Category: Astronomy

Sky map of the June 25th planetary alignment. Image credit: NASA. Click to enlarge.
Saturn, which has been prominent, in the constellation Gemini all winter is slowly exiting our skies. But the Ringed Planet has one last show to put on for us, and the stage has been set. On June 18th, Saturn was joined by Venus, followed by Mercury on the 19th. On these dates the trio formed a long string stretching from the stars Castor and Pollux to just above the horizon. As the week progressed, the two faster planets slowly drew closer to Saturn. On the evenings of the 24th and 25th the trio will form a very close conjunction with Venus being just 1 degree from Saturn and less than 1 degree from Mercury.

For the next few nights all 3 planets should be visible in the wide field of view of a pair of binoculars or small telescope. By the 27th, Mercury and Venus will have drawn away from Saturn somewhat but will lie just 8 arc-minutes from one another, nearly indistinguishable to the unaided eye.

As June turns to July, Saturn will be lost in the glare of the setting sun. But Mercury and Venus will stay in close conjunction well into the month. On July 8th look for a very slim waxing crescent moon hovering just above the pair. Around July 15th the apparent separation of Mercury and Venus will have increased to 5 degrees. At this point Mercury will begin looping back toward the sun, while Venus continues to climb higher in our evening skies.

Contrary to popular belief, planetary conjunctions are fairly common. All the planets and the sun appear to travel along an imaginary line in the sky known as the ecliptic. Because our solar system is essentially a disk, the objects in our solar system appear to follow the same path year after year after year. Since we see these objects from Earth, which is itself moving, the planets occasionally appear to get close together in the sky. Conjunctions of 2 or 3 planets happen quite often particularly when one of them is Venus. The faster planets seem to ?catch up with? and ?pass? the slower moving ones, as we see in June.

Throughout recorded history humans have observed planetary conjunctions. In ancient times they were thought to be signs or omens. Not until recent centuries have we been able to model and therefore marvel at the workings of our solar system. Even though the conjunction of Mercury, Venus and Saturn doesn?t portend events, it is nonetheless a spectacular sight to behold.

Written by Rod Kennedy

A rotating superfluid gas of fermions pierced with vortices. Image credit: MIT. Click to enlarge.
MIT scientists have brought a supercool end to a heated race among physicists: They have become the first to create a new type of matter, a gas of atoms that shows high-temperature superfluidity.

Their work, to be reported in the June 23 issue of Nature, is closely related to the superconductivity of electrons in metals. Observations of superfluids may help solve lingering questions about high-temperature superconductivity, which has widespread applications for magnets, sensors and energy-efficient transport of electricity, said Wolfgang Ketterle, a Nobel laureate who heads the MIT group and who is the John D. MacArthur Professor of Physics.

Seeing the superfluid gas so clearly is such a dramatic step that Dan Kleppner, director of the MIT-Harvard Center for Ultracold Atoms, said, “This is not a smoking gun for superfluidity. This is a cannon.”

For several years, research groups around the world have been studying cold gases of so-called fermionic atoms with the ultimate goal of finding new forms of superfluidity. A superfluid gas can flow without resistance. It can be clearly distinguished from a normal gas when it is rotated. A normal gas rotates like an ordinary object, but a superfluid can only rotate when it forms vortices similar to mini-tornadoes. This gives a rotating superfluid the appearance of Swiss cheese, where the holes are the cores of the mini-tornadoes. “When we saw the first picture of the vortices appear on the computer screen, it was simply breathtaking,” said graduate student Martin Zwierlein in recalling the evening of April 13, when the team first saw the superfluid gas. For almost a year, the team had been working on making magnetic fields and laser beams very round so the gas could be set in rotation. “It was like sanding the bumps off of a wheel to make it perfectly round,” Zwierlein explained.

“In superfluids, as well as in superconductors, particles move in lockstep. They form one big quantum-mechanical wave,” explained Ketterle. Such a movement allows superconductors to carry electrical currents without resistance.

