Omicron Aquarii

Omicron Aquarii
Location of ο Aquarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius
Right ascension 22h 03m 18.84403s[1]
Declination –02° 09′ 19.3067″[1]
Apparent magnitude (V) +4.71[2]
Characteristics
Spectral type B7 IVe[3]
U−B color index –0.39[2]
B−V color index –0.11[2]
Variable type γ Cas[4]
Astrometry
Radial velocity (Rv)+11.0[5] km/s
Proper motion (μ) RA: +24.66[1] mas/yr
Dec.: –11.16[1] mas/yr
Parallax (π)7.49 ± 0.23 mas[1]
Distance440 ± 10 ly
(134 ± 4 pc)
Absolute magnitude (MV)−0.89[6]
Details
Mass4.2[7] M☉
Radius4.0±0.3[7] R☉
Luminosity340[6] L☉
Surface gravity (log g)3.13[8] cgs
Temperature13,464±164[9] K
Metallicity [Fe/H]−0.16[6] dex
Rotational velocity (v sin i)282±20[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
SIMBADdata

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]

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