July 25, 2006
Patchy Red Giants are Common Fate of Sun-like Stars
Kamuela, Hawaii -- As astronomers increasingly link two telescopes as interferometers to reveal greater detail of distant stars, a Keck Observatory astronomer is showing the power of linking three or even more telescopes together.
Astronomer Sam Ragland used Arizona's Infrared-Optical Telescope Array (IOTA) of three linked telescopes to obtain unprecedented detail of old red giant stars that represent the eventual fate of the Sun.
Surprisingly, he found that nearly a third of the red giants he surveyed were not uniformly bright across their face, but were patchy, perhaps indicating large spots or clouds analogous to sunspots, shock waves generated by pulsating envelopes, or even planets.
"The typical belief is that stars have to be symmetric gas balls," said Ragland, an interferometer specialist. "But 30 percent of these red giants showed asymmetry, which has implications for the last stages of stellar evolution, when stars like the Sun are evolving into planetary nebulae."
The results obtained by Ragland and his colleagues also prove the feasibility of linking a trio "“ or even quintet or sextet "“ of infrared telescopes to get higher resolution images in the near-infrared than has been possible before.
"With more than two telescopes, you can explore a totally different kind of science than could be done with two telescopes," he said.
"It's a big step to go from two telescopes to three," added theoretician Lee Anne Willson, a coauthor of the study and a professor of physics and astronomy at Iowa State University in Ames. "With three telescopes you can tell not only how big the star is, but whether it's symmetric or asymmetric. With even more telescopes, you can start to turn that into a picture."
Ragland, Willson and their colleagues at institutions in the United States and France, including NASA, reported their observations and conclusions in a paper recently accepted by The Astrophysical Journal.
Ironically, the IOTA telescope array, operated jointly on Mt. Hopkins by the Smithsonian Astrophysical Observatory, Harvard University, the University of Massachusetts, the University of Wyoming, and the Massachusetts Institute of Technology's Lincoln Laboratory, was shut down July 1 to save money. The initial two-telescope interferometer went online in 1993, and the addition of a third 45-centimeter telescope in 2000 created the first optical and infrared interferometer trio.
IOTA director Wesley A. Traub, formerly of the Harvard-Smithsonian Center for Astrophysics (CfA) and now at the Jet Propulsion Laboratory, offered Ragland and his colleagues the opportunity to use the array to test the limits of multiple-telescope interferometry, and perhaps learn something about the ultimate fate of the Sun.
Interferometers combine light from two or more telescopes to see more detail, simulating the resolution of a telescope as big as the distance between the telescopes. While radio astronomers have used arrays for years to simulate much larger telescopes, they have the advantage of relatively long wavelengths "“ meters or centimeters "“ which makes it easier to detect fractional wavelength differences between the arrival times of light at separated telescopes. Doing interferometry in the near-infrared "“ at a wavelength of 1.65 microns, or about a hundredth of a millimeter, as Ragland did "“ is much harder because the wavelengths are nearly a millionth that of radio waves.
"At short wavelengths, the stability of the instrument is a major constraint," Ragland said. "Even a vibration will totally destroy the measurement."
The astronomers also employed a new technology to combine the light from the three IOTA telescopes: a half-inch wide solid-state chip, called the integrated-optics beam-combiner (IONIC), developed in France. This contrasts with the typical interferometer, which consists of many mirrors to direct the light from multiple telescopes to a common detector.
Ragland's main focus is low- to medium-mass stars "“ ranging from three-quarters the mass of the Sun to three times the mass of the Sun "“ as they approach the ends of their lives. These are stars that ballooned into red giants several billion years earlier, when they began burning the helium that had accumulated during a lifetime of hydrogen burning. By the end, though, these stars consist of a dense core of carbon and oxygen surrounded by a shell where hydrogen is converted to helium, and then helium into carbon and oxygen. In most of these stars, hydrogen and helium alternate as fuels, causing the brightness of the star to vary over a 100,000-year period as the fuel changes. In many cases, the stars spend their final 200,000 years as a Mira variable "“ a type of star whose light varies regularly in brightness over a period of 80 to 1000 days. They are named for the prototype star in the constellation of Cetus known as Mira.
"One reason I'm interested in this is that our Sun is going to take this path at some point, 4 billion years from now," Ragland said.
It's during this period that these stars begin to blow off their outer layers in a "superwind," which will eventually leave behind a white dwarf at the center of an expanding planetary nebula. Willson models the mechanisms by which these end-stage stars lose their mass, primarily though strong stellar winds.
During these waning eons, the stars also pulsate on the order of months to years, as the outer layers belch outward like a release valve, Willson said. Many of these so-called asymptotic giant branch stars are Mira variables, which vary regularly as molecules form and create a translucent or nearly opaque cocoon around the star part of the time. While some of these stars have been shown to be non-circular, any asymmetric features, such as patchy brightness, are impossible to detect with a two-telescope interferometer, Ragland said.
Ragland and his colleagues observed with IOTA a total of 35 Mira variables, 18 semi-regular variables and 3 irregular variables, all within about 1,300 light years of Earth, in our Milky Way Galaxy. Twelve of the Mira variables proved to have asymmetric brightnesses, while only three of the semi-regulars and one of the irregulars showed this patchiness.
The cause of this patchy brightness is unclear, Ragland said. Modeling by Willson has shown that a companion, such as a planet in an orbit similar to Jupiter's orbit in our own system, could generate a wake in the stellar wind that would show up as an asymmetry. Even a closer Earth-like planet could generate a detectable wake if the stellar wind was strong enough, though a planet too close to the expanded envelope would quickly be dragged inward and vaporized by the star.
Alternatively, large amounts of material expelled from the star could condense into clouds that block some or all of the light from part of the star.
Whatever the cause, Willson said, "this is telling us is that the assumption that stars are uniformly bright is wrong. We may need to develop a new generation of three-dimensional models."
"This study, the largest ever of this class of late-type stars, is the first to demonstrate the degree to which late type-stars, especially the Mira variables and carbon stars, show the effects of hot and cold spots," said coauthor William Danchi of NASA Goddard Space Flight Center. "This has implications for how we interpret observations when we use infrared interferometers to search for planets around red giants."
Ragland's coauthors are Traub; Jean-Pierre Berger, P. Kern and F. Malbet of the Laboratoire d'Astrophysique de Grenoble (LAOG) in France; Danchi; J. D. Monnier and E. Pedretti of the University of Michigan, Ann Arbor; Willson; N. P. Carleton, M. G. Lacasse and M. Pearlman of CfA; R. Millan-Gabet of the California Institute of Technology; F. P. Schloerb, M. Brewer, K. Perraut, K. Souccar and G. Wallace of the University of Massachusetts, Amherst; W. D. Cotton of the National Radio Astronomy Observatory in Virginia; Charles H. Townes of the University of California, Berkeley; P. Haguenauer of ALCATEL Space Industries of Cannes, France; and P. Labeye of the Laboratoire d'Electronique de Technologie de l'Information (LETI) in Grenoble, which is part of the French Atomic Energy Commission (CEA). The IONIC chip was developed jointly by LAOG, the Institut de Micro©lectronique, Ã“°lectromagn©tisme et Photonique (IMEP) and LETI.
The work was supported by NASA through a Michelson Postdoctoral Fellowship and by the National Science Foundation.
The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The observatory was made possible by the generous financial support of the W. M. Keck Foundation.
The paper, "First Surface-resolved results with the IOTA imaging interferometer: detection of asymmetries in AGB stars," is available for download at http://arxiv.org/PS_cache/astro-ph/pdf/0607/0607156.pdf.