The MIT team was able to view these superfluid vortices at extremely cold temperatures, when the fermionic gas was cooled to about 50 billionths of a degree Kelvin, very close to absolute zero (-273 degrees C or -459 degrees F). “It may sound strange to call superfluidity at 50 nanokelvin high-temperature superfluidity, but what matters is the temperature normalized by the density of the particles,” Ketterle said. “We have now achieved by far the highest temperature ever.” Scaled up to the density of electrons in a metal, the superfluid transition temperature in atomic gases would be higher than room temperature.

Ketterle’s team members were MIT graduate students Zwierlein, Andre Schirotzek, and Christian Schunck, all of whom are members of the Center for Ultracold Atoms, as well as former graduate student Jamil Abo-Shaeer.

The team observed fermionic superfluidity in the lithium-6 isotope comprising three protons, three neutrons and three electrons. Since the total number of constituents is odd, lithium-6 is a fermion. Using laser and evaporative cooling techniques, they cooled the gas close to absolute zero. They then trapped the gas in the focus of an infrared laser beam; the electric and magnetic fields of the infrared light held the atoms in place. The last step was to spin a green laser beam around the gas to set it into rotation. A shadow picture of the cloud showed its superfluid behavior: The cloud was pierced by a regular array of vortices, each about the same size.

The work is based on the MIT group’s earlier creation of Bose-Einstein condensates, a form of matter in which particles condense and act as one big wave. Albert Einstein predicted this phenomenon in 1925. Scientists later realized that Bose-Einstein condensation and superfluidity are intimately related.

Bose-Einstein condensation of pairs of fermions that were bound together loosely as molecules was observed in November 2003 by independent teams at the University of Colorado at Boulder, the University of Innsbruck in Austria and at MIT. However, observing Bose-Einstein condensation is not the same as observing superfluidity. Further studies were done by these groups and at the Ecole Normale Superieure in Paris, Duke University and Rice University, but evidence for superfluidity was ambiguous or indirect.

The superfluid Fermi gas created at MIT can also serve as an easily controllable model system to study properties of much denser forms of fermionic matter such as solid superconductors, neutron stars or the quark-gluon plasma that existed in the early universe.

The MIT research was supported by the National Science Foundation, the Office of Naval Research, NASA and the Army Research Office.

Original Source: MIT News Release

HESS image of binary pair PSR B-1259-63 / SS 2883. Image credit: HESS. Click to enlarge.
Binary pair PSR B-1259-63 / SS 2883 is located some 5,000 light-years distant in the general direction of the southern hemisphere constellation Crux (the Southern Cross). The duo consists of a pulsar (PSR B-1259) and massive blue giant (SS 2883) locked into a widely-swinging dance that repeats steps every 3.4 years. The pulsar?s orbit of the more massive primary is so eccentric that the pair passes within 100 million kilometers at closest approach and they separate roughly ten times that distance at their furthest point. During closest approach, signals from the pulsar drop off significantly as it is eclipsed by the massive blue giant.

Observers using the 12.5 metre High Energy Stereoscopic System (HESS) recorded the pair’s dance during moonless nights from February through April 2004, and timed them as the pulsar approached and receded from the duo’s closest point. The astronomers found that radio waves from the pulsar matched up with ultra-high gamma radiation coming from the region.

According to Felix Aharonian of the Max Plank Institute for Nuclear Physics, Heidelberg Germany, this binary system “allows ‘on-line watch’ of the extremely complex MHD (magnetohydrodynamic) processes of creation and termination of the ultrarelativistic pulsar wind, as well as particle acceleration by relativistic shock waves, through the study of spectral and temporal characteristics of the high energy gamma-radiation of the system. In this regard the binary system PSR B1259-63 is a unique laboratory to explore the physics of the pulsar winds.”

The pulsar was first detected by a team of astronomers in 1992 using the Parkes radio telescope in Australia. Its magnetic jet orients toward the Earth 20 times a second. In addition to radio emission, the pulsar broadcasts X-rays – at various energy levels – throughout its orbit. These X-rays are thought to be the result of radiation that occurs when the pulsar’s magnetic field interacts with gases released by the companion blue giant.

