Tuesday, August 28, 2012

Space-Warping White Dwarfs Produce Gravitational Waves

An international team, including the University of Texas, Harvard Smithsonian Astrophysical Observatory (SAO and the Instituto de Astrofisica de Canarias (IAC), has discovered that white dwarfs orbiting binary system J0651 are getting faster as predicted by general relativity theory of Einstein, and losing the ability to emit gravitational waves.


Credit: NASA

Gravitational waves, much like the recently discovered Higgs boson, are notoriously difficult to observe. Scientists first detected these ripples in the fabric of space-time indirectly, using radio signals from a pulsar-neutron star binary system. The find, which required exquisitely accurate timing of the radio signals, garnered its discoverers a Nobel Prize. Now a team of astronomers has detected the same effect at optical wavelengths, in light from a pair of eclipsing white dwarf stars.

"This result marks one of the cleanest and strongest detections of the effect of gravitational waves," said team member Warren Brown of the Smithsonian Astrophysical Observatory (SAO).

The team discovered the white dwarf pair last year. (White dwarfs are the remnant cores of stars like our Sun.) The system, called SDSS J065133.338+284423.37 (J0651 for short), contains two white dwarf stars so close together -- one-third of the Earth-moon distance -- that they make a complete orbit in less than 13 minutes.

Illustration of gravitational waves
Larger image


"Every six minutes the stars in J0651 eclipse each other as seen from Earth, which makes for an unparalleled and accurate clock some 3,000 light-years away," said study lead author J.J. Hermes, a graduate student working with Professor Don Winget at The University of Texas at Austin.

Einstein's theory of general relativity predicts that moving objects create subtle ripples in the fabric of space-time, called gravitational waves. Gravitational waves should carry away energy, causing the stars to inch closer together and orbit each other faster and faster. The team was able to detect this effect in J0651.

"Compared to April 2011, when we discovered this object, the eclipses now happen six seconds sooner than expected," said team member Mukremin Kilic of The University of Oklahoma.

"This is a general relativistic effect you could measure with a wrist watch," added SAO's Warren Brown.

The discovery of two white dwarfs about to merge allow scientists to test the General Theory of Relativity of Einstein.As shown in this picture (an artist's view), within 900,000 years the material will start to "travel" from one to the other, which could end in a supernova explosion. 
Larger image
Credit: David A. Aguilar / Harvard-Smithsonian Center for Astrophysics (CfA)

J0651 will provide an opportunity to compare future direct, space-based detection of gravitational waves with those inferred from the orbital decay, providing important benchmark tests of our understanding of the workings of gravity.

The team expects that the period will shrink more and more each year, with eclipses happening more than 20 seconds sooner than otherwise expected by May 2013. The stars will eventually merge, in two million years. Future observations will continue to measure the orbital decay of this system, and attempt to understand how tides affect the merger of such stars.

The team's results will be published in The Astrophysical Journal Letters and are available online.
The confirmation of the theoretical predictions has been made with several telescopes in the United States and with the GTC, which made the observations of the system faster.

The international team has now tested the theory of Einstein's general relativity by observing the progressive reduction of the orbit of a singular pair of stars: white dwarf binary system J0651. These two objects, remnants of Sun-like stars that have exhausted their nuclear fuel, currently completing an orbit every 13 minutes, with high accelerations and speeds that can reach over 600 miles per second, according to the group just released research in the journalAstrophysical Journal Letters.

According to the theory of Einstein's general relativity, the acceleration of these stars in its orbital motion cause ripples in the fabric of space-time, called gravitational waves. Although not yet directly observed, the emission of these waves takes energy to binary, which causes white dwarfs gradually approach each other and getting faster orbit.

The theory of relativity predicts that the orbit of this binary system is reduced by about 0.25 milliseconds each year. Confirmation that the stars are getting closer is given by comparing the measures taken in 2011 , when the team of astrophysicists discovered this system, with those taken today.

The GTC, the largest infrared optical telescope in the world, with a primary mirror 10.4 meters, located at the Observatorio del Roque de los Muchachos on La Palma, provided the data set with the fastest rate of these objects."Thanks to the GTC, we managed to take hundreds of pictures in a row of this interesting system without missing a beat," said Carlos Allende Prieto, IAC researcher and one of the study authors.

For Antonio Cabrera Lavers, GTC astronomer and co-author: "This is one of those cases where we have the opportunity to use telescopes to test our understanding of the most fundamental aspects of physics."

A peculiar system

J0651 fifth binary system is known with an orbital period of less than 15 minutes. In the other four cases, however, mass transfer occurs from one star to another, causing brightness variations and complicating observations of reduced orbital period and their interpretation in terms of gravitational waves.

