New telescope techniques allow for imaging binary stars, may soon image exoplanets

December 18, 2013

Franck Marchis and colleague mount the FIRST instrument on the Shane 3-meter telescope at Lick Observatory (credit: UC Berkeley)

A new instrument that combines two high-resolution telescope techniques — adaptive optics and interferometry — has for the first time distinguished and studied the individual stars in a nearby binary star system, demonstrating promise for eventually picking out planets that have been indistinguishable from the stars they orbit.

In the December issue of the journal Astronomy & Astrophysics (open access), an international team of astronomers report that they were able to resolve in visible light the two stars in the binary star system Capella, which orbit one another at about the distance of Venus from the sun and until now have been indistinguishable from Earth.

Capella is 43 light years from Earth and the brightest star in the constellation Auriga.

Most stars in the galaxy are surrounded by swirling disks of dust, circling planets or other orbiting stars, yet astronomers have a hard time studying these companions because of glare from the main star.

The team, led by University of California, Berkeley assistant research astronomer Gaspard Duchêne, used a prototype instrument called the Fibered Imager foR Single Telescope (FIRST) that was mounted three years ago on the Shane 3-meter telescope at the University of California Lick Observatories in San Jose.

“This really is a window on a unique combination of contrast and resolution that is not available today for viewing the close environment around stars,” Duchêne said.

Earlier this year, the astronomers mounted an improved instrument on the Subaru 8-meter telescope in Hawaii that has the potential to one day distinguish or resolve Earth-size exoplanets around M-type “dwarf” stars, which are slightly smaller and cooler than the sun. Imaging exoplanets is a hot field for astronomers, who learned last month that our galaxy may contain 40 billion or more potentially habitable planets circling M stars or stars like the sun.

“With the FIRST instrument at Subaru telescope, we expect to be able to resolve giant and super giant stars and observe the close environment of debris disks around young stars,” said coauthor Franck Marchis, a research astronomer at the SETI Institute in Mountain View, Calif. Marchis initiated the Lick project in 2009 while at UC Berkeley.

Interferometry and adaptive optics

The FIRST instrument at Lick Observatory uses fiber optic communication cables to channel visible light from 18 different spots on the main mirror of the telescope to a detector, where the light beams interfere to reveal high-resolution detail in the same way radio telescope arrays use interferometry to achieve high-resolution radio images of the sky. The FIRST instrument on the Subaru telescope also uses 18 fiber optic cables to sample spots on a larger 8-meter main mirror.

The 3-meter Lick and 8-meter Subaru telescopes are already equipped with adaptive optics, which creates sharper images by removing the jiggle in stars caused by turbulence in the atmosphere. The adaptive optics system provides a very stable image, which is key to allowing FIRST to inject the light from the star into the precise center of the fibers.

The key advantage of FIRST is that it can resolve very close objects, such as close binary stars or the disks of dust and gas that circle stars in the process of forming planets. It can even resolve the surface features on red super giant stars, which bloat to the diameter of Earth’s orbit. Other techniques are limited by the turbulent glare from the stars, which is effectively removed by the use of fibers in FIRST.

At the moment, however, FIRST cannot resolve objects that differ in brightness by more than 50–100 times. Planets the size of Jupiter are typically 10,000–100,000 times fainter than their stars, while Earth-size planets are a million times fainter.

“If we could add enough fibers, we could get very high contrast; that is the goal,” Duchêne said. “If we can scale this up to look for planets, it would be very, very exciting.”

FIRST also can simultaneously obtain a spectrum of each object, providing critical information on the chemical composition and temperature of the stars, debris disks or planets, he said.

One of the key components of the system is a tiny movable mirror, a microelectromechanical systems or MEMS device, that directs starlight into the optical fibers, which channel the light without much loss to the image.

“The innovation in our instrument consists in combining the adaptive optics technology with some data acquisition and processing techniques from long baseline interferometry to achieve a unique combination of angular resolution and contrast,”  Duchêne explained to KurzweilAI in an interview.

“Specifically, the FIRST instrument enables the detection of very faint sources in the immediate vicinity of bright stars. With other astronomical instruments, the glare from the bright star completely precludes this, whereas FIRST provides a very efficient means of rejecting that glare, thanks in large part to the use of single-mode optical fibers. Indeed, the FIRST instrument is a significant improvement over the already used technique of ‘aperture masking,’ which provides similar resolution but has limited contrast.

“We hope to develop higher-performance versions of this instrument for existing and future large astronomical telescopes, with which the performance of the instrument would be enhanced. We have already deployed FIRST on the Lick Observatory 3m telescope and, subsequently, the 8m Subaru Telescope. We are also involved in the development of a potential instrument for the future Thirty Meter Telescope with our Japanese collaborators.”

Astronomers at the Observatoire de Paris, the National Astronomical Observatory of Japan, Lick Observatory, the Gemini Observatory in Hawaii, and Université de Lyon were also involved.

The FIRST project on the Lick 3-meter Shane telescope was funded by Programme National de Physique Stellaire (PNPS) in France, a Small Research Grant of the American Astronomical Society, NASA, and the National Science Foundation.

Abstract of Astronomy & Astrophysics paper

Aims. FIRST is a prototype instrument built to demonstrate the capabilities of the pupil remapping technique, using single-mode fibers and working at visible wavelengths. Our immediate objective is to demonstrate the high angular resolution capability of the instrument and to show that the spectral resolution of the instrument enables characterization of stellar companions.

Methods. The FIRST-18 instrument is an improved version of FIRST-9 that simultaneously recombines two sets of nine fibers instead of one, thus greatly enhancing the (u, v) plane coverage. We report on observations of the binary system Capella at three epochs over a period of 14 months (≳4 orbital periods) with FIRST-18 mounted on the 3 m Shane telescope at Lick Observatory. The binary separation during our observations ranges from 0.8 to 1.2 times the diffraction limit of the telescope at the central wavelength of the spectral band.

Results. We successfully resolved the Capella binary system at all epochs, with an astrometric precision as good as 1 mas under the best observing conditions. FIRST also gives access to the spectral flux ratio between the two components directly measured with an unprecedented spectral resolution of R ~ 300 over the 600−850 nm range. In particular, our data allow detection of the well-known overall slope of the flux ratio spectrum, leading to an estimation of the “pivot” wavelength of 0.64 ± 0.01  μm, at which the cooler component becomes the brightest. Spectral features arising from the difference in effective temperature of the two components (specifically the Hα line, TiO, and CN bands) have been used to constrain the stellar parameters. The effective temperatures we derive for both components are slightly lower (5−7%) than the well-established properties for this system. This difference mainly comes from deeper molecular features than those predicted by state-of-the-art stellar atmospheric models, suggesting that molecular line lists used in the photospheric models are incomplete and/or oscillator strengths are underestimated, most likely concerning the CN molecule.

Conclusions. These results demonstrate the power of FIRST, which is a fibered pupil remapping-based instrument, in terms of high angular resolution and show that the direct measurement of the spectral flux ratio provides valuable information to characterize little known companions.