The blue giant SS 2883 was first discovered to be a companion with the pulsar in 1992. It’s ten times the mass of the Sun, but has high temperatures and a rapidly burning fusion engine. It rotates very quickly and ejects material from its equator on a sporadic basis. According to the paper ‘Discovery of the Binary Pulsar PSR B-1259-63 … with H.E.S.S.’, “Be stars are known to have non-isotropic stellar winds forming an equatorial disk with enhanced mass outflow.”

The paper goes on to say that “timing measurements suggest that the disk is inclined with respect to the orbital plane…” such an orbital inclination causes the “pulsar to cross the disk two times near periastron.” And it is at these crossings that things really get souped up as the pulsar’s magnetic field begins to interact with charged particles in the reverse shock region of the stellar ejecta.

As a result, this system is said to be a ‘binary plerion’ where “The intense photon field provided by the companion star not only plays an important role in the cooling of relativistic electrons but also serves as the perfect target for the production of high-energy gamma rays through inverse Compton (IC) scattering.” Felix expands on this notion by saying that “the pulsar is not isolated, but located in a binary system close to a powerful optical star. In this case, because of interaction with the stellar wind under high gas pressure, the pulsar wind terminates within the binary system where the magnetic field is quite high (approximately 1 G, i.e. 10,000 to 100,000 times larger than in standard plerions). Furthermore, because of the optical star’s presence, the electrons suffer severe losses during interactions (Compton scattering) with starlight. This makes the lifetime of electrons very short, 1 hour or less. High energy gamma-rays can be produced also by interactions of electrons (and perhaps also protons) with the dense gas of the stellar disk (also on quite short timescales!).”

As a binary plerion, the star system displays a wide-ranging energy signature based on the pulsar’s eccentric orbit and broad variations in the density of circumstellar matter around SS 2883 with which it interacts. Near periastron, The “cold” pulsar wind interacting with the ambient plasma, terminates with the creation of a relativistic shock wave which in turn accelerate particles to extremely high energies, 1 TeV or more. Heat in these particles is then ‘cooled’ as photons strike fast-moving electrons and positrons. This inverse Compton scattering effect carries off energy by amplifying photon frequencies wildly. Simply said, photons of low-energy “visible light” are boosted to much higher energy levels – some achieving the terra-electron volt region of the upper gamma ray / lower cosmic ray domain.

Meanwhile as the pulsar moves away from the stellar primary, it encounters fewer and fewer charged particles, meanwhile the density of visible light photons from the central star also falls off. As this occurs, scattering of photons is reduced and synchrotron radiation begins to dominate. Because of this, lower power-level X-rays begin to dominate the energy signature of the system as the pulsar slows and moves away from the star.

Finally, there are two periods in the pulsars orbit where it crosses the equatorial plane of the blue giant’s circumstellar disk. These transition points can result in the creation of numerous super-energized photons, electrons, positrons and even some protons. As relativistically accelerated particles are created, they in turn interact with a region able to spawn a multitude of other particles capable of breaking down into high-energy photons and other particles.

From the paper published June 13, 2005, “Up to now the theoretical understanding of this complex system, involving pulsar and stellar winds interacting with each other is quite limited because of the lack of constraining observations.” But now because of IACTS (Imaging Atmospheric Cherenkov Telescopes) such as H.E.S.S., astronomers are now able to resolve many new near-point sources of high energy gamma rays from other systems such as PSR B-1259-63 / SS 2883.

In the PSR B-1259-63 / SS 2883 system, nature seems to have provided astronomers – and physicists – with her very own version of a super-high energy particle accelerator – one that is thankfully well contained and a safe distance from Earth.

Written by Jeff Barbour

Examples of Bok globules. Image credit: SAAO. Click to enlarge.
Our Sun has been around for almost five billion years. Throughout most of its history the Sun has pretty much appeared the way it does today – a vast sphere of radiant gas and dust lit to incandescence by heat liberated through hydrogen fusion near its core. But before our Sun took form, matter had to be drawn together from the interstellar medium (ISM) and compacted in a small enough region of space to pass a critical balance between further condensation and stability. For this to occur, a delicate balance between outwardly exerted internal pressure and inward moving gravitational influence had to be overcome.

