Armagh Students Observe Unusual Double Star

HW Vir

Artist’s conception of the view from a circumbinary planet;
image courtesy NASA/JPL-Caltech.

Three sixth-form students from St Patrick's Grammar School, Armagh, used the Faulkes Telescope South in Australia in April to observe and measure HW Virginis, an unusual "binary" or double star system suspected to host one or two extrasolar planets orbiting the central stellar pair. Conor Kane, Owen McBrien and Ryan Maxwell controlled the telescope via the Internet during a work-experience project at the Armagh Observatory. The project represented a highly successful collaboration, being initiated by astronomers at Armagh and at the Korea Astronomy and Space Science Institute, and made possible by the Cardiff-based Faulkes Telescope Project and the Las Cumbres Observatory Global Telescope Network.

"The binary system HW Virginis, usually abbreviated HW Vir by astronomers, is interesting because it may host two planet-size objects orbiting the two stars on so-called circumbinary orbits", said Tobias C. Hinse from the Korea Astronomy and Space Science Institute, located in the Republic of Korea. The presence of two planets, with masses around 10 to 14 times the mass of Jupiter, was first proposed by Korean researchers J. W. Lee and colleagues in 2009 using ground-based optical telescopes. The proposed planets have since been given the names HW Virginis b and HW Virginis c.

The discovery of these giant planets, following recent discoveries made by the NASA-led KEPLER space telescope, is of great importance. By continuously monitoring the same star field on the sky, KEPLER scientists noticed three binary star systems showing an unusual characteristic in their brightness variations, the star system Kepler-16 being the first to be announced with this observed oddity. In addition to the normal brightness variations, there was a minute, but detectable, brightness change. "This discovery was only possible due to KEPLER's ability to observe stars with high time resolution and high photometric precision", said Tobias C. Hinse. The small features in the brightness variations of these stellar systems provide evidence for additional, otherwise unseen, objects orbiting the double star. Today, these objects have been studied in detail and astronomers have managed to pin down their nature as objects of planetary size. The planets called Kepler-16 b, Kepler-34 b and Kepler-35 b became the first known circumbinary extrasolar planets.

The new Faulkes Telescope observations carried out from Armagh support the use of a different technique in the search for such planets. The possible detection of the two objects HW Vir b and HW Vir c by Lee and co-workers used the so-called light-travel time effect. If no additional bodies were present, the two stars in the binary system HW Vir would pass in front of one another and eclipse each other in a regular manner, the eclipse events repeating at a fixed time interval of just under 3 hours. However, if additional bodies – planets or low-mass stellar objects – exist, they will pull the binary system one way and another, causing the position of the central binary to "wobble" around the system’s overall centre of mass. This small motion causes a change in the distance between the central binary stars and the telescope on Earth, which in turn leads to a change in the observed eclipse timing. "This so-called eclipse timing variation can be detected using medium-sized ground-based telescopes like the Faulkes Telescopes", said Tobias C. Hinse, adding "the strictly periodic nature of the brightness variations of eclipsing binaries makes it possible to test the planet hypothesis, ruling out other astrophysical mechanisms that might in principle produce a similar effect".

Carrying out follow-up observations using suitable ground-based telescopes makes sense when it comes to characterize the exact nature of planet-hosting binary star systems. The Faulkes Telescope Project, an official partner of the Las Cumbres Observatory Global Telescope Network, which operates the telescopes, aims to involve researchers and students across the world within the framework of a research-based education. "Many concepts in astronomy have direct analogies to what we see and experience on Earth and furthermore the science learnt in school physics classes can be seen to apply in the wider Universe," said David Asher from the Armagh Observatory.

The observations of HW Vir in 2012 April were coordinated by Alison Tripp of the Faulkes Telescope Project. On the scheduled day of the observations, the Armagh students were joined by Ana Gavrila (University of Glamorgan), Peter Phelps (Hazelmere CoE School) and Peter Hill (Paulet High School), in turn observing HW Vir through their registered Faulkes Telescope accounts. The obtained data were made available to the public and the St Patrick's Grammar School students succeeded in measuring the obtained images, demonstrating the brightness variations of the target star. David Asher added, "Over the course of four hours the joint efforts of different observers from across the country resulted in valuable data that will help to further understand the cause of HW Vir’s mysterious timing variations."

Armagh Observatory and associated researchers hope to develop this work to look for further evidence of planet-hosting binary star systems. "We have gained valuable experience on how to make these observations, and the Faulkes Telescopes are perfect for this", said David Asher. Future observations will reveal if these "two-Sun" circumbinary extra-solar planetary systems occur frequently in the Galaxy. The recent discovery by the KEPLER telescope indicates that these systems might exist more abundantly than previously thought. The HW Vir data collected by the Faulkes Telescope will most likely appear in a peer-reviewed astronomical journal in the near future highlighting the very essence of research-based education.

FOR FURTHER INFORMATION PLEASE CONTACT: David Asher or John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; E-mail: djaat signarm.ac.uk or jmfat signarm.ac.uk; URL: www.arm.ac.uk.

Additional Information

Last Revised: 2012 May 3rd