The Thüringer Landessternwarte has been involved in a close collaboration
comprising scientists from 10 European countries, Australia, USA,
Canada, and Chile that have detected a unique stellar system. It consists of
three stars and its orientation to the earth is in a way that mutual eclipses
can be observed as each of the stars gets behind or in
front of the others.
Although the object, HD 181068, is a very bright, 7th magnitude star that is almost visible to the naked eye, it needed the NASA's Kepler space telescope with its ultra-precise photometric measurements to detect the very faint dips in the light curve caused by the eclipses, and ground-based follow-up observations to reveal the physical nature of the system. The Thüringer Landessternwarte did an important contribution by spectroscopic observations with the high-resolution Coude-Echelle spectrograph attached to its 2-m telescope to the determination of the orbits, the mass ratio and the stellar parameters of the components. In the result of the combined effort of Kepler light curve analysis and ground-based photometry, spectroscopy, and interferometry we now know that HD 181068 consists of a red giant star around which a close pair of two red dwarfs orbits with a period of 45.5 days. The close pair itself is in a short-period orbit with a 0.9 days period. The figure to the left compares the sizes of the components, the radii are given in solar units. The animation shows the stars moving in their orbits and the corresponding parts of the Kepler light curve. It starts with a perpendicular view onto the orbital planes, turning then into the position as the system is seen from earth. Credits: D. Huber, Univ. of Sydney.
The discovery of this rare and complex system is important because HD 181068 is a real astrophysical laboratory where changes in the orbital elements, unlike in the usual cases in astronomy, can be detected in a few years from now, i.e. we can compare theoretical predictions and observed changes on human timescale. Moreover, there are hints of surface waves on the red giant that may be tidally induced by the close pair of dwarf stars.