In 1947, Harvard observational astronomer Bart Jan Bok announced the result of years of study of an important subset of cold gases and dust often associated with extended nebulosity. Bok suggested that certain isolated and distinct globules obscuring background light in space were in fact evidence of an important preliminary stage in the formation of protostellar disks leading to the birth of stars such as our sun.

Subsequent to Bok’s announcement, many physical models emerged to explain how Bok globules could come to form stars. Typically, such models begin with the notion that matter comes together in regions of space where the interstellar medium is especially dense (in the form of nebulosity), cold, and subject to radiation pressure from neighboring stars. At some point enough matter may condense into a small enough region that gravitation overcomes gas pressure and the balance tips in favor of star formation.

According to the paper “Near Infrared Imaging Survey of Bok Globules: Density Structure”, published June 10, 2005 Ryo Kandori and a team of fourteen other investigators “suggest that a nearly critical Bonner-Ebert sphere characterizes the critical density of starless globules.”

The concept of a Bonner-Ebert sphere originates with the idea that a balance of forces can exist within an idealized cloud of gas and dust. Such a sphere is held to have a constant internal density while maintaining equilibrium between the expansionary pressure caused by gases of a given temperature and density and the gravitational influence of its total mass assisted by any gas or radiation pressure exerted from neighboring stars. This critical state relates to the diameter of the sphere, its total mass, and the amount of pressure generated by latent heat within it.

Most astronomers have assumed that the Bonner-Ebert model – or some variation thereof – would ultimately prove accurate in describing the point when a particular Bok globule crosses the line to become a protostellar disk. Today, Ryo Kandori et al have gathered enough evidence from a variety of Bok globules to strongly suggest that this notion is correct.

The team started by selecting ten Bok globules for observation based on small apparent size, near-circular shape, distance from neighboring nebulosity, proximity to the Earth (less than 1700 LYs away), and accessibility to near-infrared and radio wave collecting instruments located in both the northern and southern hemispheres. From a list of nearly 250 such globules, only those meeting the above criteria were included. Among those selected only one showed evidence of a protostellar disk. This one disk took the form of a point source of infrared light detected during an all-sky survey performed by IRAS (Infrared Astronomy Satellite – a joint project of the US, UK, and Netherlands). All ten globules were located in star and nebulosity rich regions of the Milky Way.

Once candidate Bok globules were selected, the team subjected each of them to a battery of observations designed to determine their mass, density, temperature, size, and if possible, the amount of pressure applied on them by the ISM and neighboring starlight. One important consideration was to get a sense if there were any variations in density throughout the globule. The presence of uniform pressure is particularly important when it comes to determining which of a variety of theoretical models best mapped against the constitution of the modules themselves.

Using a ground-based instrument (the 1.4 meter IRSF at the South African Astronomical Observatory) in 2002 and 2003, near-infrared light in three different bands (J, H, & K) was collected from each globule to magnitude 17 plus. The images were then integrated and compared to light originating from the background star region. This data was subjected to several analysis methods to allow the team to derive the density of gas and dust across each globule down to the level of resolution supported by seeing conditions (roughly one arc second). That work basically determined that each globule showed a uniform density gradient based on its projected three-dimensional distribution. The Bonner-Ebert sphere model looked like a very good match.

The team also observed each globule using the 45 meter radio telescope of the Nobeyama Radio Observatory in Minamisaku, Nagano, Japan. The idea here was to collect specific radio frequencies associated with excited N2H+ and C18O. By looking at the amount of blur in these frequencies the team was able to determine the internal temperature of each globule which, along with the density of the gas, can be used to approximate the gas pressure internal to each globule.

After gathering the data, subjecting it to analysis, and quantifying the results, the team “found that more then half of the starless globules (7 out of 11 sources) are located near the (Bonner-Ebert) critical state. Thus we suggest that a nearly critical Bonner-Ebert sphere characterizes the typical density structure of starless globules.” In addition the team determined that three Bok globules (Coalsack II, CB87 & Lynds 498) are stable and clearly not in process of star formation, four (Barnard 66, Lynds 495, CB 161 & CB 184) are poised near the stable Bonner-Ebert state but tending toward star formation based on that model. Finally the remaining six (FeSt 1-457, Barnard 335, CB 188, CB 131, CB 134) are clearly moving toward gravitation collapse. Those six “stars in the making” include globules CB 188 and Barnard 335 already known to possess protostellar disks.

