Epsilon Aquilae
Epsilon Aquilae is a binary star system in the equatorial constellation of Aquila, near the western constellation boundary with Hercules. Its name is a Bayer designation that is Latinized from ε Aquilae, and abbreviated Epsilon Aql or ε Aql. The system has an apparent visual magnitude of 4.02 and is visible to the naked eye. Based upon an annual parallax of, Epsilon Aquilae lies at a distance of approximately from Earth, but is drifting closer with a radial velocity of −46 km/s.
It has the traditional name Deneb el Okab, from an Arabic term ذنب العقاب ðanab al-ʽuqāb "the tail of the eagle", and the Mandarin names Woo and Yuë, derived from and represent the state Wú, an old state was located at the mouth of the Yangtze River, and Yuè, an old state in Zhejiang province. According to the R.H. Allen's works, it shares names with ζ Aquilae. Epsilon Aquilae could be more precisely called Deneb el Okab Borealis, because is situated to the north of Zeta Aquilae, which can therefore be called Deneb el Okab Australis.
Properties
The binary nature of this system was reported by German astronomer F. Kustner in 1914, but it was not confirmed until 1974. It is a single-lined spectroscopic binary system; the pair orbit each other over a period of 1,271 days with an eccentricity of 0.27. There are two visual companions to Epsilon Aquilae, both reported by German astronomer R. Engelmann in 1887. Component B is a magnitude 10.56 star at an angular separation of along a position angle of 184° relative to the primary, as of 2014. At magnitude 11.25, component C is at a separation of with a PA of 159°, as of 2015.The primary component of this system is an evolved giant star with a stellar classification of, showing a mild overabundance of the CN molecule in the spectrum. The chemical abundances of the star suggest it has gone through first dredge-up. It has more than double the mass of the Sun and has expanded to ten times the Sun's radius. The star shines with 54 times the Sun's luminosity, which is being radiated from its outer envelope at an effective temperature of 4,760 K. At this heat, it glows with the orange-hue of a K-type star.
This has been designated a barium star, meaning its atmosphere is extremely enriched with barium and other heavy elements. However, this is disputed, with astronomer Andrew McWilliam finding normal abundances from an s-process.