Category: Astronomy

Computer illustration of microquasar LS5039. Image credit: PPARC. Click to enlarge.
In a recent issue of Science Magazine, the High Energy Stereoscopic System (H.E.S.S.) team of international astrophysicists reports the discovery of another new type of very high energy (VHE) gamma ray source.

Gamma-rays are produced in extreme cosmic particle accelerators such as supernova explosions and provide a unique view of the high energy processes at work in the Milky Way. VHE gamma-ray astronomy is still a young field and H.E.S.S. is conducting the first sensitive survey at this energy range, finding previously unknown sources.

The object that is producing the high energy radiation is thought to be a ‘microquasar’. These objects consist of two stars in orbit around each other. One star is an ordinary star, but the other has used up all its nuclear fuel, leaving behind a compact corpse. Depending on the mass of the star that produced it, this compact object is either a neutron star or a black hole, but either way its strong gravitational pull draws in matter from its companion star. This matter spirals down towards the neutron star or the black hole, in a similar way to water spiraling down a plughole.

However, sometimes the compact object receives more matter than it can cope with. The material is then squirted away from the system in a jet of matter moving at speeds close to that of light, resulting in a microquasar. Only a few such objects are known to exist in our galaxy and one of them, an object called LS5039, has now been detected by the H.E.S.S. team.

In fact, the real nature LS5039 is something of a mystery. It is not clear what the compact object is. Some of the characteristics suggest it is a neutron star, some that it is a black hole. Not only that, but the jet isn’t much of a jet; although it is moving at about 20% of the speed of light, which might seem a lot, in the context of these objects it’s actually quite slow.

Nor is it clear how the gamma rays are being produced. As Dr. Guillaume Dubus of the Ecole Polytechnique points out “We really shouldn’t have detected this object. Very high energy gamma rays emitted close to the companion star are more likely to be absorbed, creating a matter/antimatter cascade, than escape from the system.”

Dr Paula Chadwick of the University of Durham adds “It’s very exciting to have added another class of object to the growing catalogue of gamma ray sources. It’s an intriguing object – it will take more observations to work out what is going on in there.”

The H.E.S.S. array is ideal for finding new VHE gamma ray objects; because it’s wide field of view (ten times the diameter of the Moon) means that it can survey the sky and discover previously unknown sources.

The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of VHE gamma-rays – radiation that is a million, million times more energetic than the visible light. These high energy gamma rays are quite rare even for relatively strong sources; only about one gamma ray per month hits a square metre at the top of the Earth’s atmosphere. Also, since they are absorbed in the atmosphere, a direct detection of a significant number of the rare gamma rays would require a satellite of huge size. The H.E.S.S. telescopes employ a trick – they use the atmosphere as detector medium. When gamma rays are absorbed in the air, they emit short flashes of blue light, named Cherenkov light, lasting a few billionths of a second. This light is collected by the H.E.S.S. telescopes with large mirrors and extremely sensitive cameras and can be used to create images of astronomical objects as they appear in gamma-rays.

The H.E.S.S. telescopes represent several years of construction effort by an international team of more than 100 scientists and engineers from Germany, France, the UK, Ireland, the Czech Republic, Armenia, South Africa and the host country Namibia. The instrument was inaugurated in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab, and its first data have already resulted in a number of important discoveries, including the first astronomical image of a supernova shock wave at the highest gamma-ray energies.

Original Source: PPARC News Release

One of the meteorites analyzed to help pinpoint the age of Milky Way. Image credit: Nicolas Dauphas, University of Chicago. Click to enlarge.
The University of Chicago?s Nicolas Dauphas has developed a new way to calculate the age of the Milky Way that is free of the unvalidated assumptions that have plagued previous methods. Dauphas? method, which he reports in the June 29 issue of the journal Nature, can now be used to tackle other mysteries of the cosmos that have remained unsolved for decades.

?Age determinations are crucial to a fundamental understanding of the universe,? said Thomas Rauscher, an assistant professor of physics and astronomy at the University of Basel in Switzerland. ?The wide range of implications is what makes Nicolas? work so exciting and important.?