On any relatively cloudless day it doesn’t take much in the way of instrumentation to prove that one very unique and important ‘Bok globule’ existing some 5 billion years ago did manage to tip the scales and become a star in the making. Our Sun is firey proof that matter – once adequately condensed – can begin a process that leads to some extraordinary new possibilities.

Written by Jeff Barbour

Herbig-Haro 211 consists of two jets of material, visible at lower right. Image credit: A.A. Muench-Nasrallah, CfA. Click to enlarge.
Astronomers find jets everywhere when they look into space. Small jets spout from newborn stars, while huge jets blast out of the centers of galaxies. Yet despite their commonness, the processes that drive them remain shrouded in mystery. Even relatively nearby stellar jets hide their origins behind almost impenetrable clouds of dust. All stars, including our sun, pass through a jet phase during their “childhood,” so astronomers are eager to understand how jets form and how they may influence star and planet formation.

At this week’s meeting on submillimeter astronomy in Cambridge, Mass., astronomers described the latest results from an international collaboration using the Submillimeter Array (SMA) atop Mauna Kea, Hawaii. The SMA has begun to peer through the dust and home in on the sources of nearby stellar jets.

“Using the SMA, we can stare into the throat of the jet,” said SMA project scientist Paul Ho of the Harvard-Smithsonian Center for Astrophysics (CfA). “We’re getting close to seeing its launching point.”

Astronomer Hsien Shang of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) and her colleagues have created a model of jet formation that calculates temperatures, densities and brightnesses within stellar jets. SMA observations of a young star system prosaically named Herbig-Haro (HH) 211 have confirmed the validity of the model.

“Our model predicts what we will see about 100 astronomical units from the star,” Shang said. (One astronomical unit is the average Earth-Sun distance of 93 million miles.) “With the SMA, we can begin to look at the HH 211 system at the scale of the model and test those predictions. So far, everything checks out.”

HH 211 is located about 1,000 light-years away in the constellation Perseus. Astronomers estimate that the small protostar hidden within HH 211 is less than 1,000 years old-a mere baby by astronomical standards, so young that it is still growing by accumulating matter from a surrounding disk of gas and dust. The protostar eventually will become a low-mass star similar to the sun.

Although most of the matter in the disk will flow onto the star, some must be ejected outward to carry away excess angular momentum. Complex physical processes funnel that ejected matter into dual jets that shoot outward in opposite directions.

“Jets form very close to a protostar, within about 5 million miles of its surface according to the model we applied” said researcher Naomi Hirano (ASIAA). “The SMA can help test the jet model on the youngest protostars using molecular tracers from within that innermost region.”

SMA’s successor, the planned ALMA project, should finally reveal the nature of the engine powering these jets by peering into the core where they form.

“The SMA has brought us tantalizingly close to our goal-the answer to the question of how jets form,” said Ho. “ALMA will take us those final few steps.”

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

Original Source: Harvard CfA News Release

A cosmic view of neutrino ripples. Image credit: Oxford. Click to enlarge.
Astrophysicists from the Universities of Oxford and Rome have for the first time found evidence of ripples in the Universe?s primordial sea of neutrinos, confirming the predictions of both Big Bang theory and the Standard Model of particle physics.

Neutrinos are elementary particles with no charge and very little mass, which are extremely difficult to study due to their very weak interaction with matter. Yet pinning down the physical properties of neutrinos is of paramount importance to scientists attempting to understand the fundamental building blocks of Nature. According to the standard Big Bang model, neutrinos permeate the Universe at a density of about 150 per cubic centimetre. The Earth is therefore immersed in an ocean of neutrinos, without us ever noticing.

Although it is impossible to measure this ?Cosmic Neutrino Background? directly with present-day technology, physicists predict that ripples or waves in it have an impact on the growth of structures in the Universe.

