results later this month.
15 Temmuz 2009 Çarşamba
Fermilab's CDF observes Omega-sub-b baryon
Fermilab's CDF observes Omega-sub-b baryonAt a recent physics seminar at the Department of Energy's Fermi NationalAccelerator Laboratory, Fermilab physicist Pat Lukens of the CDFexperiment announced the observation of a new particle, the Omega-sub-b.The particle contains three quarks: two strange quarks and a bottom quark(s-s-b). It is an exotic relative of the much more common proton and hasabout six times the proton's mass.The observation of this "doubly strange" particle, predicted by theStandard Model, is significant because it strengthens physicists'confidence in their understanding of how quarks form matter. In addition,it conflicts with a 2008 result announced by CDF's sister experiment,DZero.The Omega-sub-b is the latest entry in the "periodic table of baryons."Baryons are particles formed of three quarks, the most common examplesbeing the proton and neutron. The Tevatron particle accelerator atFermilab is unique in its ability to produce baryons containing the bquark, and the large data samples now available after many years ofsuccessful running enable experimenters to find and study these rareparticles. The observation opens a new window for scientists toinvestigate its properties and better understand this rare object.Combing through almost half a quadrillion (1000 billion) proton-antiprotoncollisions produced by Fermilab's Tevatron particle collider, the CDFcollaboration isolated 16 examples in which the particles emerging from acollision revealed the distinctive signature of the Omega-sub-b. Onceproduced, the Omega-sub-b travels a fraction of a millimeter before itdecays into lighter particles. This decay, mediated by the weak force,occurs in about a trillionth of a second. In fact, CDF has performed thefirst ever measurement of the Omega-sub-b lifetime and obtained 1.13+0.53-0.40(stat.) ± 0.02(syst.) trillionths of a second.In August 2008, the DZero experiment announced its own observation of theOmega-sub-b based on a smaller sample of Tevatron data. Interestingly,the new CDF observation announced here is in direct conflict with theearlier DZero result. The CDF physicists measured the Omega-sub-b mass tobe 6054.4 ± 6.8(stat.) ± 0.9(syst.) MeV/c^2, compared to DZero's 6165 ±10(stat.) ± 13(syst.) MeV/c^2. These two experimental results arestatistically inconsistent with each other, leaving scientists from bothexperiments wondering whether they are measuring the same particle.Furthermore, the experiments observed different rates of production ofthis particle. Perhaps most interesting is that neither experiment sees ahint of evidence for the particle at the other's measured value.Although the latest result announced by CDF agrees with theoreticalexpectation for the Omega-sub-b both in the measured production rate andin the mass value, further investigation is needed to solve the puzzle ofthese conflicting results.The Omega-sub-b discovery follows the observation of the Cascade-b-minusbaryon, first observed at the Tevatron in 2007, and two types ofSigma-sub-b baryons, discovered at the Tevatron in 2006.The CDF collaboration submitted a paper that summarizes the details of itsdiscovery to the journal Physical Review D. It is available online at:http://arxiv.org/abs/0905.3123CDF is an international experiment of about 600 physicists from 62institutions in 15 countries. It is supported by the U.S. Department ofenergy, the National Science Foundation and a number of internationalfunding agencies. Fermilab is a national laboratory funded by the Officeof Science of the U.S. Department of Energy, operated under contract byFermi Research Alliance, LLC.
a galaxy collision in action
A compact group of galaxies, discovered about 130 years ago, about 280 million light years from Earth.
One galaxy is passing through a core of four other galaxies.
A shock wave generated from this motion heats the gas and produces X-rays detected by Chandra.
This beautiful image gives a new look at Stephan's Quintet, a compact group of galaxies discovered about 130 years ago and located about 280 million light years from Earth. The curved, light blue ridge running down the center of the image shows X-ray data from the Chandra X-ray Observatory. Four of the galaxies in the group are visible in the optical image (yellow, red, white and blue) from the Canada-France-Hawaii Telescope. A labeled version (roll over the image above) identifies these galaxies (NGC 7317, NGC 7318a, NGC 7318b and NGC 7319) as well as a prominent foreground galaxy (NGC 7320) that is not a member of the group. The galaxy NGC 7318b is passing through the core of galaxies at almost 2 million miles per hour, and is thought to be causing the ridge of X-ray emission by generating a shock wave that heats the gas.
Additional heating by supernova explosions and stellar winds has also probably taken place in Stephan's Quintet. A larger halo of X-ray emission - not shown here - detected by ESA's XMM-Newton could be evidence of shock-heating by previous collisions between galaxies in this group. Some of the X-ray emission is likely also caused by binary systems containing massive stars that are losing material to neutron stars or black holes.
Stephan's Quintet provides a rare opportunity to observe a galaxy group in the process of evolving from an X-ray faint system dominated by spiral galaxies to a more developed system dominated by elliptical galaxies and bright X-ray emission. Being able to witness the dramatic effect of collisions in causing this evolution is important for increasing our understanding of the origins of the hot, X-ray bright halos of gas in groups of galaxies.
Stephan's Quintet shows an additional sign of complex interactions in the past, notably the long tails visible in the optical image. These features were probably caused by one or more passages through the galaxy group by NGC 7317.
One galaxy is passing through a core of four other galaxies.
A shock wave generated from this motion heats the gas and produces X-rays detected by Chandra.
