Iota Boötis

Iota Boötis is a wide binary star system in the constellation Boötes, approximately 96 light-years from Earth. Its name is a Bayer designation that is Latinized from ι Boötis, and abbreviated Iota Boo or ι Boo. The brighter component has the traditional name Asellus Secundus, pronounced /əˈsɛləs sɪˈkʌndəs/, which is Latin for "second donkey colt", and the Flamsteed designation 21 Boötis.^[13] It is faintly visible to the naked eye with a typical apparent visual magnitude of +4.75.^[2] Based on parallax measurements, it is located at a distance of 96 light-years (29 pc) from the Earth. The star is drifting closer to the Sun with a radial velocity of −19 km/s.^[7]

ι Boötis

Location of ι Boötis (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Boötes
Right ascension	14h 16m 09.930s[1]
Declination	+51° 22′ 02.029″[1]
Apparent magnitude (V)	4.75[2] (4.73–4.78[3])
Characteristics
Spectral type	A7 V[4] + K0 V[5]
U−B color index	+0.06[6]
B−V color index	+0.20[6]
R−I color index	+0.09[6]
Variable type	Delta Scuti variable[3]
Astrometry
Radial velocity (Rv)	−18.7[7] km/s
Proper motion (μ)	RA: −149.277 mas/yr[1] Dec.: +89.135 mas/yr[1]
Parallax (π)	33.8856±0.0820 mas[1]
Distance	96.3 ± 0.2 ly (29.51 ± 0.07 pc)
Absolute magnitude (MV)	+2.38[8]
Details
ι Boo A
Mass	1.650±0.04[1] M☉
Radius	1.715+0.055 −0.021[1] R☉
Luminosity	8.90+0.06 −0.07[1] L☉
Surface gravity (log g)	4.14+0.01 −0.02[1] cgs
Temperature	7,764+3 −8[1] K
Metallicity [Fe/H]	+0.19[9] dex
Rotational velocity (v sin i)	144[10] km/s
Age	785±223[1] Myr
HD 234121
Mass	0.807+0.042 −0.041[11] M☉
Radius	0.824±0.017[11] R☉
Luminosity	0.411+0.051 −0.041[11] L☉
Surface gravity (log g)	4.511+0.018 −0.005[11] cgs
Temperature	5,090+17 −4[11] K
Other designations
Asellus Secondus[12], ι Boötis, 21 Boötis, BD+52°1784, FK5 528, GC 19269, HD 125161, HIP 69713, HR 5350, SAO 29071, PPM 34432, WDS J14162+5122[13]
Database references
SIMBAD	data
Data sources:
Hipparcos Catalogue, CCDM (2002), Bright Star Catalogue (5th rev. ed.)

ι Boötis and its nearby companion HD 234121 (the faint component C is also visible to the south)

The companion is HD 234121, a K-type main-sequence star at an angular distance of 38.6 arcseconds; easily separated with binoculars.

Components

A visual band light curve for Iota Boötis, adapted from Kiss (1995)^[14]

The primary component is a white hued A-type main-sequence star with a stellar classification of A7V.^[4] It is classified as a Delta Scuti-type variable star and its brightness varies from magnitude +4.73 to +4.78 with a stable period of 38 minutes.^[8] This star is 785^[1] million years old and has a high rate of spin, showing a projected rotational velocity of 144 km/s.^[10] It has 1.8^[4] times the mass of the Sun and 1.7 times the Sun's radius.^[1] Iota Boötis is radiating 8.8^[1] times the luminosity of the Sun from its photosphere at an effective temperature of 7,764 K.^[1]

The common proper motion companion, HD 234121, is a magnitude 7.3 main-sequence star belonging to spectral class K0V.^[5][15] It has a projected separation from ι Boo of 1,100 AU.^[4] HD 234121 has a mass of 0.8 M_☉, a luminosity of 0.4 L_☉, a temperature of 5,090 K, and a radius of 0.8 R_☉.^[11]

The Washington Double Star Catalog lists a third component, a 14th-magnitude star at 90 arcseconds,^[15] but it is an unrelated background star.^[16]

Nomenclature

This star, along with the other Aselli (θ Boo and κ Boo) and λ Boo, were Aulād al Dhiʼbah (أولاد الضّباع - awlād al-ḍibā‘), "the Whelps of the Hyenas".^{[the transcription does not match the Arabic][17]}

In Chinese, 天槍 (Tiān Qiāng), meaning Celestial Spear, refers to an asterism consisting of ι Boötis, κ² Boötis and θ Boötis.^[18] Consequently, the Chinese name for ι Boötis itself is 天槍二 (Tiān Qiāng èr, English: the Second Star of Celestial Spear).^[19]