Iota Hydrae

Iota Hydrae (ι Hydrae, abbreviated Iota Hya, ι Hya), formally named Ukdah /ˈʌkdə/,^[9] is a star in the constellation of Hydra, about 8° to the north-northwest of Alphard (Alpha Hydrae)^[10] and just to the south of the celestial equator.^[11] 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 distant.

Iota Hydrae

Location of Iota Hydrae (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	09h 39m 51.36145s[1]
Declination	−01° 08′ 34.1135″[1]
Apparent magnitude (V)	3.91[2]
Characteristics
Spectral type	K2.5 III[2]
B−V color index	1.32
Variable type	Suspected[3]
Astrometry
Radial velocity (Rv)	+24.19±0.36[4] km/s
Proper motion (μ)	RA: +46.96[1] mas/yr Dec.: −62.39[1] mas/yr
Parallax (π)	12.39±0.14 mas[1]
Distance	263 ± 3 ly (80.7 ± 0.9 pc)
Absolute magnitude (MV)	−0.63[5]
Details
Mass	1.92[2] M☉
Radius	30±0.04[6] R☉
Luminosity	241±7[7] L☉
Surface gravity (log g)	1.78±0.04[7] cgs
Temperature	4,238±22[7] K
Metallicity [Fe/H]	−0.06±0.05[7] dex
Rotational velocity (v sin i)	4.5[4] km/s
Age	2.47[2] Gyr
Other designations
Ukdah, ι Hya, 35 Hydrae, BD−00°2231, FK5 1250, HD 83618, HIP 47431, HR 3845, SAO 137035.[8]
Database references
SIMBAD	data

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".^[12] According to a 1971 NASA memorandum,^[13] 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)^[14] 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.^[9]

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.^[15] Consequently, ι Hydrae are known as 星宿四 (Xīng Sù sì, English: the Fourth Star of Star).^[16]

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.^[17] Iota Hydrae has nearly twice the mass of the Sun^[2] and has expanded to 30 times the Sun's radius.^[6] It is around 2.5 billion years old^[2] and is spinning with a leisurely projected rotational velocity of 4.5 km/s.^[4] It may be a member of the Wolf 630 moving group of stars that share a common trajectory through space.^[18]