Dauphas, an Assistant Professor in Geophysical Sciences, operates the Origins Laboratory at the University of Chicago. His wide-ranging interests include the origins of Earth?s atmosphere, the oldest rocks that may contain evidence for life on Earth and what meteorites reveal about the formation of the solar system.

In his latest work, Dauphas has honed the accuracy of the cosmic clock by comparing the decay of two long-lived radioactive elements, uranium-238 and thorium-232. According to Dauphas? new method, the age of the Milky Way is approximately 14.5 billion years, plus or minus more than 2 billion years.

That age generally agrees with the estimate of 12.2 billion years?nearly as old as the universe itself? as determined by previously existing methods. Dauphas? finding verifies what was already suspected, despite the drawbacks of existing methods: ?After the big bang, it did not take much time for large structures to form, including our Milky Way galaxy,? he said.

The age of 12 billion years for the galaxy relied on the characteristics of two different sets of stars, globular clusters and white dwarfs. But this estimate depends on assumptions about stellar evolution and nuclear physics that scientists have yet to substantiate to their complete satisfaction.

Globular clusters are clusters of stars that exist on the outskirts of a galaxy. The processes of stellar evolution suggested that most of the stars in globular clusters are nearly as old as the galaxy itself. When the big bang occurred 13.7 billion years ago, the only elements in the universe were hydrogen, helium and a small quantity of lithium. The Milky Way?s globular clusters have to be nearly that old because they contain mostly hydrogen and helium. Younger stars contain heavier elements that were recycled from the remains of older stars, which initially forged these heavier elements in their cores via nuclear fusion.

White dwarf stars, meanwhile, are stars that have used up their fuel and have advanced to the last stage of their lives. ?The white dwarf has no source of energy, so it just cools down. If you look at its temperature and you know how fast it cools, then you can approximate the age of the galaxy, because some of these white dwarfs are about as old as the galaxy,? Dauphas said.

A more direct way to calculate the age of stars and the Milky Way depends on the accuracy of the uranium/thorium clock. Scientists can telescopically detect the optical ?fingerprints? of the chemical elements. Using this capability, they have measured the uranium/thorium ratio in a single old star that resides in the halo of the Milky Way.

Original Source: University of Chicago News Release

The white dwarf star Gliese 86B is the tiny dot to the left of the bright star. Image credit: ESO. Click to enlarge.
The team has found that a star known as Gliese 86 – part of the southern constellation Erinadus, and just visible to the unaided eye – has another companion in addition to the gas giant planet that was found in a tight orbit around it seven years ago. However, this more distant companion is not another planet, but a white dwarf star that is about as far from Gliese 86 as is Uranus from the sun. The discovery marks the first time a planet has been found in the vicinity of a white dwarf, and could have implications for our own solar system – which will itself be centered around a white dwarf in a few billion years.

“This is the first observational evidence that planets can survive the white dwarf formation process of a star several astronomical units away,” said researcher team member Markus Mugrauer, a doctoral student at the Astrophysical Institute and University Observatory, University of Jena, Germany. “In theory, nearby planets should not survive the formation process, but this finding is evidence that, if they are sufficiently distant, they can. This is of interest because most stars in the galaxy, including our own, will eventually evolve into white dwarfs.”

The study, which Mugrauer conducted with Dr. Ralph Neuhaeuser, director of observations at the university’s astrophysics institute, were published as a letter in the May issue of “Monthly Notices of the Royal Astronomical Society.”

The planet itself was discovered in late 1998 at Switzerland’s La Silla observatory, and was the first exoplanet to be found using a telescope at La Silla that had been fitted with a spectrograph for the express purpose of searching for planets around other stars. Further analysis of Gliese 86’s movements indicated that the star also had a faint stellar companion that had not yet been observed, possibly a brown dwarf — an object with insufficient mass to sustain fusion in its core.

“No one was sure what it was, however,” Mugrauer said. “Just as the planet itself had been found by its influence on Gliese 86 but had not actually been ‘seen,’ the companion was tugging on the star but it was difficult to separate from background light.”

