AC Herculis, is an RV Tauri variable and

spectroscopic binary A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

star in the constellation of

Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the Gr ...

. It varies in brightness between

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

s 6.85 and 9.0. AC Her is an RVa star, meaning it is an RV Tauri variable whose maximum and minimum magnitudes do not slowly vary over hundreds of days. It also is a very clear example of a common type of RV Tauri light curve where the maximum following a deep minimum is brighter than the maximum following a shallow minimum. In each period of 75.46 days it has two maxima and two minima. AC Her is also a binary star, its companion was detected with spectroscopy and long baseline interferometry. Interferometric observations from the CHARA Array enabled its three dimensional orbit with a semimajor axis of 2.01 ± 0.01 mas, equivalent to 2.83 ± 0.08 au. The orbit has an inclination of 142.9 ± 1.1 degrees and a longitude of the ascending node of 155.1 ± 1.8 degrees. The invisible secondary is more massive () than the supergiant primary ( ). The orbital period is 1187.7 days. The two stars are also surrounded by a dusty disc filling the region between 34 and 200

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

s (AU). Little is known of the secondary star except that its mass is around , deduced from the mass ratio of the binary system and the modelled mass of the primary star. The primary itself is calculated to have a mass of , but a luminosity of . It is slightly cooler than the sun, although this varies by over a thousand K as the star pulsates. The total system mass can be estimated from the dynamics of the disc, and this gives a value of , slightly lower than from other methods.

References

External links

AAVSO article
{{DEFAULTSORT:AC Herculis Hercules (constellation) Herculis, AC RV Tauri variables F-type supergiants 090697 170756 BD+21 3459 IRAS catalogue objects J18301623+2152007 Spectroscopic binaries