Astronomy:HK Aquarii

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Short description: Star in the constellation Aquarius
HK Aquarii
HKAqrLightCurve.png
A visual band light curve for HK Aquarii, adapted from Young et al. (1990)[1]
Observation data
Equinox J2000.0]] (ICRS)
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[2] mas
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
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]

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. doi:10.1086/168347. Bibcode: 1990ApJ...349..608Y. http://articles.adsabs.harvard.edu/pdf/1990ApJ...349..608Y. Retrieved 17 October 2021. 
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Brown, A. G. A. (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics 616: A1. doi:10.1051/0004-6361/201833051. Bibcode: 2018A&A...616A...1G.  Gaia DR2 record for this source at VizieR.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 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. doi:10.1093/mnras/stu1313. Bibcode: 2014MNRAS.443.2561G. 
  4. ↑ 4.0 4.1 4.2 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. doi:10.1134/S1063772917010085. Bibcode: 2017ARep...61...80S. 
  5. ↑ 5.0 5.1 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. doi:10.1051/0004-6361:20065602. Bibcode: 2006A&A...460..695T. 
  6. ↑ 6.0 6.1 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. doi:10.1111/j.1365-2966.2009.16182.x. Bibcode: 2010MNRAS.403.1949K. 
  7. ↑ 7.0 7.1 Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system", Astronomy Letters 32 (11): 759–771, doi:10.1134/S1063773706110065, Bibcode: 2006AstL...32..759G 
  8. ↑ 8.0 8.1 8.2 Strassmeier, Klaus G. (September 2009). "Starspots". The Astronomy and Astrophysics Review 17 (3): 251–308. doi:10.1007/s00159-009-0020-6. Bibcode: 2009A&ARv..17..251S. 
  9. ↑ 9.0 9.1 9.2 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. doi:10.1046/j.1365-8711.2001.04649.x. Bibcode: 2001MNRAS.326..950B. 
  10. ↑ 10.0 10.1 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. doi:10.1051/0004-6361/201628253. A13. Bibcode: 2016A&A...590A..13E. 
  11. ↑ 11.0 11.1 Crosswell, Ken (1992). A Star that Breaks all the Rules. Kalmbach Books. pp. 54–59. 
  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. doi:10.1111/j.1365-2966.2004.07949.x. Bibcode: 2004MNRAS.352..589B. 
  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. doi:10.1093/mnras/stw1922. Bibcode: 2016MNRAS.463..965L. 



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