January 28, 2008
MAUNA KEA (January 28th, 2008) First results from a new scientific instrument at W. M. Keck Observatory are helping scientists understand the physics behind recurrent novae, a type of cataclysmic star system. The results are overturning long-standing assumptions about powerful explosions called novae and have produced the first unified model for a nearby nova called RS Ophiuchi.
“We were getting ready for a routine engineering run when all of a sudden the nova went off. It was very bright and easy to observe, so we took this opportunity and turned it into gold,” says team member Marc Kuchner of NASA’s Goddard Space Flight Center in Greenbelt, Md.
The “nulling” mode of the Keck Interferometer is part of the NASA-funded Keck Interferometer, which combines starlight using two 10-meter (33 feet) telescopes. In this mode, the interferometer suppresses the blinding light of a star so researchers can study the surrounding environment. The instrument helps researchers observe very faint objects near bright sources and produces ten times more resolving power than a single Keck telescope working alone. It is the only instrument of its kind in operation.
The nulling mode was developed to search for dust around nearby stars, which make finding planets around these stars more difficult. “If the dust were by itself, it would be easy to detect with Keck,” explains Dr. Rachel L. Akeson, Keck Interferometer project scientist at the Michelson Science Center at Caltech. “But the star is so much brighter, that something has to be done to block the light, which is what the nuller does. But this technique turns out to be useful for lots of other kinds of objects, including this one, where dust is near a star that just went nova.”
The Keck Nuller was undergoing tests February 12, 2006, when a nova flared up in the constellation Ophiuchus. The system, known as RS Ophiuchi, consists of a white dwarf and a red giant. The red giant is gradually shedding its massive gaseous outer layers, and the white dwarf is sweeping up much of this wind, growing in mass over time. As the matter builds up on the white dwarf’s surface it eventually reaches a critical point that ignites a thermonuclear explosion that causes the system to brighten 600-fold. RS Ophiuchi was previously seen to blow its stack in 1898, 1933, 1958, 1967, and 1985, so astronomers were eagerly anticipating the 2006 eruption.
Just 3.8 days after the nova was detected, the group observed the explosion with the Keck Nuller. The team set the instrument to cancel out the nova’s light, allowing the group to see the much fainter surrounding material. The group next adjusted the nuller to observe the extremely bright blast zone.
The instrument’s versatility was key to a surprising discovery. The nuller saw no dust in the bright zone, presumably because the nova’s blast wave vaporized dust particles. But farther from the white dwarf, at distances starting around 20 times the Earth-Sun distance, the nuller recorded the spectral signature of silicate dust. The blast wave had not yet reached this zone, so the dust must have pre-dated the explosion.
“This flies in the face of what we expected. Astronomers had previously thought that nova explosions actually create dust,” says Richard Barry of NASA Goddard, lead author of a paper on the Keck observations that will be published in the Astrophysical Journal. The team thinks the dust is created as the white dwarf plows through the red giant’s wind, creating a pinwheel pattern of higher-density regions that is reminiscent of galaxy spiral arms. Inside these spiral arms, atoms reach low enough temperatures and high enough densities to allow atoms to stick together to form dust particles. The nova’s blast wave has since destroyed RS Ophiuchi’s pinwheel pattern, but it should re-form over the next few years, and future Spitzer Space Telescope observations could see it.
Most studies of RS Ophiuchi have relied on spectroscopic models, but those methods have not been able to distinguish various nova components with as much detail as the interferometer. The Keck Nuller measured one component of the RS Ophiuchi system to an accuracy of just 4 milliarcseconds, or about the size of a basketball at 7,500 miles. The findings led to a new physical model of the system.
Barry is also coauthor of a paper based on Spitzer observations of RS Ophiuchi. This paper, which appeared in the December 20, 2007 issue of Astrophysical Journal, reports independent evidence for silicate dust that predates the 2006 explosion.
“The RS Ophiuchi observations are just a small taste of the power and potential we expect from the Keck Nuller,” says coauthor William Danchi of NASA Goddard. “But ultimately we want to launch a nulling interferometer into space to image extrasolar planets. These Keck results are a technological and scientific pathfinder toward that future.”
The paper, “Milliarcsecond N-Band Observations of the Nova RS Ophiuchi: First Science with the Keck Interferometer Nuller” will be published in the May 1st issue of the Astrophysical Journal, with co-authors from Goddard, Jet Propulsion Laboratory, Michelson Science Center, W. M. Keck Observatory and Columbia University.
Observations were conducted at the W. M. Keck Observatory (http://www.keckobservatory.org) in Hawaii, a non-profit 501 (c) (3) organization. The Michelson Science Center manages the Keck Interferometer’s science operations for NASA’s Science Mission Directorate from its offices at Caltech in coordination with the Jet Propulsion Laboratory. The W. M. Keck Observatory is governed by a board 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. For more information about the Keck Interferometer Nuller, visit: http://msc.caltech.edu/ missions/KI/.
Astrophysical Journal preprint: Milliarcsecond N-Band Observations of the Nova RS Ophiuchi: First Science with the Keck Interferometer Nuller