HD 181068

HD 181068 is a star system in the constellation of Lyra. With an apparent magnitude of 7.09,^[3] the system is not visible to the naked eye but may be viewed with a pair of binoculars. Based on parallax measurements made by the Hipparcos spacecraft, the system is some 810 light years (250 parsecs) away from Earth.^[2]

HD 181068

A light curve for HD 181068, adapted from Derekas et al. (2011)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Lyra
Right ascension	19h 17m 08.97863s[2]
Declination	+41° 15′ 53.3103″[2]
Apparent magnitude (V)	7.09[3]
Characteristics
Spectral type	G8III / G8V / K1V[1]
Variable type	Eclipsing binary
Astrometry
Radial velocity (Rv)	5.83 km/s
Proper motion (μ)	RA: 0.60[2] mas/yr Dec.: -8.07[2] mas/yr
Parallax (π)	4.02 ± 0.40 mas[2]
Distance	810 ± 80 ly (250 ± 20 pc)
Absolute magnitude (MV)	–0.3 / 5.6 / 6.1[1]
Orbit[4]
Primary	HD 181068 A
Companion	HD 181068 B
Period (P)	45.4711 ± 0.0002 d
Semi-major axis (a)	90.31 ± 0.72 R☉
Eccentricity (e)	0
Inclination (i)	87.5 ± 2°
Periastron epoch (T)	Tmin = 2455499.9962
Orbit[4]
Primary	HD 181068 Ba
Companion	HD 181068 Bb
Period (P)	0.9056768 ± 0.0000002 d
Semi-major axis (a)	4.777 ± 0.039 R☉
Eccentricity (e)	0
Inclination (i)	87.6 ± 1.4°
Periastron epoch (T)	Tmin = 2455051.23623
Details[5]
HD 181068 A
Mass	3 ± 0.1 M☉
Radius	12.46 ± 0.15 R☉
Luminosity (bolometric)	92.8 ± 7.6 L☉
Surface gravity (log g)	2.73 cgs
Temperature	5100 ± 100 K
HD 181068 Ba
Mass	0.915 ± 0.034 M☉
Radius	0.865 ± 0.01 R☉
Luminosity (bolometric)	0.447 ± 0.037 L☉
Surface gravity (log g)	4.53 cgs
Temperature	5100 ± 100 K
HD 181068 Bb
Mass	0.870 ± 0.043 M☉
Radius	0.8 ± 0.02 R☉
Luminosity (bolometric)	0.27 ± 0.027 L☉
Surface gravity (log g)	4.58 cgs
Temperature	4675 ± 100 K
Other designations
HD 181068, BD+41° 3292, HIP 94780, SAO 48282, KIC 5952403[6]
Database references
SIMBAD	data
KIC	data

HD 181068 is in the Kepler spacecraft's field of view, and its unique properties were first observed by the satellite's photometer. It consists of a red giant, designated HD 181068 A, along with two main-sequence stars, designated HD 181068 Ba and HD 181068 Bb, respectively. Normal eclipsing binaries have two components that pass in front of each other while eclipsing. However, all three components of HD 181068 orbit each other in such a way that they eclipse each other, forming a rare triply eclipsing system.^[7]

The primary, HD 181068 A, has a spectral type of G8III,^[1] meaning it is a red giant that has used up its core hydrogen and has expanded to a radius of 12.46 R_☉.^[5] The primary star is also unusual in that it does not exhibit internal seismic oscillations as have been detected in other red giants, although tidal forces from the closer pair may possibly be causing other variability in the light curve of the system.^[7]

HD 181068 Ba and Bb have spectral types of G8V and K1V respectively, indicating their location on the main sequence, slightly later than the Sun. They are in a close orbit and complete an orbit once every 0.906 days (about 21.7 hours), while they orbit HD 181068 A every 45.5 days.^[4] All three stars have similar surface brightnesses and colors, so when the two companions eclipse the red giant, the change in brightness is very slight and hard to detect.^[7]

See also

• HD 188753, a triple star system with a possible exoplanet companion

References

1. Derekas, A.; et al. (2011). "HD 181068: A Red Giant in a Triply Eclipsing Compact Hierarchical Triple System". Science. 332 (6026): 216–218. arXiv:1202.2196. Bibcode:2011Sci...332..216D. doi:10.1126/science.1201762. PMID 21474755. S2CID 14180715.
2. van Leeuwen, F.; et al. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
3. Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27 – L30. Bibcode:2000A&A...355L..27H.
4. Borkovits, T.; Derekas, A.; Kiss, L. L.; Forgács-Dajka, E.; Bíró, I. B.; Bedding, T. R.; Bryson, S. T.; Huber, D.; Szabó, R. (2013). "Dynamical masses, absolute radii and 3D orbits of the triply eclipsing star HD 181068 from Kepler photometry". Monthly Notices of the Royal Astronomical Society. 428 (2): 1656–1672. arXiv:1210.1061. Bibcode:2013MNRAS.428.1656B. doi:10.1093/mnras/sts146.
5. Czesla, S.; Huber, K. F.; Schneider, P. C.; Schmitt, J. H. M. M. (2014). "A multiwavelength study of the hierarchical triple HD 181068. A test bed for studying star-planet interaction?". Astronomy & Astrophysics. 570: A115. arXiv:1408.2988. Bibcode:2014A&A...570A.115C. doi:10.1051/0004-6361/201423879. S2CID 119307151.
6. "HD 181068". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 28 February 2017.
7. "Kepler discovery of a unique triply eclipsing triple star". NASA Kepler News. April 7, 2011. Archived from the original on May 10, 2011.