Space Daily, CA
07-21-06
Three-Telescope Interferometer Shows Patchy Red Giants Common Fate Of
Sun-Like Stars
Aerial view of the recently decommissioned IOTA array atop Mt. Hopkins in
Arizona. Image credit: IOTA/CfA
by Staff Writers
Kamuela HI (SPX)
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.
Sam Ragland used Arizona's Infrared-Optical Telescope Array of three linked
telescopes to obtain unprecedented detail of old red giant stars that represent the
eventual fate of the Sun.
He found nearly one-third of the red giants he surveyed were not uniformly bright
across their faces, 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," she added.
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.
Ironic, but 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 in Cambridge, Mass., and now at the Jet Propulsion Laboratory in
Pasadena, Calif., 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.
Radio astronomers have used arrays for years to simulate much larger
telescopes, but 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 integratedoptics 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 threequarters 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.
During this period, these stars begin to blow off their outer layers in a
"superwind," which eventually will 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.
Some of these stars have been shown to be non-circular, but any asymmetric
features, such as patchy brightness, are impossible to detect with a twotelescope interferometer, Ragland said.
Ragland and 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, but within the Milky Way Galaxy.
Twelve of the Mira variables proved to have asymmetric brightness, 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, 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.
An alternative scenario suggests 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 co-author William
Danchi of NASA's Goddard Space Flight Center in Greenbelt, Md.
"This has implications for how we interpret observations when we use infrared
interferometers to search for planets around red giants," Danchi added.