HK Aquarii
Rapidly rotating red dwarf with strong magnetic activity.
HK Aquarii is a solitary variable star located in the equatorial constellation Aquarius. This M-type main-sequence red dwarf, classified as M0Ve, is too dim to be seen without a telescope, with its average apparent visual magnitude hovering near 10.99. Based on parallax measurements, it lies about 81 light-years from the Sun, and its radial velocity, though not well determined, suggests it is moving away at roughly 2 km/s.
The star was identified as a variable in 1983 by Arthur Young and colleagues, who also noted its extremely rapid spin. It received its variable star designation, HK Aquarii, in 1987. With 57% of the Sun's mass and 53% of its diameter, this star rotates with a projected rotational velocity of 69 km/s, completing a full rotation in just 10.34 hours. Its photosphere radiates about 5% of the Sun's luminosity at an effective temperature of 3,800 K, and its atmosphere shows a high abundance of iron, indicating elevated metallicity.
HK Aquarii is classified as a BY Draconis variable, with observed brightness ranging from magnitude 10.72 to 10.94. It stands out for its exceptional activity given its isolation, driven by its rapid rotation and strong magnetic field, which produce large starspots and powerful flares. Starspots have been detected at various latitudes but not at the poles. Flaring was reported in 1987, and steady X-ray emission has been recorded. Such activity is typical of very young stars, and while HK Aquarii is not near any young open cluster, it may be an ejected member of the Pleiades; a 2016 study instead suggests membership in the Octans association.
The star's atmosphere also shows signs of prominences, which exhibit periodic oscillations and can reach heights exceeding two-thirds of the star's radius. These prominences can be more massive and voluminous than similar features on the Sun.
- Constellation
- Aquarius
- Apparent magnitude range
- 10.72 to 10.94
- Average apparent magnitude
- 10.99
- Distance
- 81 light years
- Spectral type
- M0Ve
- Mass
- 57% of the Sun
- Radius
- 53% of the Sun
- Rotation period
- 10.34 hours
- Projected rotational velocity
- 69 km/s
- Effective temperature
- 3800 K
Lore & Background
In 1983, Arthur Young et al. discovered that the star, then called GLS 890, is a variable star, also noting its very rapid rotation. It was given its variable star designation, HK Aquarii, in 1987. The star is located at a distance of 81 light years from the Sun based on parallax, and its radial velocity is poorly constrained but appears to be drifting further away at a rate of ~2 km/s.
HK Aquarii is an M-type main-sequence star with 57% of the Sun's mass and 53% of its radius. It spins with a projected rotational velocity of 69 km/s and a rotation period of just 10.34 hours. Based on iron abundance, it has high metallicity. The star radiates about 5% of the Sun's luminosity from its photosphere at an effective temperature of 3,800 K.
Classified as a BY Draconis variable, it has been observed ranging in brightness from visual magnitude 10.72 down to 10.94. Starspots have been reported at a variety of latitudes but not at the poles. Flaring activity was reported in 1987, and steady X-ray emission has been detected. These characteristics are typical of very young stars; although not close to any youthful open cluster, it is a possible ejected member of the Pleiades. A 2016 study instead suggests it is a member of the Octans association. The signature of prominences has been detected, displaying periodic oscillations and reaching altitudes greater than two-thirds of the star's radius, with higher mass and volume than similar features on the Sun.
Reader's Guide
HK Aquarii is significant as an example of an unusually active isolated red dwarf, demonstrating how rapid rotation can drive strong magnetic fields, large starspots, and powerful flares even in the absence of a binary companion or nearby star cluster. Its classification as a BY Draconis variable highlights the role of starspot modulation in its brightness variations. The detection of prominences reaching altitudes over two-thirds of the star's radius, with greater mass and volume than solar prominences, provides insight into stellar magnetic activity at high rotation rates. The star's possible membership in the Pleiades or Octans association, though disputed, ties it to broader questions of stellar youth and dynamical ejection from open clusters. Its high metallicity and emission-line spectrum further mark it as a chemically enriched, magnetically active low-mass star.
Did You Know?
- HK Aquarii rotates once every 10.34 hours, far faster than the Sun.
- The star was discovered to be variable in 1983 by Arthur Young et al., when it was called GLS 890.
- It is a possible ejected member of the Pleiades, though a 2016 study suggests membership in the Octans association.
More in Variable Stars, Part 3 1-24
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