Jul 13 2007
The National Science Foundation (NSF) has awarded the W. M. Keck Observatory $2 million to improve the sensitivity and resolution of the Keck Interferometer.
The improvements will enable the instrument to detect Jupiter-sized planets around other stars and test predictions of Einstein's general theory of relativity in the chaotic core of our galaxy.
The three-year grant is from NSF's Major Research Instrumentation Program (MRI), which each year funds more than 200 proposals to develop or purchase scientific instrumentation. Typically, less than one-half of one percent of all submitted proposals receive the maximum award of $2 million, and only a couple go to astronomical observatories.
“The interferometer improvements will make Keck Observatory a unique instrument for measuring the position, velocity and acceleration of stars near the massive black hole at the center of our own galaxy, allowing us to look for the distortions in space predicted by general relativity,” said Principal Investigator Dr. Peter Wizinowich, a senior scientist at the W. M. Keck Observatory.
The money will be used to boost the sensitivity of the 85-m baseline Keck Interferometer which combines the light from the two 10-meter diameter Keck telescopes. Combined with Laser Guide Star Adaptive Optics on both Keck telescopes, the improvements will allow the linked Keck telescopes to observe objects 100 times fainter than the existing interferometer and measure the apparent positions of celestial objects with 10 times more accuracy than a single Keck telescope working alone.
Observations near the black hole at the center of the galaxy “are at the core of the project and will be difficult and technically challenging,” said Project Scientist James R. Graham, professor of astronomy at the University of California, Berkeley.
Even before this goal is achieved, the upgrades will allow the Keck Interferometer to help determine the mass of extra-solar planets by measuring the periodic change in the position of parent stars caused by the tug of unseen planets. Currently, more than 200 extra-solar planets have been detected due to the radial velocity or “wobble effect” they induce on their parent star. About two-thirds of all known extra-solar planets have been confirmed at the W. M. Keck Observatory. The Interferometer will add precise orbital measurements to the existing catalogue of radial velocity data to help precisely determine the mass of extra-solar planets the size of Jupiter and larger.
Extra-solar planets and dusty stellar disk observations were a major goal of the interferometer when NASA's Origins program funded its development in support of upcoming planet-finding missions. Now that NASA has temporarily shelved these missions—the Space Interferometer Mission and the Terrestrial Planet Finder— the Keck Interferometer will be the only instrument in the world capable of measuring accurate masses for planets around distant stars.
The grant funds two major telescope improvements: installation of a phase referencing system on the interferometer that will allow longer exposures, and thus detection of fainter objects; and upgrading of the interferometer to be able to perform accurate measurements of a star's position.
Phase referencing is an interferometric technique in which two or more receivers simultaneously look at the same reference star or galaxy and compares signals. The process makes it possible for the instrument to adjust the signal from each telescope just the right amount to cancel out any wavelength differences caused by atmospheric turbulence, or “seeing.”
Phase referencing provides a stable image which will allow the Keck Interferometer to track an object 100 to 500 times longer than before. The extended exposure time will allow the instrument to study much fainter objects, such as the cores of active galactic nuclei that signal the presence of a central black hole.
The second part of the project will develop the interferometer's ability to accurately measure positions of celestial objects. Improvements will be made to the existing metrology systems and the instrument's ability to accurately measure the relative positions of the two Keck telescopes.
A key goal of the project, Dr. Wizinowich added, is to demonstrate the power of combining laser guide star adaptive optics with interferometry to observe faint science objects.
With the added improvements, the Keck Interferometer will resolve objects on the sky to an accuracy of 30 microarcseconds, compared to about 300-microarcsecond resolution of each telescope alone. Such fine measurements will allow scientists to measure the velocities of stars orbiting the black hole at the center of the galaxy.
The hope, Dr. Graham said, is to detect in the stellar orbits the effect of the dragging of “inertial reference frames” predicted to occur near a rapidly rotating black hole. This effect is predicted by Newton's laws of motion for mass located very near a spinning black hole. Scientists using the Keck Interferometer may be able to see this effect, which would be major breakthrough in tests of general relativity and other theories of gravity. The observations could also prove that black holes spin, thus constraining theories of their formation.
“This is a major opportunity to show astronomers what interferometry can do for them,” Wizinowich said. “Every time astronomers look in more detail at the sky, they learn something new.”
Scientific collaborators on the NSF proposal for development of the Keck Interferometer with Laser Guide Star Adaptive Optics for Microarcsecond Astronmetry—from Exoplanets to Black Holes— include: Dr. Julien Woillez at the W. M. Keck Observatory, Dr. Andrea Ghez at the University of California at Los Angeles; Dr. Rachel Akeson and Dr. Lynne Hillenbrand of the California Institute of Technology; Dr. Josh Eisner and Dr. Eliot Quataert of University of California at Berkeley; Dr. Nevin Weinberg, University of California at Santa Barbara and Dr. John Monnier at the University of Michigan.
The W. M. Keck Observatory is operated by the California Association for Research in Astronomy (CARA), a non-profit 501 (c) (3) corporation whose governing board consists of directors from the California Institute of Technology and the University of California. In addition, the National Aeronautics and Space Administration and the W. M. Keck Foundation each have liaisons to the board. Construction of the twin Keck telescopes and domes was made possible with generous grants totaling more than $140 million from the W. M. Keck Foundation in Los Angeles.