This binary system is also peculiar in its orientation relative to the earth, as the orbital plane is aligned with our line of sight. "Every six minutes, one of the stars in J0651 eclipses the other, providing extremely accurate clock to 3,000 light years," says doctoral student at the University of Texas at Austin (USA), and first author Article, JJ Hermes. "Now eclipses occur about six seconds earlier than expected from measures months ago," says the professor of the University of Oklahoma and a member of the team, Mukremin Kilic.

The results of this study have been possible thanks to the more than 200 hours of observations, in addition to the GTC, with the 2.1-meter telescope Otto Stuve, at the McDonald Observatory in Texas, with the Gemini telescope of 8.2 meters, in Hawaii, and with the 3.5-meter telescope at Apache Point Observatory in New Mexico, all located in the U.S..

Directly detect gravitational waves is extremely difficult. Specifically, measuring the effect of gravitational waves produced by the solar system J0651 from satellites require several millions of miles and reported by lasers. Although physicists have spent years planning a system of this type, there is still no definite space mission and funding of these features.

"Thus we have an easier, albeit indirect, to detect the effects of gravitational waves," says Allende Prieto.

Einstein’s theory of general relativity  

The team of astronomers led by researchers from The University of Texas at Austin has confirmed the emission of gravitational waves from the second-strongest known source in our galaxy by studying the shrinking orbital period of a unique pair of burnt-out stars. Their observations tested Albert Einstein's theory of general relativity in a new regime. The results will be published soon in The Astrophysical Journal Letters.

Einstein’s theory of general relativity predicts that moving objects create subtle ripples in the fabric of space-time, called gravitational waves. Though not yet directly observed, gravitational waves should carry away energy, causing the stars to inch closer together and orbit each other faster and faster.

Evidence for gravitational waves
Credit: McDonald Observatory

"Every six minutes the stars in J0651 eclipse each other as seen from Earth, which makes for an unparalleled and accurate clock some 3,000 light-years away," said study lead author J.J. Hermes, a graduate student working with Professor Don Winget at The University of Texas at Austin.

Einstein's theory predicts that the orbital period of this binary system loses about 0.25 milliseconds every year, less than one-thousandth of a second.

The team has just tested that prediction using more than 200 hours of observations from the 2.1-meter Otto Struve Telescope at the university’s McDonald Observatory in West Texas, the 8.2-meter Gemini North Telescope in Hawaii, the 10.4-meter Gran Telescopio Canarias in the Canary Islands of Spain, and the 3.5-meter Apache Point telescope in New Mexico.

"Compared to April 2011, when we discovered this object, the eclipses now happen six seconds sooner than expected," said team member Mukremin Kilic of The University of Oklahoma.

This confirms that the two stars are getting closer and that the orbital period is shrinking at nearly 0.25 ms each year. By April 2013, the eclipses should happen roughly 20 seconds sooner than they did relative to the group's first observations in April 2011.

"These compact stars are orbiting each other so closely that we have been able to observe the usually negligible influence of gravitational waves using a relatively simple camera on a 75-year-old telescope in just 13 months," added Hermes. The Struve Telescope, which came into service in the late 1930s, was the first at McDonald Observatory.

Astronomers know of just four other binary systems with orbits under 15 minutes, and all of those systems are transferring mass from one star to the other, which complicates observations of orbital decay and the interpretation of these changes in terms of gravitational waves.

"This result marks one of the cleanest and strongest detection of the effect of gravitational waves," said team member Warren Brown of the Smithsonian Astrophysical Observatory.

The direct detection of gravitational waves is notoriously hard. Gravitational waves from J0651 are predicted to change two points in space an inch apart by less than a billionth of a trillionth of an inch. To detect such a tiny effect requires satellites that shoot lasers at each other from millions of miles apart. No such mission is currently funded by NASA or the European Space Agency.

"Here we have an easier way to detect the effects of gravitational waves, though indirectly," added team member Carlos Allende Prieto of the Instituto de Astrofísica de Canarias.

J0651 will provide an opportunity to compare future direct, space-based detection of gravitational waves with those inferred from the orbital decay, providing important benchmark tests of our understanding of the workings of gravity.

The team expects that the period will shrink each year, with eclipses happening more than 20 seconds sooner than expected by May 2013. The stars will eventually merge. Future observations will continue to measure the orbital decay of this system, and attempt to understand how tides affect the mergers of such stars.

"It's exciting to confirm predictions Einstein made nearly a century ago by watching two stars bobbing in the wake caused by their sheer mass," Hermes said. The two stars in this system are both less massive than our sun; one has half the sun's mass and the other other a quarter.



Contact and sources:
Harvard Smithsonian Astrophysical Observatory
Instituto de Astrofisica de Canarias (IAC)
Carlos Allende Prieto (IAC)
Antonio Cabrera Lavers (GTC, IAC)
University of Texas
McDonald Observatory

Related links:
Publication: http://adsabs.harvard.edu/abs/2012arXiv1208.5051H
CaosyCiencia items (includes video): http://www.caosyciencia.com/ideas/articulo.php?id=310112

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