Universe Today

Image credit: NASA

It’s possible that the spin rate of pulsars is limited by gravitational radiation according to new data gathered by NASA’s Rossi X-ray Timing Explorer – a phenomenon predicted by Albert Einstein. Pulsars are the core remains of exploded stars, no larger than 15 kilometres across, and some rotate as fast as once/millisecond. Scientists believe that as a pulsar speeds up, it flattens out, and the distortions in its shape cause it to emanate waves of gravity which stop it from rotating so fast it flies apart.

Gravitational radiation, ripples in the fabric of space predicted by Albert Einstein, may serve as a cosmic traffic enforcer, protecting reckless pulsars from spinning too fast and blowing apart, according to a report published in the July 3 issue of Nature.

Pulsars, the fastest spinning stars in the Universe, are the core remains of exploded stars, containing the mass of our Sun compressed into a sphere about 10 miles across. Some pulsars gain speed by pulling in gas from a neighboring star, reaching spin rates of nearly one revolution per millisecond, or almost 20 percent light speed. These “millisecond” pulsars would fly apart if they gained much more speed.

Using NASA’s Rossi X-ray Timing Explorer, scientists have found a limit to how fast a pulsar spins and speculate that the cause is gravitational radiation: The faster a pulsar spins, the more gravitational radiation it might release, as its exquisite spherical shape becomes slightly deformed. This may restrain the pulsar’s rotation and save it from obliteration.

“Nature has set a speed limit for pulsar spins,” said Prof. Deepto Chakrabarty of the Massachusetts Institute of Technology, lead author on the journal article. “Just like cars speeding on a highway, the fastest-spinning pulsars could technically go twice as fast, but something stops them before they break apart. It may be gravitational radiation that prevents pulsars from destroying themselves.”

Chakrabarty’s co-authors are Drs. Edward Morgan, Michael Muno, and Duncan Galloway of MIT; Rudy Wijnands, University of St. Andrews, Scotland; Michiel van der Klis, University of Amsterdam; and Craig Markwardt, NASA Goddard Space Flight Center. Wijnands also leads a second Nature letter complementing this finding.

Gravitational waves, analogous to waves upon an ocean, are ripples in four-dimensional spacetime. These exotic waves, predicted by Einstein’s theory of relativity, are produced by massive objects in motion and have not yet been directly detected.

Created in a star explosion, a pulsar is born spinning, perhaps 30 times per second, and slows down over millions of years. Yet if the dense pulsar, with its strong gravitational potential, is in a binary system, it can pull in material from its companion star. This influx can spin up the pulsar to the millisecond range, rotating hundreds of times per second.

In some pulsars, the accumulating material on the surface occasionally is consumed in a massive thermonuclear explosion, emitting a burst of X-ray light lasting only a few seconds. In this fury lies a brief opportunity to measure the spin of otherwise faint pulsars. Scientists report in Nature that a type of flickering found in these X-ray bursts, called “burst oscillations,” serves as a direct measure of the pulsar’s spin rate. Studying the burst oscillations from 11 pulsars, they found none spinning faster than 619 times per second.

The Rossi Explorer is capable of detecting pulsars spinning as fast as 4,000 times per second. Pulsar break-up is predicted to occur at 1,000 to 3,000 revolutions per second. Yet scientists have found none that fast. >From statistical analysis of 11 pulsars, they concluded that the maximum speed seen in nature must be below 760 revolutions per second.

This observation supports the theory of a feedback mechanism involving gravitational radiation limiting pulsar speeds, proposed by Prof. Lars Bildsten of the University of California, Santa Barbara. As the pulsar picks up speed through accretion, any slight distortion in the star’s dense, half-mile-thick crust of crystalline metal will allow the pulsar to radiate gravitational waves. (Envision a spinning, oblong rugby ball in water, which would cause more ripples than a spinning, spherical basketball.) An equilibrium rotation rate is eventually reached where the angular motion shed by emitting gravitational radiation matches the angular momentum being added to the pulsar by its companion star.

Bildsten said that accreting millisecond pulsars could eventually be studied in greater detail in an entirely new way, through the direct detection of their gravitational radiation. LIGO, the Laser Interferometer Gravitational-Wave Observatory now in operation in Hanford, Washington, and in Livingston, Louisiana, will eventually be tunable to the frequency at which millisecond pulsars are expected to emit gravitational waves.

