HK Aquarii

HK Aquarii

A visual band light curve for HK Aquarii, adapted from Young et al. (1990)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius
Right ascension	23h 08m 19.55110s[2]
Declination	15° 24′ 35.7682″[2]
Apparent magnitude (V)	10.99±0.02[3] 10.72 - 10.94[4]
Characteristics
Evolutionary stage	Main sequence
Spectral type	M0Ve[5]
U−B color index	+1.118[6]
B−V color index	+1.31[3]
Variable type	BY Dra + UV Cet[4]
Astrometry
Radial velocity (Rv)	+2.7±1.6[7] km/s
Proper motion (μ)	RA: 106.955[2] mas/yr Dec.: −18.738[2] mas/yr
Parallax (π)	40.1410 ± 0.0436 mas[2]
Distance	81.25 ± 0.09 ly (24.91 ± 0.03 pc)
Absolute magnitude (MV)	+9.13[6]
Details[3]
Mass	0.57±0.07 M☉
Radius	0.53±0.04 R☉
Luminosity	0.05±0.01 L☉
Temperature	3,800±76 K
Metallicity [Fe/H]	0.27±0.12 dex
Rotation	0.4307 d[8]
Rotational velocity (v sin i)	69.0±0.1 km/s[9] km/s
Age	30–40[10] Myr
Other designations
HK Aqr, NSV 14434, BD−16°6218, GJ 890, HIP 114252
Database references
SIMBAD	data

HK Aquarii is a single^[8] variable star in the equatorial constellation of Aquarius. It is invisible to the naked eye, having an average apparent visual magnitude that fluctuates around 10.99.^[3] The star is located at a distance of 81 light years from the Sun based on parallax.^[2] The radial velocity is poorly constrained but it appears to be drifting further away at a rate of ~2 km/s.^[7]

In 1983, Arthur Young et al. discovered that the star, called GLS 890 at that time, is a variable star. They also noted its very rapid rotation.^[11] It was given its variable star designation, HK Aquarii, in 1987.^[12]

This is a small red dwarf star; an M-type main-sequence star with a stellar classification of M0Ve,^[5] where the 'e' indicates emission lines in the spectrum. It has 57% of the mass of the Sun and has 53% of the Sun's girth. The star is spinning with a projected rotational velocity of 69 km/s^[9] and has a rotation period of just 10.34 hours.^[8] Based on the abundance of iron in the atmosphere, it has a high metallicity; what astronomers term the abundances of elements with a higher atomic number than helium. The star is radiating around 5% of the luminosity of the Sun from its photosphere at an effective temperature of 3,800 K.^[3]

HK Aquarii is classified as a BY Draconis variable and has been observed ranging in brightness from visual magnitude 10.72 down to 10.94.^[4] This star is noteworthy for being unusually active for an isolated red dwarf; it rotates rapidly, generating a strong magnetic field that creates large starspots and powerful flares.^[13] Star spots have been reported at a variety of latitudes, but not at the poles.^[14] Flaring activity was reported in 1987, and a steady X-ray emission has been detected.^[9] These are characteristic of very young stars; although it is not close to any youthful open cluster, it is a possible ejected member of the Pleiades.^[13] (A 2016 study instead suggests it is a member of the Octans association.^[10])

The signature of prominences has been detected in the star's atmosphere. These display evidence of periodic oscillations and can reach altitudes greater than two-third's of the star's radius. The prominences can obtain a higher mass and volume compared to similar features on the Sun.^[15]