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
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