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Omicron Aquarii
	Location of ο Aquarii (circled)
Observation data; Epoch J2000 Equinox J2000
Constellation	Aquarius
Right ascension	22h 03m 18.84403s
Declination	–02° 09′ 19.3067″
Apparent magnitude (V)	+4.71
Characteristics
Spectral type	B7 IVe
U−B color index	–0.39
B−V color index	–0.11
Variable type	γ Cas
Astrometry
Radial velocity (Rv)	+11.0 km/s
Proper motion (μ)	RA: +24.66 mas/yr ; Dec.: –11.16 mas/yr
Parallax (π)	7.49 ± 0.23 mas
Distance	440 ± 10 ly ; (134 ± 4 pc)
Absolute magnitude (MV)	−0.89
Details
Mass	4.2 M☉
Radius	4.0±0.3 R☉
Luminosity	340 L☉
Surface gravity (log g)	3.13 cgs
Temperature	13,464±164 K
Metallicity [Fe/H]	−0.16 dex
Rotational velocity (v sin i)	282±20 km/s
Other designations
	ο Aqr, 31 Aquarii, BD–02 5681, FK5 3765, HD 209409, HIP 108874, HR 8402, SAO 145837
Database references
SIMBAD	data

Omicron Aquarii, Latinized from ο Aquarii, is the Bayer designation for a variable star in the equatorial constellation of Aquarius. Visible to the naked eye, it has an apparent visual magnitude of +4.71.^[2] Parallax measurements put it at a distance of roughly 440 light-years (130 parsecs) from Earth.^[1] It is drifting further away with a radial velocity of +11 km/s.^[5] The star is a candidate member of the Pisces-Eridanus stellar stream.^[12]

It has the traditional star name Kae Uh, from the Chinese 蓋屋 (Mandarin pronunciation Gài Wū).^[13] In Chinese astronomy, 蓋屋 is the rooftop, an asterism consisting of ο Aquarii and 32 Aquarii.^[14] Consequently, the Chinese name for ο Aquarii itself is 蓋屋一 (Gài Wū yī, English: the First Star of Roofing.)^[15]

A light curve for Omicron Aquarii, plotted from *TESS* data^[16]

The spectrum of Omicron Aquarii fits a stellar classification of B7 IVe;^[3] the luminosity class of IV suggests that this is a subgiant star that is exhausting the supply of hydrogen at its core and is in the process of evolving into a giant star. The 'e' suffix on the class indicates that the spectrum shows emission lines of hydrogen, thus categorizing this as a Be star. It is rotating rapidly with an equatorial rotational velocity of 368 km/s, which is ~96% of the star's critical rotation velocity of 391 km/s.^[7] The emission lines are being generated by a decreted circumstellar disk of hot hydrogen gas.^[10] This disk has been globally stable for at least twenty years, as of 2020.^[7] It is likely a single star, with no stellar companions.^[17]

Omicron Aquarii has 4.2 times the mass of the Sun, four^[7] times the Sun's radius, and is radiating 340^[6] times the luminosity of the Sun from its photosphere at an effective temperature of 13,464 K.^[9] It is classified as a Gamma Cassiopeiae type^[4] variable star and its brightness varies from magnitude +4.68 down to +4.89.^[18]

References

^ ^a ^b ^c ^d ^e ^f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
^ ^a ^b ^c ^d Feinstein, A.; Marraco, H. G. (November 1979), "The photometric behavior of Be Stars", Astronomical Journal, 84: 1713–1725, Bibcode:1979AJ.....84.1713F, doi:10.1086/112600.
^ ^a ^b Lesh, Janet Rountree (December 1968). "The Kinematics of the Gould Belt: an Expanding Group?". Astrophysical Journal Supplement. 17: 371. Bibcode:1968ApJS...17..371L. doi:10.1086/190179.
^ ^a ^b "omi Aqr", General Catalogue of Variable Stars, Sternberg Astronomical Institute, retrieved 2012-07-03. Note: type = GCAS.
^ ^a ^b Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veroeffentlichungen des Astronomischen Rechen-Instituts Heidelberg, 35 (35), Astronomisches Rechen-Institut Heidelberg: 1, Bibcode:1999VeARI..35....1W.
^ ^a ^b ^c ^d Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
^ ^a ^b ^c ^d ^e de Almeida, E. S. G.; et al. (April 2020), "Visible and near-infrared spectro-interferometric analysis of the edge-on Be star o Aquarii", Astronomy & Astrophysics, 636: 23, arXiv:2002.09552, Bibcode:2020A&A...636A.110D, doi:10.1051/0004-6361/201936039, S2CID 211258993, A110
^ Soubiran, C.; Le Campion, J.-F.; Cayrel de Strobel, G.; Caillo, A. (June 2010), "The PASTEL catalogue of stellar parameters", Astronomy and Astrophysics, 515: A111, arXiv:1004.1069, Bibcode:2010A&A...515A.111S, doi:10.1051/0004-6361/201014247, S2CID 118362423.
^ ^a ^b Underhill, A. B.; et al. (November 1979), "Effective temperatures, angular diameters, distances and linear radii for 160 O and B stars", Monthly Notices of the Royal Astronomical Society, 189 (3): 601–605, Bibcode:1979MNRAS.189..601U, doi:10.1093/mnras/189.3.601.
^ ^a ^b Meilland, A.; Millour, F.; Kanaan, S.; Stee, Ph.; Petrov, R.; Hofmann, K.-H.; Natta, A.; Perraut, K. (February 2012), "First spectro-interferometric survey of Be stars. I. Observations and constraints on the disk geometry and kinematics", Astronomy & Astrophysics, 538: A110, arXiv:1111.2487, Bibcode:2012A&A...538A.110M, doi:10.1051/0004-6361/201117955, S2CID 64777633.
^ "omi Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-07-03.
^ Curtis, Jason L.; et al. (August 2019), "TESS Reveals that the Nearby Pisces-Eridanus Stellar Stream is only 120 Myr Old", The Astronomical Journal, 158 (2): 11, arXiv:1905.10588, Bibcode:2019AJ....158...77C, doi:10.3847/1538-3881/ab2899, S2CID 166228270, 77.
^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc, p. 53, ISBN 0-486-21079-0, retrieved 2010-12-12.
^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 16 日
^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
^ Hutter, D. J.; Tycner, C.; Zavala, R. T.; Benson, J. A.; Hummel, C. A.; Zirm, H. (2021). "Surveying the Bright Stars by Optical Interferometry. III. A Magnitude-limited Multiplicity Survey of Classical Be Stars". The Astrophysical Journal Supplement Series. 257 (2): 69. arXiv:2109.06839. Bibcode:2021ApJS..257...69H. doi:10.3847/1538-4365/ac23cb. S2CID 237503492.
^ Ruban, E. V.; et al. (September 2006), "Spectrophotometric observations of variable stars", Astronomy Letters, 32 (9): 604–607, Bibcode:2006AstL...32..604R, doi:10.1134/S1063773706090052, S2CID 121747360.