To resolve Gliese 86’s companion, the pair used high contrast observations using the 8m Very Large Telescope at La Silla together with a new simultaneous differential imaging device.

“With these instruments, we can resolve objects about 150,000 times fainter than the central star, but which are still very close to them,” Mugrauer said. “This allows us to search for close and very faint companions of our target stars.”

After filtering out the background noise, they found Gliese’s companion orbiting at a distance of about 21 AU, but were surprised to find it hotter than expected — at least 3700 Kelvin, too warm to be a brown dwarf. Judging by its velocity and distance from Gliese 86, they also found that the white dwarf has about 55 percent the mass of our sun, making it smaller than Gliese 86, which has 70 percent of our sun’s mass.

“But since a star loses a good deal of its mass as it evolves into a white dwarf, this companion was once much larger than Gliese 86, perhaps as large as our own sun or even larger,” Mugrauer said. “It was much closer to Gliese 86 before it became a white dwarf, perhaps 15 AU, or a distance about halfway between the orbits of Saturn and Uranus in our own system. It migrated outward after it lost mass during its evolution into a white dwarf.”

Because of the planet’s size and distance from the red giant, Mugrauer said, the companion’s evolution wouldn’t have dramatically affected the planet’s size.

“The planet’s gravity is simply too strong to lose mass because of the impacting material and due to its large separation,” he said. “However, during the red giant phase, the companion would have swollen up and become 10,000 more luminous. It would also have become the dominant heat source of the planet, heating it 1000K or more.”

Nowadays, he said, the companion would probably appear as a very bright star in the planet’s night sky, but would provide it with very little additional heat in comparison with Gliese 86, which the giant planet circles at about a tenth the distance of the Earth to the sun.

“We expect that distant planets — those farther than Jupiter is from our sun — can survive the evolution of a star from red giant to white dwarf. These observations tend to confirm that expectation,” Mugrauer said. “In the Gliese 86 system in particular, the separation between the white dwarf and the exoplanet is large enough that it seems very possible that a planet can survive the red giant phase of a G dwarf such as our sun.”

But Mugrauer said that he and Neuhaeuser would continue to search for companion stars in this and other exoplanetary systems because, despite the number of planets that have been found circling other stars, little is known about the properties of planets in binary systems. Planets in close binaries, like Gliese 86, are rare. “Gliese 86 is one of the closest binary systems hosting a planet,” Mugrauer said.

“These systems provide important information about the planet formation process and how the multiplicity of the host star may effect it,” he said. “Gliese 86 is only about 35 light years from earth, so it was near the top of our list of stars to explore. But we are on our way to checking out a lot more.”

Written by Chad Boutin

Artist illustratino of a planetary zone filled with pebbles. Image credit: CfA. Click to enlarge.
Interstellar travelers might want to detour around the star system TW Hydrae to avoid a messy planetary construction site. Astronomer David Wilner of the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues have discovered that the gaseous protoplanetary disk surrounding TW Hydrae holds vast swaths of pebbles extending outward for at least 1 billion miles. These rocky chunks should continue to grow in size as they collide and stick together until they eventually form planets.

“We’re seeing planet building happening right before our eyes,” said Wilner. “The foundation has been laid and now the building materials are coming together to make a new solar system.”

Wilner used the National Science Foundation’s Very Large Array to measure radio emissions from TW Hydrae. He detected radiation from a cold, extended dust disk suffused with centimeter-sized pebbles. Such pebbles are a prerequisite for planet formation, created as dust collects together into larger and larger clumps. Over millions of years, those clumps grow into planets.

“We’re seeing an important step on the path from interstellar dust particles to planets,” said Mark Claussen (NRAO), a co-author on the paper announcing the discovery. “No one has seen this before.”

A dusty disk like that in TW Hydrae tends to emit radio waves with wavelengths similar to the size of the particles in the disk. Other effects can mask this, however. In TW Hydrae, the astronomers explained, both the relatively close distance of the system and the stage of the young star’s evolution are just right to allow the relationship of particle size and wavelength to prevail. The scientists observed the young star’s disk with the VLA at several centimeter-range wavelengths. “The strong emission at wavelengths of a few centimeters is convincing evidence that particles of about the same size are present,” Claussen said.