In research to be published in the journal Physical Review Letters, Dr. Roberto Trotta, Lockyer Fellow of the Royal Astronomical Society at Oxford?s Department of Physics, and Dr. Alessandro Melchiorri of La Sapienza University in Rome were able to demonstrate for the first time the existence of ripples of primordial origin in the Cosmic Neutrino Background.

The discovery, made by combining data produced by the NASA WMAP (Wilkinson Microwave Anisotropy Probe) satellite and the Sloan Digital Sky Survey, confirms the predictions of both the Big Bang theory and the Standard Model of particle physics. The research has important implications for the study of neutrinos, showing that theories of the infinitely large (cosmology) and the infinitely small (particle physics) are in agreement.

Dr. Trotta said: ?This research provides important new evidence in favour of the current cosmological model, unifying it with fundamental physics theories. Cosmology is becoming a more and more powerful laboratory where physics not easily accessible on Earth can be tested and verified. The high quality of recent cosmological data allows us to investigate neutrinos in the cosmological framework, obtaining measurements which are competitive with ? if not superior to ? particle accelerator findings.?

Original Source: Oxford News Release

Artist interpretation of protoplanetary systems forming inside a nebula. Image credit: CfA. Click to enlarge.
Meeting this week in Cambridge, Mass., astronomers using the Submillimeter Array (SMA) on Mauna Kea, Hawaii, confirmed, for the first time, that many of the objects termed “proplyds” found in the Orion Nebula do have sufficient material to form new planetary systems like our own.

“The SMA is the only telescope that can measure the dust within the Orion proplyds, and thereby assess their true potential for forming planets. This is critical in our understanding of how solar systems form in hostile regions of space,” said Jonathan Williams of the University of Hawaii Institute for Astronomy, lead author on a paper submitted to The Astrophysical Journal.

Surviving in the chaotic regions within the Orion Nebula where stellar winds can reach a staggering two million miles per hour and temperatures exceed a searing 18,000 degrees Fahrenheit, the question remained – would enough material endure to form a new solar system or would it be eroded away into space like wind and sand eroding away desert cliffs? It now appears that these protoplanetary disks are quite tenacious, bringing new grounds for optimism in the search for planetary systems.

Imaged by the Hubble Space Telescope back in the early 1990s as misshapen silhouettes against the nebular background, the most spectacular proplyds appear bright. Their surrounding ionized cocoons glow due to their close proximity to a nearby hot star formation called the Trapezium. The Trapezium is a star cluster consisting of more than 1,000 young, hot stars that are only 1 million years old. They condensed out of the original cold, dark cloud of gas that now glows from their ionizing light. They are crowded into a space about 4 light-years in diameter, the same as the distance between the Sun and Proxima Centauri, the next closest star in space.

Blasted by the solar winds of the Trapezium, the proplyds are the next generation of smaller stars to arise in Orion, this time with visible discs that may be forming planets. It has remained unclear, however, whether they contained enough material to form stable planetary systems. Using the SMA, astronomers now have been able to probe deep inside these disks to measure their mass and to unravel the formation process presented by these potential infant solar systems.

“While the Hubble pictures were spectacular, they revealed only disk-like shapes that did not tell us the amount of material present,” said David Wilner, of the Harvard-Smithsonian Center for Astrophysics (CfA). Since some of the discs appear to be comparable in size and mass to our own solar system, this strengthens the connection between the Orion proplyds and our origins.

Since most Sun-like stars in the Galaxy eventually form in environments like the Orion Nebula, the SMA results suggest that the formation of solar systems like our own is common and a continuing event in the Galaxy.

“The same cycle of birth, life and death we experience here on Earth is repeated in the stars overhead. Now, the SMA provides us with a front-row seat in unraveling the wonder of these cosmic events,” reflected Wilner.

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

Original Source: CfA News Release

Supernova remnant Cassiopeia A. Image credit: NASA/JPL. Click to enlarge.
An enormous light echo etched in the sky by a fitful dead star was spotted by the infrared eyes of NASA’s Spitzer Space Telescope.

The surprising finding indicates Cassiopeia A, the remnant of a star that died in a supernova explosion 325 years ago, is not resting peacefully. Instead, this dead star likely shot out at least one burst of energy as recently as 50 years ago.