“The waves are subtle, altering spacetime and the distance between objects as far apart as the Earth and the Moon by much less than the width of an atom,” said Prof. Barry Barish of the California Institute of Technology, the LIGO director. “As such, gravitational radiation has not been directly detected yet. We hope to change that soon.”

Original Source: NASA News Release

Image credit: NASA

Astronomers have directly observed a hot disc of dust and gas surrounding a protostar using the twin W.M. Keck telescopes in Hawaii. This is the first published science observation using a technology called interferometry, which combines the light from several telescopes to act as a larger observatory – the twin 10-metre Keck telescopes act as a virtual 85 metre telescope. The observation was of DG Tau, a T-Tauri object which is so young its centre star hasn’t begun burning hydrogen; it’s surrounded by a disc of dust and gas that could form planets.

Astronomers have observed a young star ringed by a swirling disc that may spin off planets, marking the first published science observation using two linked 10-meter (33- foot) telescopes in Hawaii.

The linked telescopes at the W.M. Keck Observatory on Mauna Kea, known as the Keck Interferometer, comprise the world’s largest optical telescope system. The observation was made of DG Tau, a young star that has not yet begun to burn hydrogen in its core. Such stars are called T-Tauri objects. Observations of DG Tau were made on October 23, 2002, and February 13, 2003, and the findings will appear in an upcoming issue of the Astrophysical Journal Letters.

“We’re trying to measure the size of the hot material in the dust disc around DG Tau, where planets may form,” said Dr. Rachel Akeson, leader of the study team and an astronomer at the Michelson Science Center at the California Institute of Technology in Pasadena. “Studies like this teach us more about how stars form, either alone or in pairs, and how planets eventually form in discs around stars.”

The Keck Interferometer observations revealed a gap of 18 million miles between DG Tau and its orbiting dust disc. Akeson notes that of the extra-solar planets – planets orbiting other stars – discovered so far, roughly one in four lies within 10 million miles of the parent star. Since planets are believed to form within a dust disc, either DG Tau’s disc has a larger-than-usual gap, or the close-in planets form farther from the star and migrate inward.

Since 1995, astronomers have detected more than 100 extra- solar planets, many considered too large and close to their hot, parent stars to sustain life. By measuring the amount of dust around other stars, where planets may form, the Keck Interferometer will pave the way for NASA’s Terrestrial Planet Finder mission. Terrestrial Planet Finder will look for smaller, Earth-like planets that may harbor life. The Keck Interferometer and Terrestrial Planet Finder are part of NASA’s Origins Program, which seeks to answer the questions: Where did we come from? Are we alone?

“T-Tauri objects had been observed with other instruments, but only the brightest ones were detectable until now,” Akeson said. “With the larger telescopes and greater sensitivity of the Keck Interferometer, we can look at fainter T-Tauri objects, like this one.”

The Keck Interferometer gathers light waves with two telescopes and then combines the waves so they interact, or “interfere” with each other. It’s like throwing a rock into a lake and watching the ripples, or waves, and then throwing in a second rock. The second set of waves either bumps against the first set and changes its pattern, or both sets join together to form larger, more powerful waves. With interferometry, the idea is to combine light waves from multiple telescopes to simulate a much larger, more powerful telescope.

In its ability to resolve fine details, the Keck Interferometer is equivalent to an 85-meter (279-foot) telescope. “The system transports the light gathered by the two telescopes to an optical laboratory located in the central building,” said Dr. Mark Colavita of NASA’s Jet Propulsion Laboratory (JPL), Pasadena, interferometer system architect and lead author of the paper. “In the lab, a beam combiner and infrared camera combine and process the collected light to make the science measurement.”

To make these measurements, the interferometer’s optical system adjusts the light paths to a fraction of a wavelength of light, and adaptive optics on the telescopes remove the distortion caused by Earth’s atmosphere.

“This research represents the first scientific application of an interferometer with telescopes that use adaptive optics,” said Dr. Peter Wizinowich, interferometer team lead for the W.M. Keck Observatory and co-author of the paper.

The development of the Keck Interferometer is managed by JPL for NASA’s Office of Space Science, Washington. JPL is a division of the California Institute of Technology in Pasadena. The W.M. Keck Observatory is funded by Caltech, the University of California and NASA, and is managed by the California Association for Research in Astronomy, Kamuela, Hawaii.

Original Source: NASA News Release