self lensing binary starThe first self-lensing binary star system, one in which the mass of the closer star can be measured by how powerfully it magnifies light from its more distant companion star, has been confirmed by researchers at the University of Washington.

The possibility of such a system was predicted by an astronomer in 1973, working with stellar evolution models of the time.

The discoveries happened mainly by accident. What initially appeared to be a sort of upside-down planet instead uncovered a new technique to study binary star systems. Our own sun stands alone, but around 40 percent of similar stars are in binary two-star (binary) or multi-star systems, orbiting their companions in a gravitational dance.

Star System KOI-3278

Astronomers become aware of planets too far away for direct observation via the dimming of light when a world passes in front of, or transits, its home star. The UW’s Ethan Kruse, a doctoral student, was searching for transits other astronomers may have overlooked, using data from the Kepler Space Telescope. He saw something in binary star system KOI-3278 that didn’t make sense.

“I found what essentially looked like an upside-down planet,” Kruse said. “What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite — it looks like an anti-transit.”

KOI-3278’s two stars are about 2,600 light-years (one light-year being equal to 5.88 trillion miles) away from Earth, in the Lyra constellation. They take turns being nearer to Earth as they orbit each other every 88.18 days.

The stars are about 43 million miles apart, around same the distance the planet Mercury is from the sun. The white dwarf, a cooling star thought to be in the final stage of life, is about Earth’s size but 200,000 times more massive.

Gravitational Lensing

The increase in light, instead of than the dip Kruse thought he’d see, was the white dwarf bending and magnifying light from its more distant neighbor through gravitational lensing, like a magnifying glass.

“The basic idea is fairly simple,” UW astronomer Eric Agol said. “Gravity warps space and time and as light travels toward us it actually gets bent, changes direction. So, any gravitational object — anything with mass — acts as a magnifying glass,” though a weak one. “You really need large distances for it to be effective.”

“The cool thing, in this case, is that the lensing effect is so strong, we are able to use that to measure the mass of the closer, white dwarf star. And instead of getting a dip now you get a brightening through the gravitational magnification.”

A common tool in astronomy, gravitational lensing has been used to detect planets around distant stars within the Milky Way galaxy, and was among the first methods used to confirm Albert Einstein’s general theory of relativity. Lensing within the Milky Way galaxy, such as this, is called microlensing.

But until now, the process had only been used in the fleeting instances of a nearby and distant star, not otherwise associated in any way, aligning just right, before going their separate ways again.

“The chance is really improbable,” said Agol. “As those two stars go through the galaxy they’ll never come back again, so you see that microlensing effect once and it never repeats. In this case, though, because the stars are orbiting each other, it repeats every 88 days.”

White dwarfs are significant in astronomy. They are used as indicators of age in the galaxy, according to the UW researchers. Essentially embers of burned-out stars, white dwarfs cool off at a specific rate over time. With this lensing, astronomers can learn with much greater precision what its mass and temperature are, and follow-up observations may yield its size.


E. Kruse, E. Agol.
KOI-3278: A Self-Lensing Binary Star System.
Science, 2014; 344 (6181): 275 DOI: 10.1126/science.1251999

graphene carbon spaserThe first ever spaser to be made completely of carbon has been modelled by researchers from Monash University. The technology could lead to mobile phones becoming so small, efficient, and flexible they could be printed on clothing.

A spaser (an acronym for Surface Plasmon Amplification by Stimulated Emission of Radiation) is essentially a nanoscale laser or nanolaser. It emits a beam of light through the vibration of free electrons, instead of the space-consuming electromagnetic wave emission process of a traditional laser.

“Other spasers designed to date are made of gold or silver nanoparticles and semiconductor quantum dots while our device would be comprised of a graphene resonator and a carbon nanotube gain element,” said lead researcher Chanaka Rupasinghe. “The use of carbon means our spaser would be more robust and flexible, would operate at high temperatures, and be eco-friendly. Because of these properties, there is the possibility that in the future an extremely thin mobile phone could be printed on clothing.”

Graphene and Carbon Nanotubes

Spaser-based devices are able be used as a substitute for current transistor-based devices like microprocessors, memory, and displays in order to surmount current miniaturisation and bandwidth limitations.

The researchers opted to develop their spaser using graphene and carbon nanotubes. These materials are more than a hundred times stronger than steel and can conduct heat and electricity much better than copper. They can also withstand higher temperatures.

The research demonstrated for the first time that graphene and carbon nanotubes can interact and transfer energy to each other through light. These optical interactions are very fast and energy-efficient, and so are suitable for applications such as computer chips.

“Graphene and carbon nanotubes can be used in applications where you need strong, lightweight, conducting, and thermally stable materials due to their outstanding mechanical, electrical and optical properties. They have been tested as nanoscale antennas, electric conductors and waveguides,” Chanaka said.

Spasers generate high-intensity electric fields, concentrated into a nanoscale space. The fields are much stronger than those generated by illuminating metal nanoparticles by a laser in applications such as cancer therapy.

“Scientists have already found ways to guide nanoparticles close to cancer cells. We can move graphene and carbon nanotubes following those techniques and use the high concentrate fields generated through the spasing phenomena to destroy individual cancer cells without harming the healthy cells in the body,” Chanaka said.


Spaser Made of Graphene and Carbon Nanotubes.
Chanaka Rupasinghe, Ivan D. Rukhlenko, and Malin Premaratne.
ACS Nano 2014 8 (3), 2431-2438. DOI: 10.1021/nn406015d

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