HK Aquarii

HK Aquarii is a single[7] 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.[2] The star is located at a distance of 81 light years from the Sun based on parallax.[1] The radial velocity is poorly constrained but it appears to be drifting further away at a rate of ~2 km/s.[6]

HK Aquarii
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius
Right ascension 23h 08m 19.55110s[1]
Declination 15° 24′ 35.7682″[1]
Apparent magnitude (V) 10.99±0.02[2]
10.72 - 10.94[3]
Characteristics
Evolutionary stage Main sequence
Spectral type M0Ve[4]
U−B color index +1.118[5]
B−V color index +1.31[2]
Variable type BY Dra + UV Cet[3]
Astrometry
Radial velocity (Rv)+2.7±1.6[6] km/s
Proper motion (μ) RA: 106.955[1] mas/yr
Dec.: −18.738[1] mas/yr
Parallax (π)40.1410 ± 0.0436[1] mas
Distance81.25 ± 0.09 ly
(24.91 ± 0.03 pc)
Absolute magnitude (MV)+9.13[5]
Details[2]
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[7]
Rotational velocity (v sin i)69.0±0.1 km/s[8] km/s
Age30–40[9] Myr
Other designations
HK Aqr, NSV 14434, BD−16°6218, GJ 890, HIP 114252
Database references
SIMBADdata

This is a small red dwarf star; an M-type main-sequence star with a stellar classification of M0Ve,[4] 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[8] and has a rotation period of just 10.34 hours.[7] 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.[2]

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.[3] 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.[10] Star spots have been reported at a variety of latitudes, but not at the poles.[11] Flaring activity was reported in 1987, and a steady X-ray emission has been detected.[8] 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.[10] (A 2016 study instead suggests it is a member of the Octans association.[9])

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.[12]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Gaidos, E.; et al. (September 2014). "Trumpeting M dwarfs with CONCH-SHELL: a catalogue of nearby cool host-stars for habitable exoplanets and life". Monthly Notices of the Royal Astronomical Society. 443 (3): 2561–2578. arXiv:1406.7353. Bibcode:2014MNRAS.443.2561G. doi:10.1093/mnras/stu1313.
  3. Samus', N. N; Kazarovets, E. V; Durlevich, O. V; Kireeva, N. N; Pastukhova, E. N (2017). "General catalogue of variable stars: Version GCVS 5.1". Astronomy Reports. 61 (1): 80. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869.
  4. Torres, C. A. O.; et al. (2015). "Search for associations containing young stars (SACY). I. Sample and searching method". Astronomy and Astrophysics. 460 (3): 695. arXiv:1505.07837. Bibcode:2006A&A...460..695T. doi:10.1051/0004-6361:20065602. S2CID 16080025.
  5. Koen, C.; et al. (2010). "UBV(RI)C JHK observations of Hipparcos-selected nearby stars". Monthly Notices of the Royal Astronomical Society. 403 (4): 1949. Bibcode:2010MNRAS.403.1949K. doi:10.1111/j.1365-2966.2009.16182.x.
  6. Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065, S2CID 119231169
  7. Strassmeier, Klaus G. (September 2009). "Starspots". The Astronomy and Astrophysics Review. 17 (3): 251–308. Bibcode:2009A&ARv..17..251S. doi:10.1007/s00159-009-0020-6.
  8. Barnes, J. R.; Collier, Cameron, A. (September 2001). "Starspot patterns on the M dwarfs HK Aqr and RE 1816 +541". Monthly Notices of the Royal Astronomical Society. 326 (3): 950–958. Bibcode:2001MNRAS.326..950B. doi:10.1046/j.1365-8711.2001.04649.x.
  9. Elliott, P.; et al. (May 2016). "Search for associations containing young stars (SACY). VII. New stellar and substellar candidate members in the young associations". Astronomy & Astrophysics. 590: 28. arXiv:1604.03550. Bibcode:2016A&A...590A..13E. doi:10.1051/0004-6361/201628253. A13.
  10. Crosswell, Ken (1992). A Star that Breaks all the Rules. The New Cosmos, Astronomy Magazine. Kalmbach Books. pp. 54–59.
  11. Barnes, J. R.; et al. (August 2004). "Differential rotation and star-spot evolution on HK Aqr in 2001 and 2002". Monthly Notices of the Royal Astronomical Society. 352 (2): 589–599. Bibcode:2004MNRAS.352..589B. doi:10.1111/j.1365-2966.2004.07949.x.
  12. Leitzinger, M.; et al. (November 2016). "Indications of stellar prominence oscillations on fast rotating stars: the cases of HK Aqr and PZ Tel". Monthly Notices of the Royal Astronomical Society. 463 (1): 965–979. arXiv:1608.00453. Bibcode:2016MNRAS.463..965L. doi:10.1093/mnras/stw1922.
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