NOAO: Half of all Exoplanet Host Stars are Binaries

The Kepler field of view, located between two bright stars in the summer triangle, rising over the WIYN telescope in southern Arizona.

Imagine living on an exoplanet with two suns. One, you orbit and the other is a very bright, nearby neighbor looming large in your sky. With this “second sun” in the sky, nightfall might be a rare event, perhaps only coming seasonally to your planet. A new study suggests that this could be far more common than we realized.

The NASA Kepler Space Telescope has confirmed about 1000 exoplanets, as well as thousands more stars considered “Kepler objects of interest”, dubbed KOIs – stars that could possibly host planets. Until now, there has been an unanswered question about exoplanet host stars; how many host stars are binaries? Binary stars have long been known to be commonplace – about half the stars in the sky are believed to consist of two stars orbiting each other. So, are stars with planets equally likely to have a companion star, or do companion stars affect the formation of planets? A team of astronomers, led by Dr. Elliott Horch, Southern Connecticut State University, have shown that stars with exoplanets are just as likely to have a binary companion: that is, 40% to 50% of the host stars are actually binary stars. As Dr. Horch said, “It’s interesting and exciting that exoplanet systems with stellar companions turn out to be much more common than was believed even just a few years ago.”

Their study makes use of very high spatial resolution observations that were carried out on the WIYN telescope located on Kitt Peak in southern Arizona and the Gemini North telescope located on Mauna Kea in Hawaii. The technique used by the team is called speckle imaging and consists of obtaining digital images of a small portion of the sky surrounding a star of interest, 15 to 25 times a second. The images are then combined in software using a complex set of algorithms, yielding a final picture of the star with a resolution better that that of the Hubble space telescope. By using this technique, the team can detect companion stars that are up to 125 times fainter than the target, but only 0.05 arcseconds away. For the majority of the Kepler stars, this means companion stars with a true separation of a few to about 100 times the Sun-Earth distance. By noting the occurrence rate of these true binary companion stars, the discoveries can be extended to show that half of the stars that host exoplanets are probably binaries.

Co-author of the study, Dr. Steve B. Howell (NASA Ames Research Center), commented, “An interesting consequence of this finding is that in the half of the exoplanet host stars that are binary we can not, in general, say which star in the system the planet actually orbits.”

Kepler has discovered a number of circumbinary planets, that is, a planet that orbits both stars in very close binary systems. There also exist exoplanets that are known to orbit one of the stars in very wide binary systems. If the two stars are very close to each other and the planet far away, a circumbinary planet will be reminiscent of Tatooine in Star Wars. If instead the exoplanet orbits one of the stars in a very wide pair, the companion star might appear simply as a bright star among others in the night sky. “Somewhere there will be a transition between these two scenarios,” Howell said,” but we are far from knowing where.”

The accompanying figure shows the Kepler field of view, located between two bright stars in the summer triangle, rising over the WIYN telescope in southern Arizona. 

In a study like this, it is critical to rule out faint companions that are only in the line of sight with the KOI star. To allow for these possibilities, the team performed a model simulation that relied on known statistical properties of binary star systems and line of sight companions. The results suggest that the large majority of the stellar companions to KOIs are true bound companions, not line of sight stars unconnected with the system. 

This work has been accepted for publication in the Astrophysical Journal. The additional authors are Dr. Mark E. Everett, National Optical Astronomy Observatory and Dr. David R. Ciardi, NASA Exoplanet Science Institute, California Institute of Technology. 

The WIYN telescope is operated by the WIYN Consortium, which consists of the University of Wisconsin, Indiana University and the National Optical Astronomy Observatory (NOAO). Kitt Peak National Observatory is a division of NOAO, which is operated by the Association of Universities for Research in Astronomy Inc. under a cooperative agreement with the National Science Foundation.

Science Contact

Dr. Steve B. Howell
NASA Ames Research Center
PO Box 1
M/S 244-30
Moffett Field, CA 94035
steve.b.howell@nasa.gov
650.604.4238

Source: National Optical Astronomy Observatory (NOAO)

WIYN/NOAO: M51, the Whirlpool Galaxy, seen with new ODI Camera on WIYN Telescope

Full frame image of Spiral Galaxy M51 (Image gallery page), demonstrating the wide field of view of the One Degree Imager (ODI) on the WIYN 3.5-m telescope on Kitt Peak. Image credit: K. Rhode, M. Young and WIYN/NOAO/AURA/NSF. 

