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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
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]

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]

References

  1. ^ Young, Arthur; Skumanich, Andrew; MacGregor, Keith B.; Temple, Scott (February 1990). "High-Latitude SPOT and Plage Activity on the Rapidly Rotating M Dwarf Star Gliese 890". The Astrophysical Journal. 349: 608–619. Bibcode: 1990ApJ...349..608Y. doi: 10.1086/168347. Retrieved 17 October 2021.
  2. ^ a b c d e f 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.
  3. ^ a b c d e 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.
  4. ^ a b c 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.
  5. ^ a b 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.
  6. ^ a b 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.
  7. ^ a b 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
  8. ^ a b c 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.
  9. ^ a b c 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.{{ cite journal}}: CS1 maint: multiple names: authors list ( link)
  10. ^ a b 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. S2CID  53138126. A13.
  11. ^ a b Crosswell, Ken (1992). A Star that Breaks all the Rules. Kalmbach Books. pp. 54–59. {{ cite book}}: |work= ignored ( help)
  12. ^ 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.
  13. ^ 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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