Split the Difference

In 2011 NASA’s Kepler satellite, a space observatory launched in 2009 to discover Earth-like planets, identified a pair of stars that are orbited by a planet—much like the stellar system of the fictional planet Tatooine in the movie Star Wars. Named Kepler-16, it was the first “eclipsing binary” star system conclusively shown to host a planet orbiting two stars.

artist’s conception of the Kepler-16 system

An artist’s conception of the Kepler-16 system, shown during one of its transit events. The larger star, with mass about 70 percent that of the Sun, contributes most of the total light from the system. The smaller star, with mass about 20 percent that of the Sun, contributes only 3 percent of the light. The planet is thought to be a gas giant, with mass roughly equal to that of Neptune.

Now a team of Penn State astronomers using the Hobby-Eberly Telescope (HET) in Texas has obtained precise measurements of these stars, which will aid astronomers in understanding how stars and planetary systems form.

The orbits of the stars and planet in Kepler-16 are aligned so that they eclipse or transit each other when observed from Earth. In 2011, a Kepler science team combined timing of these eclipses with a sophisticated modeling technique to derive the masses of the two stars, which are about 70 percent and 20 percent that of the Sun, and of the orbiting planet.

However, as is the case with close binary stars, the light from the Kepler-16 pair is blended together. “We wanted to separate the light from each star so they could be studied individually,” says Jason Wright, assistant professor of astronomy and a member of the research team.

To do so, the Penn State astronomers used the spectroscopic capabilities of the HET. The resulting measurements are among the most precise ever made for low mass stars of this type, and also provide an important independent test of the mass-measuring technique.

“As the two Kepler-16 stars move in their 41-day orbit, their velocity relative to the Earth periodically changes, and we were able to track these motions by obtaining six high-resolution-spectroscopic observations with the Hobby-Eberly Telescope,” explains Suvrath Mahadevan, co-investigator and assistant professor of astronomy and astrophysics.

The team then used these data to disentangle the light from the individual stars. “The resulting velocity measurements, combined with Kepler’s Laws of Motion, directly give the masses of stars Kepler-16A and Kepler-16B with precisions of 2.5 percent and 1.5 percent, respectively,” Mahadevan says.

“Precise measurements of stellar masses, radii and system architectures provide important insights into how planets form,” says the leader of the team, Postdoctoral Fellow Chad Bender.

“Understanding the radii and masses of low-mass stars such as these is critical for the search for planets in habitable zones,” Mahadevan adds. The Kepler-16 measurements are the initial results from a much larger survey being led by Penn State to measure masses of more than 100 eclipsing binary stars discovered by Kepler.

“The Kepler mission is revealing at least as much about Sun-like and low-mass stars as it is about planets; it’s really revolutionizing the field of stellar astrophysics,” notes Wright. “These observations illustrate how measurements with the Hobby-Eberly Telescope and other ground-based telescopes amplify and extend the fantastic science Kepler is doing, and can teach us more about these stars and the planets that orbit them.”

Chad Bender, Ph.D., is a postdoctoral fellow, and Suvrath Mahadevan, Ph.D., and Jason Wright, Ph.D., are assistant professors, all in the Department of Astronomy and Astrophysics. The three are also members of the Penn State Center for Exoplanets and Habitable Worlds. Their email addresses are cfb12@psu.edu, suvrath@astro.psu.edu, and jtwright@astro.psu.edu.

Other members of the Penn State Kepler-16 team include Associate Professor Steinn Sigurdsson; Distinguished Senior Scholar and Professor Larry Ramsey; Distinguished Professor Donald Schneider; postdoctoral scholars Rohit Deshpande and Scott Fleming; and graduate students Arpita Roy and Ryan Terrien.

Funding for this research was provided by the Center for Exoplanets and Habitable Worlds, NASA Astrobiology Institute, Penn State Astrobiology Research Center and National Science Foundation. The Hobby-Eberly Telescope is a joint project of the University of Texas at Austin, Penn State, Stanford University, Ludwig-Maximilians-Universitat Munchen and Georg-August-Universitat Gottingen.

Last Updated August 22, 2012