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Iota Hydrae
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	09h 39m 51.36145s
Declination	−01° 08′ 34.1135″
Apparent magnitude (V)	3.91
Characteristics
Spectral type	K2.5 III
B−V color index	1.32
Variable type	Suspected
Astrometry
Radial velocity (Rv)	+24.19±0.36 km/s
Proper motion (μ)	RA: +46.96 mas/yr ; Dec.: −62.39 mas/yr
Parallax (π)	12.39 ± 0.14 mas
Distance	263 ± 3 ly ; (80.7 ± 0.9 pc)
Absolute magnitude (MV)	−0.63
Details
Mass	1.92 M☉
Radius	30±0.04 R☉
Luminosity	83 L☉
Surface gravity (log g)	2.2 cgs
Temperature	4,244±32 K
Metallicity [Fe/H]	−0.05 dex
Rotational velocity (v sin i)	4.5 km/s
Age	2.47 Gyr
Other designations
	Ukdah, ι Hya, 35 Hydrae, BD−00° 2231, FK5 1250, HD 83618, HIP 47431, HR 3845, SAO 137035.
Database references
SIMBAD	data

Iota Hydrae (ι Hydrae, abbreviated Iota Hya, ι Hya), formally named Ukdah /ˈʌkdə/,^[8] is a star in the constellation of Hydra, about 8° to the north-northwest of Alphard (Alpha Hydrae)^[9] and just to the south of the celestial equator.^[10] Visible to the naked eye, it is a suspected variable star with an apparent visual magnitude that ranges between 3.87 and 3.91.^[3] Based upon an annual parallax shift of 12.39 mas measured during the Hipparcos mission,^[1] it is located around 263 light-years from the Sun.

Nomenclature

ι Hydrae (Latinised to Iota Hydrae) is the star's Bayer designation.

This star along with Tau¹ Hydrae, Tau² Hydrae and 33 Hydrae (A Hydrae), were Ptolemy's Καμπή (Kampē); but Kazwini knew them as عقدة ʽuqdah (or ʽuḳdah) "knot".^[11] According to a 1971 NASA memorandum,^[12] Ukdah was the name of an asterism of four stars: Tau¹ Hydrae as Uḳdah I, Tau² Hydrae as Uḳdah II, 33 Hydrae as Uḳdah III and Iota Hydrae as Uḳdah IV. In 2016, the IAU organized a Working Group on Star Names (WGSN)^[13] to catalog and standardize proper names for stars. The WGSN approved the name Ukdah for Iota Hydrae on 1 June 2018 and it is now so included in the List of IAU-approved Star Names.^[8]

In Chinese, 星宿 (Xīng Sù), meaning Star or asterism, refers to an asterism consisting of ι Hydrae, Alphard, τ¹ Hydrae, τ² Hydrae, 26 Hydrae, 27 Hydrae, HD 82477 and HD 82428.^[14] Consequently, ι Hydrae are known as 星宿四 (Xīng Sù sì, English: the Fourth Star of Star).^[15]

Properties

This is an evolved K-type giant star with a stellar classification of K2.5 III.^[2] It is a Barium star, which means that, for a giant star, it displays unusually strong absorption lines of singly-ionized barium and strontium.^[16] Iota Hydrae has nearly twice^[2] the mass of the Sun and has expanded to 30 times the Sun's radius.^[6] It is around 2.5 billion years old and is spinning with a leisurely projected rotational velocity of 4.5^[4] km/s. It may be a member of the Wolf 630 moving group of stars that share a common trajectory through space.^[17]

References

^ ^a ^b ^c ^d ^e ^f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Luck, R. Earle (September 2015), "Abundances in the Local Region. I. G and K Giants", The Astronomical Journal, 150 (3): 23, arXiv:1507.01466, Bibcode:2015AJ....150...88L, doi:10.1088/0004-6256/150/3/88, S2CID 118505114, 88.
^ ^a ^b Samus, N. N.; Durlevich, O. V.; et al. (2009), "General Catalogue of Variable Stars", VizieR On-line Data Catalog, 1, Bibcode:2009yCat....102025S.
^ ^a ^b ^c ^d Massarotti, Alessandro; et al. (January 2008), "Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209.
^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
^ ^a ^b Baines, Ellyn K.; Armstrong, J. Thomas; Schmitt, Henrique R.; Zavala, R. T.; Benson, James A.; Hutter, Donald J.; Tycner, Christopher; Van Belle, Gerard T. (2018), "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer", The Astronomical Journal, 155 (1): 30, arXiv:1712.08109, Bibcode:2018AJ....155...30B, doi:10.3847/1538-3881/aa9d8b, S2CID 119427037.
^ "iot Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-04.{{cite web}}: CS1 maint: postscript (link)
^ ^a ^b "Naming Stars". IAU.org. Retrieved 18 June 2018.
^ O'Meara, Steve (2007), Herschel 400 Observing Guide, Cambridge University Press, p. 84, ISBN 978-0521858939.
^ Moore, Patrick (2013), The Observer's Year: 366 Nights of the Universe, The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 71, ISBN 978-1447136132.
^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 250. ISBN 0-486-21079-0. Retrieved 2010-12-12.
^ Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
^ (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.
^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 29 日 Archived 2011-05-22 at the Wayback Machine
^ Lu, Phillip K. (June 1991), "Taxonomy of barium stars", Astronomical Journal, 101: 2229–2254, Bibcode:1991AJ....101.2229L, doi:10.1086/115845.
^ McDonald, A. R. E.; Hearnshaw, J. B. (August 1983), "The Wolf 630 moving group of stars", Monthly Notices of the Royal Astronomical Society, 204 (3): 841–852, Bibcode:1983MNRAS.204..841M, doi:10.1093/mnras/204.3.841.