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]
Distance81.25 ± 0.09 ly
(24.91 ± 0.03 pc)
Absolute magnitude (MV)+9.13[6]
Details[3]
Mass0.57±0.07 M☉
Radius0.53±0.04 R☉
Luminosity0.05±0.01 L☉
Temperature3,800±76 K
Metallicity [Fe/H]0.27±0.12 dex
Rotation0.4307 d[8]
Rotational velocity (v sin i)69.0±0.1 km/s[9] km/s
Age30–40[10] Myr
Other designations
HK Aqr, NSV 14434, BD−16°6218, GJ 890, HIP 114252
Database references
SIMBADdata

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]

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.[11] Star spots have been reported at a variety of latitudes, but not at the poles.[12] 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.[11] (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.[13]

  1. ^ Cite error: The named reference Young was invoked but never defined (see the help page).
  2. ^ a b c d e f Cite error: The named reference dr2 was invoked but never defined (see the help page).
  3. ^ a b c d e Cite error: The named reference Gaidos_et_al_2014 was invoked but never defined (see the help page).
  4. ^ a b c Cite error: The named reference Samus_et_al_2017 was invoked but never defined (see the help page).
  5. ^ a b Cite error: The named reference sacy was invoked but never defined (see the help page).
  6. ^ a b Cite error: The named reference koen was invoked but never defined (see the help page).
  7. ^ a b Cite error: The named reference Gontcharov2006 was invoked but never defined (see the help page).
  8. ^ a b c Cite error: The named reference Strassmeier2009 was invoked but never defined (see the help page).
  9. ^ a b c Cite error: The named reference Barnes_Collier_2001 was invoked but never defined (see the help page).
  10. ^ a b Cite error: The named reference Elliott2016 was invoked but never defined (see the help page).
  11. ^ a b Cite error: The named reference crosswell was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference Barnes_et_al_2004 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference Leitzinger_et_al_2016 was invoked but never defined (see the help page).