Cropped view of Spiral Galaxy M51, demonstrating the excellent sharpness of the One Degree Imager (ODI) on the WIYN 3.5-m telescope on Kitt Peak. Image credit: K. Rhode, M. Young and WIYN/NOAO/AURA/NSF. 

Stars over the WIYN 3.5-meter telescope on Kitt Peak National Observatory. Image credit: P. Marenfeld/NOAO/AURA/NSF 

The Whirlpool Galaxy (Messier 51) has been a popular night sky target for astronomers for centuries. Charles Messier first identified it in 1773 and listed it as number 51 in his catalog. To him, it looked like a faint, fuzzy object that might be a comet. William Parsons, the 3rd Earl of Rosse, used his 72-inch telescope “Leviathan” to observe the Whirlpool in 1845. Since then, Messier 51 has likely been targeted by virtually every telescope in the northern hemisphere. It is found in the constellation Canes Venatici (the Hunting Dogs) and is a classic example of a spiral galaxy. 

Now, a new camera on the WIYN 3.5-meter telescope at Kitt Peak National Observatory has imaged the Whirlpool Galaxy anew. The wide field of the One Degree Imager (ODI) camera makes it possible to capture the entire galaxy and its companion in one pointing, something that even the Hubble Space Telescope cannot do. 

Indiana University (IU) astronomy professor Katherine Rhode led this effort as part of an imaging survey of spiral and elliptical galaxies. The survey is aimed at understanding how these so-called “giant galaxies” form and evolve. 

“The WIYN telescope is an ideal telescope for the survey because of its wide field and because it produces some of the sharpest, highest-quality images possible with a ground-based telescope”, explained Rhode. “WIYN’s 3.5-meter mirror is also very efficient at gathering light from astronomical objects, so it allows us to image faint objects, like individual star clusters within the galaxies.”

This new image, as well as over one thousand others, can be found on the National Optical Astronomy Observatory (NOAO) image gallery http://www.noao.edu/image_gallery/. The gallery contains images taken with all the telescopes supported by NOAO, selected videos, and pictures of telescopes and instruments.

An important consideration for imaging with ground-based telescopes is what astronomers refer to as “seeing”, and most people think of as stars twinkling. Twinkling is caused by movement of air in the Earth’s atmosphere, and it can be minimized at a good telescope site, like on a mountaintop in a dry climate. As WIYN Interim Director Dr. Eric Hooper said, “The WIYN telescope on Kitt Peak is known for producing excellent, steady images with high resolution, or sharpness.”

The WIYN ODI camera spent about an hour observing M51 through three different filters: blue, green, and red. These digital images were later combined to construct a “true-color” image: redder objects in the image are cooler, emitting most of their light at longer optical wavelengths, while bluer objects in the image are bluer and hotter in reality. Objects that glow green are somewhere in between. Even though the galaxy is almost 30 million light years away, the image clearly shows clusters of young, hot stars that light up the spiral arms. Threaded through the arms are dark “dust lanes”, where sooty material left over from previous generations of stars has settled. More dust lanes can be seen in the bridge of luminous stars and gas that connects Messier 51 to its companion, the peculiar galaxy NGC 5195, in the upper part of the image. 

The images were taken by Dr. Rhode in May 2013 and then processed by the ODI Portal, Pipeline, and Archive (ODI-PPA) project team at IU. The ODI-PPA project is a collaboration between IU’s Pervasive Technology Institute (PTI), the Science Data Management group at NOAO, and WIYN. Arvind Gopu, ODI-PPA project manager, noted: “When requested by a pipeline operator, ODI data are processed and archived using NSF-funded cyberinfrastructure located at IU. In the case of the M51 images, our lead developer Michael Young ran the raw images through the calibration pipeline and used that data to make the final true-color images.”

The ODI camera is funded by the WIYN partners, and the National Science Foundation, through its Telescope System Instrumentation Program. The WIYN partners are University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatory (NOAO). NOAO is operated by Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

Science Contact:

Dr. Katherine L. Rhode
Indiana University Dept. of Astronomy
Bloomington, IN 47405
e-mail: rhode@astro.indiana.edu