“We had thought the stellar remains inside Cassiopeia A were just fading away,” said Dr. Oliver Krause, University of Arizona, Tucson. “Spitzer came along and showed us this exploded star, one of the most intensively studied objects in the sky, is still undergoing death throes before heading to its final grave.”

Infrared echoes trace the dusty journeys of light waves blasted away from supernova or erupting stars. As the light waves move outward, they heat up clumps of surrounding dust, causing them to glow in infrared light. The echo from Cassiopeia A is the first witnessed around a long-dead star and the largest ever seen. It was discovered by accident during a Spitzer instrument test.

“We had no idea that Spitzer would ever see light echoes,” said Dr. George Rieke of the University of Arizona. “Sometimes you just trip over the biggest discoveries.”

To view the echoes dancing through clouds of dust surrounding Cassiopeia A, visit:
http://www.spitzer.caltech.edu/Media/releases/ssc2005-14/visuals.shtml.

A supernova remnant like Cassiopeia A typically consists of an outer, shimmering shell of expelled material and a core skeleton of a once-massive star, called a neutron star. Neutron stars come in several varieties, ranging from intensely active to silent. Typically, a star that has recently died will continue to act up. Consequently, astronomers were puzzled that the star responsible for Cassiopeia A appeared to be silent so soon after its death.

The new infrared echo indicates the Cassiopeia A neutron star is active and may even be an exotic, spastic type of object called a magnetar. Magnetars are like screaming dead stars, with eruptive surfaces that rupture and quake, pouring out tremendous amounts of high-energy gamma rays. Spitzer may have captured the “shriek” of such a star in the form of light zipping away through space and heating up its surroundings.

“Magnetars are very rare and hard to study, especially if they are no longer associated with their place of origin. If we have indeed uncovered one, then it will be just about the only one for which we know what kind of star it came from and when,” Rieke said.

Astronomers first saw hints of the infrared echo in strange, tangled dust features that showed up in the Spitzer test image. When they looked at the same dust features again a few months later using ground-based telescopes, the dust appeared to be moving outward at the speed of light. Follow-up Spitzer observations taken one year later revealed the dust was not moving, but was being lit up by passing light.

A close inspection of the Spitzer pictures revealed a blend of at least two light echoes around Cassiopeia A, one from its supernova explosion, and one from the hiccup of activity that occurred around 1953. Additional Spitzer observations of these light echoes may help pin down their enigmatic source.

Krause was lead author with Rieke of a study about the discovery appearing this week in the journal Science.

JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center, California Institute of Technology, Pasadena, Calif. JPL is a division of Caltech. Spitzer’s multiband imaging photometer, which made the new observations, was built by Ball Aerospace Corporation, Boulder, Colo.; the University of Arizona; and Boeing North America, Canoga Park, Calif. Its development was led by Rieke.

For additional images and information about Spitzer on the Web, visit: http://www.spitzer.caltech.edu/Media. For information about NASA and agency programs on the Web, visit: http://www.nasa.gov/home/index.html.

Original Source: NASA/JPL News Release

All-sky map of the best fitting ‘halo+disk’ model of 511 keV gamma-ray line emission. Image credit: INTEGRAL. Click to enlarge.
The positron, the anti-matter counterpart to the electron, was predicted by Paul Dirac’s – at the time revolutionary – quantum wave equation for the electron. A few years later, in 1932, Carl Anderson discovered the positron in cosmic rays, and Dirac got the Nobel Prize in 1933 and Anderson in 1936.

When a positron meets an electron, they annihilate, producing two gamma rays. Sometimes however, the annihilation is preceded by the formation of positronium, which is like a hydrogen atom with the proton replaced by a positron (positronium has its own symbol, Ps). Positronium comes in two forms, is unstable, and decays into either two gammas (within about 0.1 nanoseconds) or three (within about 100 nanoseconds).

Astronomers have known since the 1970s that there must be a lot of positrons in the universe. Why? Because when a positron and electron annihilate to give two gammas, both have the same wavelength, about 0.024 Å, or 0.0024 nm (astronomers, like particle physicists, don’t talk about the wavelengths of gamma rays, they talk about their energy; 511 keV in this case). So, if you look at the sky with gamma-ray vision – from above the atmosphere of course! – you know there was lots of positrons because you can see lots of gammas of a single ‘colour’, 511 keV (it’s similar to concluding there’s lots of hydrogen in the universe by noticing lots of the red (1.9 eV) H alpha in the night sky).

