Epsilon Aquilae

ε Aquilae
Location of ε Aquilae (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Aquila
Right ascension 18h 59m 37.36161s[1]
Declination +15° 04′ 05.8807″[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: –56.078[1] mas/yr
Dec.: –73.917[1] mas/yr
Parallax (π)23.9930 ± 0.4006 mas[1]
Distance136 ± 2 ly
(41.7 ± 0.7 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)41,718±17 MJD
Argument of periastron (ω)
(secondary)		82±5°
Semi-amplitude (K1)
(primary)		5.17±0.13 km/s
Details
ε Aql Aa
Mass2.1+0.4
−0.2[7] M☉
Radius10.14±0.70[8] R☉
Luminosity54±5[8] L☉
Surface gravity (log g)2.91[8] cgs
Temperature4,760[8] K
Metallicity [Fe/H]0.00[8] dex
Rotational velocity (v sin i)4.4[9] km/s
ε Aql Ab
Mass0.47±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[10]
Database references
SIMBADdata

Epsilon Aquilae, Latinized from ε Aquilae, is the Bayer designation for a binary star[11] system in the equatorial constellation of Aquila, near the western constellation boundary with Hercules. It has an apparent visual magnitude of 4.02[2] and is visible to the naked eye. Based upon an annual parallax of 23.993 mas,[1] Epsilon Aquilae lies at a distance of approximately 136 light-years (42 parsecs) from Earth, but is drifting closer with a radial velocity of –46 km/s.[4]

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[12] (together with 19 Capricorni in Twelve States asterism). According to the R.H. Allen's works, it shares names with ζ Aquilae.[13] Epsilon Aquilae is 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.

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.[14]

The primary component of this system is an evolved giant star with a stellar classification of K1-IIICN0.5,[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.[15] It has more than double[7] the mass of the Sun and has expanded to ten[8] times the Sun's radius. The star shines with 54–fold the Sun's luminosity, which is being radiated from its outer envelope at an effective temperature of 4,760 K.[8] At this heat, it glows with the orange-hue of a K-type star.[16] 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]

  1. ^ a b c d e f Cite error: The named reference GaiaDR2 was invoked but never defined (see the help page).
  2. ^ a b c d Cite error: The named reference clpl4_99 was invoked but never defined (see the help page).
  3. ^ a b Cite error: The named reference Keenan_McNeil_1989 was invoked but never defined (see the help page).
  4. ^ a b Cite error: The named reference Famaey_et_al_2005 was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference aj133_6_2464 was invoked but never defined (see the help page).
  6. ^ a b Cite error: The named reference Griffin1982 was invoked but never defined (see the help page).
  7. ^ a b c d e Cite error: The named reference aaa398_1163 was invoked but never defined (see the help page).
  8. ^ a b c d e f g Cite error: The named reference aaa526_A100 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference aj135_1_209 was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference SIMBAD was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference mnras389_2_869 was invoked but never defined (see the help page).
  12. ^ (in Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name Archived August 10, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  13. ^ Star Names - R.H.Allen p.61
  14. ^ Cite error: The named reference WDSC2014 was invoked but never defined (see the help page).
  15. ^ Cite error: The named reference Mishenina_et_al_1995 was invoked but never defined (see the help page).
  16. ^ Cite error: The named reference csiro was invoked but never defined (see the help page).