Not only does TW Hydrae show evidence of ongoing planet formation, it also shows signs that at least one giant planet may have formed already. Wilner’s colleague, Nuria Calvet (CfA), has created a computer simulation of the disk around TW Hydrae using previously published infrared observations. She showed that a gap extends from the star out to a distance of about 400 million miles – similar to the distance to the asteroid belt in our solar system. The gap likely formed when a giant planet sucked up all the nearby material, leaving a hole in the middle of the disk.

Located about 180 light-years away in the constellation Hydra the Water Snake, TW Hydrae consists of a 10 million-year-old star about four-fifths as massive as the Sun. The protoplanetary disk surrounding TW Hydrae contains about one-tenth as much material as the Sun – more than enough to form one or more Jupiter-sized worlds.

“TW Hydrae is unique,” said Wilner. “It’s nearby, and it’s just the right age to be forming planets. We’ll be studying it for decades to come.”

This research was published in the June 20, 2005, issue of The Astrophysical Journal Letters.

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

Simulation of interstellar grains of dust. Image credit: OSU. Click to enlarge.
Science fiction writer Harlan Ellison once said that the most common elements in the universe are hydrogen and stupidity.

While the verdict is still out on the volume of stupidity, scientists have long known that hydrogen is indeed by far the most abundant element in the universe. When they peer through their telescopes, they see hydrogen in the vast clouds of dust and gas between stars ?- especially in the denser regions that are collapsing to form new stars and planets.

But one mystery has remained: why is much of that hydrogen in molecular form ?- with two hydrogen atoms bonded together ?- rather than its single atomic form? Where did all that molecular hydrogen come from? Ohio State University researchers recently decided to try to figure it out.

They discovered that one seemingly tiny detail — whether the surfaces of interstellar dust grains are smooth or bumpy — could explain why there is so much molecular hydrogen in the universe. They reported their results at the 60th International Symposium on Molecular Spectroscopy, held at Ohio State University .

Hydrogen is the simplest atomic element known; it consists of just one proton and one electron. Scientists have always taken for granted the existence of molecular hydrogen when forming theories about where all the larger and more elaborate molecules in the universe came from. But nobody could explain how so many hydrogen atoms were able to form molecules — until now.
When it comes to making molecular hydrogen, the ideal microscopic host surface is ?less like the flatness of Ohio and more like a Manhattan skyline.?

For two hydrogen atoms to have enough energy to bond in the cold reaches of space, they first have to meet on a surface, explained Eric Herbst, Distinguished University Professor of physics at Ohio State.

Though scientists suspected that space dust provided the necessary surface for such chemical reactions, laboratory simulations of the process never worked. At least, they didn’t work well enough to explain the full abundance of molecular hydrogen that scientists see in space.

Herbst, professor of physics, chemistry, and astronomy, joined with Herma Cuppen, a postdoctoral researcher, and Qiang Chang, a doctoral student, both in physics, to simulate different dust surfaces on a computer. They then modeled the motion of two hydrogen atoms tumbling along the different surfaces until they found one another to form a molecule.

Given the amount of dust that scientists think is floating in space, the Ohio State researchers were able to simulate the creation of the right amount of hydrogen, but only on bumpy surfaces.

When it comes to making molecular hydrogen, the ideal microscopic host surface is ?less like the flatness of Ohio and more like a Manhattan skyline,? Herbst said.

The problem with past simulations, it seems, is that they always assumed a flat surface.

Cuppen understands why. ?When you want to test something, starting with a flat surface is just faster and easier,? she said

She should know. She’s an expert in surface science, yet it still took her months to assemble the bumpy dust model, and she’s still working to refine it. Eventually, other scientists will be able to use the model to simulate other chemical reactions in space.

In the meantime, the Ohio State scientists are collaborating with colleagues at other institutions who are producing and using actual bumpy surfaces that mimic the texture of space dust. Though real space dust particles are as small as grains of sand, these larger, dime-sized surfaces will enable scientists to test whether different textures help molecular hydrogen to form in the lab.

Original Source: OSU News Release

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