From the spectrum of the three-gamma decay of positronium, compared with the 511 keV line intensity, astronomers four years ago worked out that about 93% of positrons whose annihilation we see form positronium before they decay.

How much positronium? In the Milky Way bulge, about 15 billion (thousand million) tons of positrons are annihilated every second. That’s as much mass as the electrons in tens of trillions of tons of stuff we’re used to, like rocks or water; about as much as in a mid-sized asteroid, 40 km across.

By analyzing the publicly released INTEGRAL data (about one year’s worth), J?rgen Kn?dlseder and his colleagues found that:

• the positrons which are being annihilated in the Milky Way disk most likely come from the beta+ (i.e. positron) decay of the isotopes Aluminium-26 and Titanium-44, which themselves were produced in recent supernovae (remember, astronomers call even 10 million years ago ‘recent’)
• however, there are more positrons being annihilated in the Milky Way bulge than in the disk, by a factor of five
• there don’t seem to be any ‘point’ sources.

Of course, to an INTEGRAL scientist, a ‘point’ source doesn’t have quite the same meaning as it does to an amateur astronomer! Gamma-ray vision in the positronium line is incredibly blurry, an object six Moons across (3?) would look like a ‘point’! Nonetheless, Kn?dlseder and his team of astrophysics sleuths are able to say that “none of the sources we searched for showed a significant 511 keV flux”; these 40 ‘usual suspects’ include pulsars, quasars, black holes, supernovae remnants, star-forming regions, rich galaxy clusters, satellite galaxies, and blazars. But, they’re still looking, “We have indeed [planned,] dedicated INTEGRAL observations of the usual suspects, such as Type Ia supernovae (SN1006, Tycho), and LMXB (Cen X-4) which might help to solve this problem.”

So, where do the 15 billion tons of positrons being annihilated every second in the bulge come from? “For me the most important thing about the positron annihilation is that the principal source is still a mystery,” says Kn?dlseder. “We can explain the faint emission from the disk by Aluminium-26 decay, but the bulk of positrons are situated in the bulge region of the Galaxy, and we have no source that can easily explain all observational characteristics. In particular, if you compare the 511 keV sky to the sky observed at other wavelengths you recognise that the 511 keV sky is unique! There is no other sky that resembles to what we observe.”

The INTEGRAL team feel they can rule out massive stars, collapsars, pulsars, or cosmic ray interactions, for if these were the source of the bulge positrons, then the disk would be much brighter in 511 keV light.

The bulge positrons may come from low-mass X-ray binaries, classical novae, or Type 1a supernovae, through a variety of processes. The challenge in each case is to understand how sufficient positrons created by these could survive long enough afterwards and diffuse far enough from their birthplaces.

What about cosmic strings? While the recent Tanmay Vachaspati paper proposing these as a possible source of the bulge positrons came out too recently for Kn?dlseder et al. to consider for their paper, “Yet for me it is not obvious that we have enough observational constraints to state that cosmic strings make the 511 keV; we don’t even know if cosmic strings exist. One would need a unique characteristic of cosmic strings that exclude all other sources, and today I think we are far from this.”

Perhaps most excitingly, the positrons may come from the annihilation of a low-mass dark matter particle and its anti-particle, or as Kn?dlseder et al. put it “Light dark matter (1-100 MeV) annihilation, as suggested recently by Boehm et al. (2004), is probably the most exotic but also the most exciting candidate source of galactic positrons.” Dark matter is even more exotic than positronium; dark matter is not anti-matter, and no one has been able to capture it, let alone study it in a lab. Astronomers accept that it is ubiquitous and tracking down its nature is one of the hottest topics in both astrophysics and particle physics. If the billions of tons per second of positrons that are annihilated in the Milky Way bulge cannot have come from classical novae or thermonuclear supernovae, then perhaps good old dark matter is to blame.