Faulkes Telescope Observations

Eclipsing binary HW Virginis

(click on plot to enlarge)

Lightcurve of HW Vir from images obtained using the Faulkes Telescope South, operated by Las Cumbres Observatory. The blue triangles show the difference between two stars whose brightness is constant. The red squares show the difference between HW Vir and one of those stars, i.e. this is the lightcurve of HW Vir. The primary eclipse (brighter star eclipsed) and secondary eclipse (fainter star eclipsed) are clearly visible. Data reduced by Conor Kane, Owen McBrien and Ryan Maxwell (St Patrick's Grammar School, Armagh) during a work experience project (supervised by David Asher) at Armagh Observatory. Conor, Owen and Ryan also made some of the observations. Thanks to the other observers Ana Gavrila (Univ. Glamorgan), Peter Phelps (Hazelmere CofE School), Peter Hill (Paulet High School), and especially to Alison Tripp of the Faulkes Telescope Project for coordinating everything.

Next day we made a few Faulkes Telescope observations of near-Earth asteroids (NEAs) including an object from the Minor Planet Center's confirmation page which turned out to be a recently discovered NEA, now designated 2012 FM52.

Why is HW Vir so interesting?

This project was suggested by Tobias Hinse (formerly of Armagh Observatory, now Research Fellow at the Korea Astronomy and Space Science Institute), who writes:

The study of eclipsing binaries (two stars orbiting about a common centre of mass) has general astrophysical importance since we can obtain a wealth of information (from photometry & spectroscopy) about the two stars. This information can be used to test stellar evolution models and to fine tune or constrain those models.

The particular eclipsing system HW Vir is especially interesting because of a different aspect: The timing of the primary eclipse exhibits long-term variation (Lee et al. 2009). This variation is most likely associated with the presence of additional bodies which orbit around HW Vir on so-called circumbinary orbits. These additional bodies are called circumbinary companions. The two binary stars are called components. If the binary system had no companions the time of future primary (or secondary) mid-eclipses would be following a straight line with a slope corresponding to the binary system's orbital period.

However, if companions are present, they will pull the binary system and make it "wobble": sometimes the binary system is close to Earth (Sun) and sometimes it is farther away from Earth. This results in a varying light-path distance between the system's centre of mass and the observer, which is known as the light-travel time effect (LTT or LITE). This wobble effect can be detected as a sinusoidal-like variation (superimposed on the linear part) by measuring and monitoring the time of mid-eclipse of the primary (or secondary) eclipse. If there is a wobble, then it is very likely that it is caused by an additional companion (or even companions) orbiting the binary pair. For HW Vir two companions have been proposed by Korean researchers (Lee et al. 2009) at the Korea Astronomy and Space Science Institute in the Republic of Korea (South Korea).

These two companions are HW Virginis b and HW Virginis c.

The results by Lee et al. (2009) should be viewed in the context of the recent discovery by the KEPLER space telescope. The following three binary systems Kepler-16, Kepler-34b and Kepler-35b each have a circumbinary companion of planetary nature. Each planet revealed itself by direct eclipses in front of either one of the two binary components. These eclipses are very minute and their photometric detection is only possible with the KEPLER space telescope mainly due to its high photometric precision. Long-term monitoring of the three circumbinary KEPLER planets will eventually also reveal the light-travel time effect caused by each planet on the two binary components. The two proposed planets around HW Vir have not yet been revealed by direct eclipses.

Future observations will reveal if these "two-Sun" exoplanetary systems occur frequently in the universe. The recent discovery by KEPLER indicates that these systems might exist more abundantly than previously thought: imagine living on a planet facing towards west and watching a sunset made up of two stars. This is no longer fiction, but rather science.

The eclipse timing can be determined from the Faulkes Telescope South data (2012 April 2nd) at high accuracy, a valuable new input to the studies to constrain the parameters of the proposed planets. This shows the value of the Faulkes Telescopes in the follow-up observations of eclipsing binaries with the aim to detect and characterise additional circumbinary companions.

More astronomical projects with the Faulkes Telescopes

Last Revised: 2012 April 5th