Epsilon Aquilae

Epsilon Aquilae is a binary star^[12] 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^[2] and is visible to the naked eye. Based upon an annual parallax of 18.1821 mas,^[1] Epsilon Aquilae lies at a distance of approximately 179 light-years (55 parsecs) from Earth, but is drifting closer with a radial velocity of −46 km/s.^[4]

ε Aquilae

Location of ε Aquilae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Aquila
Right ascension	18h 59m 37.356s[1]
Declination	+15° 04′ 05.81″[1]
Apparent magnitude (V)	+4.02[2]
Characteristics
Spectral type	K1-IIICN0.5[3]
U−B color index	+1.04[2]
B−V color index	+1.08[2]
R−I color index	+0.52
Astrometry
Radial velocity (Rv)	−45.9±0.3[4] km/s
Proper motion (μ)	RA: −51.062 mas/yr[1] Dec.: −69.439 mas/yr[1]
Parallax (π)	18.1821±0.3319 mas[1]
Distance	179 ± 3 ly (55 ± 1 pc)
Absolute bolometric magnitude (Mbol)	+0.30[5]
Orbit[6]
Period (P)	1,270.6±1.1 d
Semi-major axis (a)	≥ 86.9 ± 2.3 Gm (0.581 ± 0.015 AU)
Eccentricity (e)	0.272±0.026
Periastron epoch (T)	41718±17 MJD
Argument of periastron (ω) (secondary)	82±5°
Semi-amplitude (K1) (primary)	5.17±0.13 km/s
Details
ε Aql Aa
Mass	2.1+0.4 −0.2[7] M☉
Radius	10.13+0.21 −0.22[8] R☉
Luminosity	54±5[9] L☉
Surface gravity (log g)	2.91[9] cgs
Temperature	4,760[9] K
Metallicity [Fe/H]	0.00[9] dex
Rotational velocity (v sin i)	4.4[10] km/s
ε Aql Ab
Mass	0.47±0.05[7] M☉
Other designations
Deneb el Okab, ε Aql, 13 Aql, BD+14 3736, FK5 712, GC 26091, HD 176411, HIP 93244, HR 7176, SAO 104318, PPM 135586, WDS J18596+1504A[11]
Database references
SIMBAD	data

It has the traditional name Deneb el Okab /ˈdɛnɛb ɛl ˈoʊkæb/, from an Arabic term ذنب العقاب ðanab al-ʽuqāb "the tail of the eagle", and the Mandarin names Woo /ˈwuː/ and Yuë /ˈjuːeɪ/, derived from and represent the state Wú (吳), an old state was located at the mouth of the Yangtze River, and Yuè (越), an old state in Zhejiang province^[13] (together with 19 Capricorni in Twelve States asterism). According to the R.H. Allen's works, it shares names with ζ Aquilae.^[14] 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.^[15]

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;^[7] the pair orbit each other over a period of 1,271 days (3.5 years) with an eccentricity (ovalness) of 0.27.^[6] 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 122.00″ along a position angle (PA) of 184° relative to the primary, as of 2014. At magnitude 11.25, component C is at a separation of 142.90″ with a PA of 159°, as of 2015.^[16]

The primary component of this system is an evolved giant star with a stellar classification of K1-III CN0.5,^[3] 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.^[17] It has more than double the mass of the Sun^[7] and has expanded to ten times the Sun's radius.^[8] 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.^[9] At this heat, it glows with the orange-hue of a K-type star.^[18]

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 (1990) finding normal abundances from an s-process.^[7]