Epsilon Aquilae
Binary star in the constellation of Aquila
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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]
 | |
| 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]