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Kepler discovery of a unique triply eclipsing triple star
Relative sizes of HD181068
Relative sizes of HD181068
While the quest for Earth-like planets around other stars using the NASA's Kepler space telescope has recently produced many exciting discoveries, other branches of stellar astrophysics also benefit from the ultraprecise space photometry offered by the revolutionary Kepler satellite. An international group of European, Australian and American researchers report on the discovery of a unique stellar system in a paper accepted for publication in the Science magazine. The object, catalogued as HD 181068 and known as Trinity' within the authorship team, is a 7th magnitude star that is almost visible to the naked eye, and the seemingly single star is in reality a complex triple system in which three stars reside in a very special geometry, showing mutual eclipses as each of the stars gets behind or in front of the others. The most luminous object is a red giant star around which a close pair of two red dwarfs orbits with a period of 45,5 days.

"Thanks to the fortunate viewing angle from Earth, the combined light from the three stars change very characteristically: there are sharp brightness decreases with a period of 0.9 days produced by the mutual eclipses of the close pair of dwarfs, while it takes 2 days for the close pair to pass in front of or behind the red giant" - says Aliz Derekas (Eotvos University and Konkoly Observatory, Budapest, Hungary), the lead author of the paper. A mind-boggling feature of the variations is that when the red dwarfs are in front of the red giant, their short-period eclipses disappear. This is because the surface brightnesses of the three stars are actually very similar, and just as a white rabbit cannot be seen in snow-fall, the red dwarfs in front of the red giant are also almost invisible, hence no light is lost when they eclipse each other.

Animation of HD181068 light curve
Animation of HD181068 system and light curve (click on image to play animation).
The authors discovered this interesting system in June 2010 and consequently took ground-based observations. "The spectroscopic measurements revealed the periodic motion of the largest star in the system with the wide orbital period of 45,5 days. The 2-day long eclipses are so similar at first sight that we thought that the outer orbital period is 22.7 days. Only after having obtained the whole radial velocity curve, we realised the tiny differences in the long period minima and that the real period was the double of it.'- says Laszlo Kiss (Konkoly Observatory), the second author of the discovery paper. Further observations using interferometry were used to measure the angular size of the red giant. "Combining the angular diameter with the known distance of the system we were able to measure the absolute radius of the red giant, which was a great achievement given its large distance of 800 light years" - adds Daniel Huber (University of Sydney, Australia), who led the interferometric observations using the Center for High-Angular Resolution Astronomy (CHARA) at Mount Wilson Observatory in California, USA. The results show that the largest star in the system is 12.4 times larger than our Sun. The scientists could also estimate the mass of the main component as 3 times that of the Sun.

The discovery of this complex system is significant because HD 181068 is a real astrophysical labaratory 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. In addition, HD 181068 has further peculiar features. Careful analyses of red giant stars observed by Kepler have shown that all red giant stars should exhibit oscillations similar to those in the Sun. The frequency of these oscillations can be theoretically determined knowing the basic physical paremeters of the red giant (mass, temperature, radius). However, there is no sign of such oscillations in the red giant component of HD 181068 which means there must be a mysterious mechanism that suppresses the pulsation. "Surprisingly, we do detect some variability but with periods that are closely linked to the orbital period of the close pair in the system" - told Dr. Derekas. This may indicate that tidal forces of the close pair induce vibes in the surface of the red giant.

The intriguing nature of this unique system remained unnoticed until now despite its brightness. We really needed Kepler with its unprecedentedly precise and uninterrupted photometric monitoring to uncover such a rare gem - she added.

See also:

  • Science article, HD 181068: A Red Giant in a Triply Eclipsing Compact Hierarchical Triple System, A. Derekas et al. - The Kepler satellite reveals details of the oscillations patterns of an evolved star in an exotic triple-star system.
  • NASA Kepler Feature story: NASA Kepler Reaching into the Stars
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