Lambda Aquarii

Lambda Aquarii, informally known as Hydor (/ˈhaɪdɔːr/),[11][12] is a variable star in the equatorial constellation of Aquarius. The name is Latinized from the Bayer designation λ Aquarii. The apparent visual magnitude of this star ranges from 3.57 down to 3.80,[3] which is bright enough to be visible with the naked eye. It lies just 0.39 degrees south of the ecliptic[13] and so is subject to lunar occultations.[14][15] The star is eclipsed by the sun from about 1-4 March;[16] thus the star can be viewed the whole night, crossing the sky, in early September, in the current epoch. Lambda Aquarii is located at a distance of approximately 310 light-years (95 pc) from the Sun based on parallax,[1] but is drifting closer with a radial velocity of −10.5 km/s.[6]

A visual band light curve for Lambda Aquarii, plotted from data published by Tabur et al. (2009).[17] The inset plot shows the same data folded with a period of 24.5 days, after a linear trend had been removed.
Lambda Aquarii
Location of λ Aquarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius
Right ascension 22h 52m 36.87723s[1]
Declination −07° 34′ 46.5830″[1]
Apparent magnitude (V) 3.722[2] (3.57 to 3.80)[3]
Characteristics
Evolutionary stage AGB[4]
Spectral type M2.5 IIIa Fe–1[5]
U−B color index +1.721[2]
B−V color index +1.641[2]
Variable type Lb[3]
Astrometry
Radial velocity (Rv)−10.46±0.53[6] km/s
Proper motion (μ) RA: +11.806[1] mas/yr
Dec.: +34.967[1] mas/yr
Parallax (π)8.9360 ± 0.2356 mas[7]
Distance365 ± 10 ly
(112 ± 3 pc)
Absolute magnitude (MV)−1.5[8]
Details
Mass3.6[9] M☉
Radius44.10+21.14
−13.20[1] R☉
Luminosity698.5±41.5[1] L☉
Temperature3,835[9] K
Metallicity [Fe/H]−2.25[8] dex
Other designations
λ Aqr, 73 Aquarii, BD–08°968, FK5 864, HD 216386, HIP 112961, HR 8698, SAO 146362[10]
Database references
SIMBADdata

The stellar classification of Lambda Aquarii is M2.5 IIIa Fe–1,[5] indicating this is an aging red giant star with an underabundance of iron showing in its spectrum. This star is on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen.[4] With 3.6[9] times the mass of the Sun, it has expanded to 44[1] times the Sun's radius. It is classified as slow irregular variable and pulsation periods of 24.5, 32.0, and 49.5 days have been identified.[17] On average, it is radiating nearly 700[1] times the luminosity of the Sun from the photosphere at an effective temperature of 3,835 K.[9]

Naming

Hydor is from Greek Ὕδωρ "water", a name given by Proclus, according to Richard Hinckley Allen. Another Greek name for the star is Ekkhysis, from εκχυσις "outpouring".[12]

In Chinese, 壘壁陣 (Lěi Bì Zhèn), meaning Line of Ramparts, refers to an asterism consisting of λ Aquarii, κ Capricorni, ε Capricorni, γ Capricorni, δ Capricorni, ι Aquarii, σ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium.[18] Consequently, λ Aquarii itself is 壘壁陣七 (Lěi Bì Zhèn qī, English: the Seventh Star of Line of Ramparts.)[19]

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. Gutierrez-Moreno, Adelina; et al. (1966). "A System of photometric standards". Publ. Dept. Astron. Univ. Chile. Publicaciones Universidad de Chile, Department de Astronomy. 1: 1–17. Bibcode:1966PDAUC...1....1G.
  3. Samus, N. N.; Kazarovets, E. V.; et al. (2017). "General Catalogue of Variable Stars". Astronomy Reports (5.1 ed.). 61 (1): 80–88. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869. Retrieved 2019-11-23.
  4. Lebzelter, T.; Hron, J. (January 2008). "BRITE stars on the AGB". Communications in Asteroseismology. 152: 178–181. Bibcode:2008CoAst.152..178L. doi:10.1553/cia152s178.
  5. Eggleton, P. P.; Tokovinin, A. A. (September 2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869–879. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. S2CID 14878976.
  6. Famaey, B.; et al. (May 2009). "Spectroscopic binaries among Hipparcos M giants. I. Data, orbits, and intrinsic variations". Astronomy and Astrophysics. 498 (2): 627–640. arXiv:0901.0934. Bibcode:2009A&A...498..627F. doi:10.1051/0004-6361/200810698. S2CID 18739721.
  7. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
  8. Dupree, A. K.; Li, Timothy Q.; Smith, Graeme H. (October 2007). "Hubble Space Telescope Observations of Chromospheres in Metal-Deficient Field Giants". The Astronomical Journal. 134 (4): 1348–1359. arXiv:0709.1709. Bibcode:2007AJ....134.1348D. doi:10.1086/520925. S2CID 14481140.
  9. Tsuji, Takashi (May 2007). Kupka, F.; Roxburgh, I.; Chan, K. (eds.). "Convection in Astrophysics, Proceedings of IAU Symposium #239 held 21-25 August, 2006 in Prague, Czech Republic". Proceedings of the International Astronomical Union. 2: 307–310. arXiv:astro-ph/0610180. Bibcode:2007IAUS..239..307T. doi:10.1017/S1743921307000622. S2CID 119362842. Isotopic abundances of Carbon and Oxygen in Oxygen-rich giant stars.
  10. "* lam Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-07-01.
  11. Kaler, James B. "Hydor". STARS. Retrieved 2017-08-17.
  12. Allen, Richard Hinckey. "Star Names: Their Lore and Meaning". Retrieved 2017-08-17.
  13. John Pratt's stars re-publication by Dr J.P. Pratt (Doctor of Astronomy, University of Arizona) of sidereal coordinate data. Note: possibly a non-book published source.
  14. Schmidtke, P. C.; Africano, J. L. (January 2011). "KPNO Lunar Occultation Summary. III". The Astronomical Journal. 141 (1): 7. Bibcode:2011AJ....141...10S. doi:10.1088/0004-6256/141/1/10. S2CID 120313180. 10.
  15. Nather, R. Edward; et al. (June 1970). "The Angular Diameter of Lambda Aquarii". Astrophysical Journal. 160: L181. Bibcode:1970ApJ...160L.181N. doi:10.1086/180555.
  16. Ford, Dominic. "In-The-Sky.org Planetarium". Retrieved 2021-06-07.
  17. Tabur, V.; et al. (December 2009). "Long-term photometry and periods for 261 nearby pulsating M giants". Monthly Notices of the Royal Astronomical Society. 400 (4): 1945–1961. arXiv:0908.3228. Bibcode:2009MNRAS.400.1945T. doi:10.1111/j.1365-2966.2009.15588.x. S2CID 15358380.
  18. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  19. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日
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