Iota Arae

Iota Arae is a binary star system in the southern constellation of Ara. Its name is a Bayer designation that is Latinized from ι Arae, and abbreviated Iota Ara or ι Ara. The system has a combined apparent visual magnitude of 5.2. Based upon the Bortle Dark-Sky Scale, this means it is visible to the naked eye from suburban skies. Parallax measurements yield a distance estimate of 920 light-years (281 parsecs), give or take a 20 light-year margin of error. It is drifting closer to the Sun with a radial velocity of around −19 km/s.^[6]

ι Arae

Location of ι Arae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Ara[1]
Right ascension	17h 23m 16.07624s[2]
Declination	−47° 28′ 05.5057″[2]
Apparent magnitude (V)	5.18–5.26[3]
Characteristics
Spectral type	B2 IVe + sdO[4]
U−B color index	−0.82[5]
B−V color index	−0.11[5]
R−I color index	−0.08
Variable type	BE[3]
Astrometry
Radial velocity (Rv)	−19.0±7.4[6] km/s
Proper motion (μ)	RA: −6.209 mas/yr[2] Dec.: −17.699 mas/yr[2]
Parallax (π)	3.5613±0.0904 mas[2]
Distance	920 ± 20 ly (281 ± 7 pc)
Absolute magnitude (MV)	−2.06[1]
Orbit[4]
Period (P)	176.17±0.04
Inclination (i)	46±6°
Periastron epoch (T)	2458654.2±0.5 HJD
Semi-amplitude (K1) (primary)	5.80±0.06 km/s
Semi-amplitude (K2) (secondary)	57.6±0.6 km/s
Details
ι Ara A
Mass	8.3±0.4[7] M☉
Radius	6.3[2] R☉
Luminosity	10,864[8] L☉
Surface gravity (log g)	4.18[8] cgs
Temperature	20,172[2] K
Rotational velocity (v sin i)	340[8] km/s
Age	30.0±7.4[7] Myr
ι Ara B
Mass	1.06±0.29[4] M☉
Radius	0.61±0.09[9] R☉
Luminosity	437[9] L☉
Temperature	33,800[9] K
Other designations
ι Ara, NSV 8566, CD−47°11484, FK5 3379, GC 23470, HD 157042, HIP 85079, HR 6451, SAO 227886, PPM 322888[10]
Database references
SIMBAD	data

The primary component is an evolved subgiant star with a stellar classification of B2 IVe.^[4] The 'e' notation indicates the spectrum displays emission lines, which means this is a Be star that is surrounded by hot, circumstellar gas. It is spinning rapidly with a projected rotational velocity of 340 km/s.^[8] The Doppler effect from this rotation is causing the absorption lines to widen and become nebulous.

A light curve for Iota Arae, plotted from TESS data^[11]

Iota Arae has around 8.3^[7] times the mass of the Sun and is shining brightly with 10,864 times the Sun's luminosity.^[8] This energy is being radiated into space from the outer atmosphere at an effective temperature of 20,172 K,^[2] giving it the characteristic blue-white hue of a B-type star.^[12] The General Catalog of Variable Stars classifies it as a BE variable star, ranging from visual magnitude 5.18 to 5.26 with a period of 13.36 hours.^[3] In a study of the Hipparcos data, it was found to vary in brightness by 0.054 in magnitude with no clear period.^[13]

This is a spectroscopic binary system with an orbital period of 176 days. The companion is a subdwarf O star with a mass similar to the Sun but only 61% of the Sun's radius.^[4]