Epsilon Aquilae

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]

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]
Distance	136 ± 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
Mass	2.1+0.4 −0.2[7] M☉
Radius	10.14±0.70[8] R☉
Luminosity	54±5[8] L☉
Surface gravity (log g)	2.91[8] cgs
Temperature	4,760[8] K
Metallicity [Fe/H]	0.00[8] dex
Rotational velocity (v sin i)	4.4[9] 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[10]
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^[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]

References

1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
2. Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J.
3. Keenan, Philip C.; McNeil, Raymond C. (1989), "The Perkins catalog of revised MK types for the cooler stars", Astrophysical Journal Supplement Series, 71: 245, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
4. Famaey, B.; et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics, 430 (1): 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272, S2CID 17804304.
5. Luck, R. Earle; Heiter, Ulrike (June 2007), "Giants in the Local Region", The Astronomical Journal, 133 (6): 2464–2486, Bibcode:2007AJ....133.2464L, doi:10.1086/513194.
6. Griffin, R. F. (June 1982), "Spectroscopic binary orbits from photoelectric radial velocities. Paper 44: epsilon Aquilae", The Observatory, 102: 82–85, Bibcode:1982Obs...102...82G.
7. Pourbaix, D.; Boffin, H. M. J. (February 2003), "Reprocessing the Hipparcos Intermediate Astrometric Data of spectroscopic binaries. II. Systems with a giant component", Astronomy and Astrophysics, 398 (3): 1163–1177, arXiv:astro-ph/0211483, Bibcode:2003A&A...398.1163P, doi:10.1051/0004-6361:20021736, S2CID 12361870.
8. Piau, L.; et al. (February 2011), "Surface convection and red-giant radius measurements", Astronomy and Astrophysics, 526: A100, arXiv:1010.3649, Bibcode:2011A&A...526A.100P, doi:10.1051/0004-6361/201014442, S2CID 118533297.
9. Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209, S2CID 121883397.
10. "eps Aql". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2011-12-29.
11. Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
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. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920.
15. Mishenina, T. V.; et al. (October 1995), "Chemical composition of five giants with positive CN-indices", Astronomy and Astrophysics Supplement, 113: 333, Bibcode:1995A&AS..113..333M.
16. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-18, retrieved 2012-01-16.

External links

• Deneb el Okab Borealis by Jim Kaler
• Image Epsilon Aquilae