Universe Today

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