This beautiful image gives a new look at Stephan's Quintet, a compact group of galaxies discovered about 130 years ago and located about 280 million light years from Earth. The curved, light blue ridge running down the center of the image shows X-ray data from the Chandra X-ray Observatory. Four of the galaxies in the group are visible in the optical image (yellow, red, white and blue) from the Canada-France-Hawaii Telescope. A labeled version (roll over the image above) identifies these galaxies (NGC 7317, NGC 7318a, NGC 7318b and NGC 7319) as well as a prominent foreground galaxy (NGC 7320) that is not a member of the group. The galaxy NGC 7318b is passing through the core of galaxies at almost 2 million miles per hour, and is thought to be causing the ridge of X-ray emission by generating a shock wave that heats the gas.
Additional heating by supernova explosions and stellar winds has also probably taken place in Stephan's Quintet. A larger halo of X-ray emission - not shown here - detected by ESA's XMM-Newton could be evidence of shock-heating by previous collisions between galaxies in this group. Some of the X-ray emission is likely also caused by binary systems containing massive stars that are losing material to neutron stars or black holes.
Stephan's Quintet provides a rare opportunity to observe a galaxy group in the process of evolving from an X-ray faint system dominated by spiral galaxies to a more developed system dominated by elliptical galaxies and bright X-ray emission. Being able to witness the dramatic effect of collisions in causing this evolution is important for increasing our understanding of the origins of the hot, X-ray bright halos of gas in groups of galaxies.
Stephan's Quintet shows an additional sign of complex interactions in the past, notably the long tails visible in the optical image. These features were probably caused by one or more passages through the galaxy group by NGC 7317.
X-Ray Absorbtion
X-Ray Absorption
Absorption by the Earth's atmosphere restricts ground-based observations to radio, near infrared, and visible wavelengths. X-rays are absorbed high above the Earth in the following way:
X-ray photons--tiny high-energy packets of electromagnetic radiation--are absorbed by encounters with individual atoms. Even though the atoms in the atmosphere are widely spaced, the total thickness of the atmosphere is large and the total number of atoms is enormous. An X-ray photon passing through the atmosphere will encounter as many atoms as it would in passing through a 5 meter (16 ft) thick wall of concrete!
What happens when an X-ray is absorbed in the atmosphere?
The energy of the X-ray goes into tearing one of the electrons away from its orbit around the nucleus of a nitrogen or an oxygen atom.
This process is called photo-electric absorption, because a photon is absorbed in the process of removing an electron from an atom. The high-energy of X-rays is necessary for photo-electric absorption to take place.
X-ray telescopes in orbit above the Earth's atmosphere can collect X-rays from energetic sources billions of light years away. These cosmic X-rays are focused by barrel-shaped mirrors onto an instrument especially designed to measure properties such as the incoming direction and energy of the X-ray photon. A gaseous or solid material in the instrument absorbs the X-rays by the photo-electric effect.
Absorption by the Earth's atmosphere restricts ground-based observations to radio, near infrared, and visible wavelengths. X-rays are absorbed high above the Earth in the following way:
X-ray photons--tiny high-energy packets of electromagnetic radiation--are absorbed by encounters with individual atoms. Even though the atoms in the atmosphere are widely spaced, the total thickness of the atmosphere is large and the total number of atoms is enormous. An X-ray photon passing through the atmosphere will encounter as many atoms as it would in passing through a 5 meter (16 ft) thick wall of concrete!
What happens when an X-ray is absorbed in the atmosphere?
The energy of the X-ray goes into tearing one of the electrons away from its orbit around the nucleus of a nitrogen or an oxygen atom.
This process is called photo-electric absorption, because a photon is absorbed in the process of removing an electron from an atom. The high-energy of X-rays is necessary for photo-electric absorption to take place.
X-ray telescopes in orbit above the Earth's atmosphere can collect X-rays from energetic sources billions of light years away. These cosmic X-rays are focused by barrel-shaped mirrors onto an instrument especially designed to measure properties such as the incoming direction and energy of the X-ray photon. A gaseous or solid material in the instrument absorbs the X-rays by the photo-electric effect.
1 Temmuz 2009 Çarşamba
A super efficient particle accelerator-chandra blog-
This image of data from NASA's Chandra X-ray Observatory and the European Southern Observatory's Very Large Telescope shows a part of the roughly circular supernova remnant known as RCW 86. This remnant is the remains of an exploded star, which may have been observed on Earth in 185 AD by Chinese astronomers. By studying this remnant, a team of astronomers was able to understand new details about the role of supernova remnants as the Milky Way's super-efficient particle accelerators. The team shows that the shock wave visible in this area is very efficient at accelerating particles and the energy used in this process matches the number of cosmic rays observed on Earth.
The VLT data (colored red in the composite) was used to measure the temperature of the gas right behind the shock wave created by the stellar explosion. Using X-ray images from Chandra (blue), taken three years apart, the researchers were also able to determine the speed of the shock wave to be between one and three percent of the speed of light. The temperature found by these latest results is much lower than expected, given the measured shock wave's velocity. The researchers conclude that the missing energy goes into accelerating the cosmic rays.
The VLT data (colored red in the composite) was used to measure the temperature of the gas right behind the shock wave created by the stellar explosion. Using X-ray images from Chandra (blue), taken three years apart, the researchers were also able to determine the speed of the shock wave to be between one and three percent of the speed of light. The temperature found by these latest results is much lower than expected, given the measured shock wave's velocity. The researchers conclude that the missing energy goes into accelerating the cosmic rays.
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