Omicron Aquarii

Short description: Variable B-type star in the constellation Aquarius

Omicron Aquarii

Location of ο Aquarii (circled)

Observation data
Equinox J2000.0]] (ICRS)
Constellation | Aquarius[1]
Right ascension |  22h 03m 18.844s[2]
Declination | −02° 09′ 19.31″[2]
Apparent magnitude (V) | +4.71[3]
Characteristics
Evolutionary stage | main sequence[4]
Spectral type | B5V[5]
U−B color index | −0.39[3]
B−V color index | −0.11[3]
Variable type | γ Cas[6]
Astrometry
Radial velocity (Rv)| +11.0[7] km/s
Proper motion (μ)|  RA: +24.593[2] mas/yr
Dec.: −11.720[2] mas/yr
Parallax (π)| 6.9984 ± 0.1424[2] mas
Distance| 466 ± 9 ly
(143 ± 3 pc)
Absolute magnitude (MV)| −0.89[1]
Details
Mass| 4.2[8] M☉
Radius| 4.0±0.3[8] R☉
Luminosity| 644[9] L☉
Surface gravity (log g)| 3.51[9] cgs
Temperature| 11,145±498[10] K
Metallicity [Fe/H]| −0.16[1] dex
Rotational velocity (v sin i)| 211±26[10] km/s
Other designations

ο Aqr, 31 Aquarii, BD−02 5681, FK5 3765, HD 209409, HIP 108874, HR 8402, SAO 145837[11]

Database references
SIMBAD| data

Omicron Aquarii is a variable star in the equatorial constellation of Aquarius. Its name is a Bayer designation that is Latinized from ο Aquarii, and abbreviated Omicron Aqr or ο Aqr. Visible to the naked eye, it has an apparent visual magnitude of +4.71.[3] Parallax measurements put it at a distance of roughly 466 light-years (143 parsecs) from Earth.[2] It is drifting further away with a radial velocity of +11 km/s.[7] The star is a candidate member of the Pisces-Eridanus stellar stream of co-moving stars.[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]

## Properties

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

The spectrum of Omicron Aquarii fits a stellar classification of B5V, indicating that this is a main sequence star.[5]

Omicron Aquarii has 4.2 times the mass of the Sun and four[8] times the Sun's radius. It is radiating 644 times the luminosity of the Sun[9] from its photosphere at an effective temperature of 11,145 K.[10] This is classified as a Gamma Cassiopeiae type[6] variable star and its brightness varies from magnitude +4.68 down to +4.89.[17] It is likely a single star, with no stellar companions.[18]

This star is spinning rapidly with an equatorial rotational velocity of 368 km/s, which is ~96% of the star's critical rotation velocity of 391 km/s.[8] This is creating an equatorial bulge with a radius of 6.8±1.0 R☉ compared to a polar radius of 6.0±0.8 R☉. As a result, the polar temperature 11,500±578 K; higher than the equator.[10]

The emission lines are being generated by a circumstellar disk of hot hydrogen gas.[19] This disk has been globally stable for at least twenty years, as of 2020.[8] It is inclined at an angle of 75°±3° to the plane of the sky. 90% of the hydrogen emission comes from within 9.5 stellar radii of the host star, and the disk has an estimated mass of ∼1.8×10−10 of the star's mass.[20]

