Omicron Aquarii
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. Parallax measurements put it at a distance of roughly from Earth. It is drifting further away with a radial velocity of +11 km/s. The star is a candidate member of the Pisces-Eridanus stellar stream of co-moving stars.
It has the traditional star name Kae Uh, from the Chinese 蓋屋. In Chinese astronomy, 蓋屋 is the rooftop, an asterism consisting of ο Aquarii and 32 Aquarii. Consequently, the Chinese name for ο Aquarii itself is 蓋屋一
Properties
The spectrum of Omicron Aquarii fits a stellar classification of B7 IVe; 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.Omicron Aquarii has 4.2 times the mass of the Sun and four times the Sun's radius. It is radiating 340 times the luminosity of the Sun from its photosphere at an effective temperature of 11,145 K. This is classified as a Gamma Cassiopeiae type variable star and its brightness varies from magnitude +4.68 down to +4.89. It is likely a single star, with no stellar companions.
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. This is creating an equatorial bulge with a radius of compared to a polar radius of. As a result, the polar temperature ; higher than the equator.
The emission lines are being generated by a circumstellar disk of hot hydrogen gas. This disk has been globally stable for at least twenty years, as of 2020. It is inclined at an angle of 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 of the star's mass.