## References

1. ↑ 1.0 1.1 1.2 Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters 38 (5): 331. doi:10.1134/S1063773712050015. Bibcode: 2012AstL...38..331A XHIP record for this object at VizieR.
2. ↑ 2.0 2.1 2.2 2.3 2.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940 Gaia DR3 record for this source at VizieR.
3. ↑ 3.0 3.1 3.2 3.3 Feinstein, A.; Marraco, H. G. (November 1979), "The photometric behavior of Be Stars", Astronomical Journal 84: 1713–1725, doi:10.1086/112600, Bibcode: 1979AJ.....84.1713F.
4. ↑ Zorec, J.; Royer, F. (2012). "Rotational velocities of A-type stars. IV. Evolution of rotational velocities". Astronomy and Astrophysics 537: A120. doi:10.1051/0004-6361/201117691. Bibcode: 2012A&A...537A.120Z.
5. ↑ 5.0 5.1 Arcos, C.; Kanaan, S.; Chávez, J.; Vanzi, L.; Araya, I.; Curé, M. (2018). "Stellar parameters and H α line profile variability of be stars in the BeSOS survey". Monthly Notices of the Royal Astronomical Society 474 (4): 5287. doi:10.1093/mnras/stx3075. Bibcode: 2018MNRAS.474.5287A.
6. ↑ 6.0 6.1 "omi Aqr", General Catalogue of Variable Stars (Sternberg Astronomical Institute), http://www.sai.msu.su/gcvs/cgi-bin/ident.cgi?cat=HD++&num=209409, retrieved 2012-07-03. Note: type = GCAS.
7. ↑ 7.0 7.1 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 (Astronomisches Rechen-Institut Heidelberg) 35 (35): 1, Bibcode: 1999VeARI..35....1W.
8. ↑ 8.0 8.1 8.2 8.3 8.4 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, doi:10.1051/0004-6361/201936039, A110, Bibcode: 2020A&A...636A.110D.
9. ↑ 9.0 9.1 9.2 Zorec, J.; Frémat, Y.; Domiciano De Souza, A.; Royer, F.; Cidale, L.; Hubert, A.-M.; Semaan, T.; Martayan, C. et al. (2016). "Critical study of the distribution of rotational velocities of be stars. I. Deconvolution methods, effects due to gravity darkening, macroturbulence, and binarity". Astronomy and Astrophysics 595: A132. doi:10.1051/0004-6361/201628760. Bibcode: 2016A&A...595A.132Z.
10. ↑ 10.0 10.1 10.2 10.3 Turis-Gallo, D. et al. (March 2025), "Unveiling stellar spin: Determining inclination angles in Be stars", Astronomy & Astrophysics 695: id. A129, doi:10.1051/0004-6361/202452825, Bibcode: 2025A&A...695A.129T.
11. ↑ "omi Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=omi+Aqr.
12. ↑ 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, doi:10.3847/1538-3881/ab2899, 77, Bibcode: 2019AJ....158...77C.
13. ↑ Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc, p. 53, ISBN 0-486-21079-0, https://archive.org/details/starnamestheirlo00alle/page/53, retrieved 2010-12-12.
14. ↑ Script error: The function "in_lang" does not exist. 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.
15. ↑ Script error: The function "in_lang" does not exist. AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 16 日
16. ↑ MAST: Barbara A. Mikulski Archive for Space Telescopes, Space Telescope Science Institute, https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html, retrieved 8 December 2021.
17. ↑ Ruban, E. V. et al. (September 2006), "Spectrophotometric observations of variable stars", Astronomy Letters 32 (9): 604–607, doi:10.1134/S1063773706090052R, Bibcode: 2006AstL...32..604R.
18. ↑ Hutter, D. J. et al. (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, doi:10.3847/1538-4365/ac23cb, Bibcode: 2021ApJS..257...69H.
19. ↑ Meilland, A. et al. (February 2012), "First spectro-interferometric survey of Be stars. I. Observations and constraints on the disk geometry and kinematics", Astronomy & Astrophysics 538: A110, doi:10.1051/0004-6361/201117955, Bibcode: 2012A&A...538A.110M.
20. ↑ Sigut, T. A. A. et al. (December 2015), "The Circumstellar Disk of the Be Star o Aquarii as Constrained by Simultaneous Spectroscopy and Optical Interferometry", The Astrophysical Journal 814 (2): id. 159, doi:10.1088/0004-637X/814/2/159, Bibcode: 2015ApJ...814..159S.

## External links

* Image Omicron Aquarii

* v
* t
* e

Constellation of Aquarius

* Aquarius in Chinese astronomy
* Aquarius Stream
* List of stars in Aquarius

Stars|

| Bayer|

* α
* β
* γ
* δ
* ε
* ζ
* η
* θ
* ι
* κ
* λ
* μ
* ν
* ξ
* ο
* π
* ρ
* σ
* τ1
* τ2
* υ
* φ
* χ
* ψ2
* ψ3
* ω1
* ω2

|

Flamsteed|

* 1
* 3
* 4
* 5
* 7
* 8
* 9
* 10
* 11
* 12
* 14
* 15
* 16
* 17
* 18
* 19
* 20
* 21
* 24
* 25
* 26
* 28
* 29
* 30
* 32
* 35
* 36
* 37
* 38
* 39
* 40
* 41
* 42
* 44
* 45
* 47
* 49
* 50
* 51
* 53
* 54
* 56
* 58
* 60
* 61
* 64
* 65
* 66
* 67
* 68
* 70
* 72
* 74
* 75
* 77
* 78
* 81
* 82
* 83
* 84
* 85
* 86
* 87
* 88
* 89
* 91
* 94
* 96
* 97
* 98
* 99
* 100
* 101
* 103
* 104
* 106
* 107
* 108

Variable|

* R
* U
* AE
* DV
* EW
* EZ
* FO
* HK
* HU
* IZ
* LP

HD|

* 199442
* 200375
* 202259
* 205765
* 208177
* 210277
* 210434
* 211392
* 211575
* 213429
* 214448
* 214810
* 215114
* 215152
* 216718
* 218061
* 219279
* 220035
* 220466
* 220689
* 220766
* 221148
* 222093
* 222582
* 223311

Other|

* 2MASS J21392676+0220226
* BD−22 5866
* Fomalhaut C
* Gliese 849
* Gliese 876
* K2-28
* K2-72
* K2-138
* LS IV-14 116
* Sneden's Star
* TRAPPIST-1
* WASP-6
* WASP-47

| Exoplanets|

* 91 Aquarii b
* Gliese 849 b
* Gliese 876 b
* c
* d
* e
* HD 206610 b
* HD 210277 b
* HD 212771 b
* HD 222582 b
* K2-21b
* c
* K2-72b
* c
* d
* e
* K2-138b
* TRAPPIST-1b
* c
* d
* e
* f
* g
* h
* WASP-6b

|

| Star clusters|

* Messier 2
* Messier 72
* Messier 73

|

| Nebulae|

* Helix
* Saturn

|

Galaxies|

| NGC|

* 6975
* 7001
* 7010
* 7047
* 7051
* 7065
* 7069
* 7077
* 7081
* 7184
* 7252
* 7257
* 7301
* 7302
* 7600
* 7606
* 7723
* 7727
* 7759

|

Other|

* Aquarius Dwarf
* Lyman-alpha blob 1
* SMM J2135-0102
* WISE J224607.57−052635.0

| Galaxy clusters|

* Abell 2597
* XMMXCS 2215-1738

|

| Astronomical events|

* SN 2213-1745

|

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