Iota Cassiopeiae

Iota Cassiopeiae (ι Cas, ι Cassiopeiae) is a star system in the constellation Cassiopeia. The system has a combined apparent magnitude of about 4.5,^[6] making it visible to the naked eye. Based on its parallax, it is located about 133 light-years (41 parsecs) from Earth.^[8]

ι Cassiopeiae

The Bayer-designated stars in Cassiopeia. Iota Cassiopeiae is circled.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cassiopeia[1]
ι Cas A
Right ascension	02h 29m 03.960s[2]
Declination	+67° 24′ 08.70″[2]
Apparent magnitude (V)	4.61[2] (4.65 / 8.48)‍[3]
ι Cas B
Right ascension	02h 29m 03.567s[2]
Declination	+67° 24′ 07.01″[2]
Apparent magnitude (V)	6.87[2]
ι Cas C
Right ascension	02h 29m 05.086s[2]
Declination	+67° 24′ 05.53″[2]
Apparent magnitude (V)	9.05[4] (9.14 / 11.84)‍[4]
Characteristics
U−B color index	+0.03[5]
B−V color index	+0.12[5]
ι Cas A
Spectral type	A3p / G6[3]
Variable type	α2 CVn[6]
ι Cas B
Spectral type	F5[3]
ι Cas C
Spectral type	K4 / M2[4]
U−B color index	+0.18[5]
B−V color index	+0.72[5]
Astrometry
Radial velocity (Rv)	1.2 ± 2[7] km/s
Proper motion (μ)	RA: -26.61[8] mas/yr Dec.: 38.21[8] mas/yr
Parallax (π)	22.22±0.08 mas[9]
Distance	146.8 ± 0.5 ly (45.0 ± 0.2 pc)
Absolute magnitude (MV)	1.62±0.07 (Aa) 5.60±0.17 (Ab)‍[10]
Orbit[9]
Primary	ι Cas Aa
Companion	ι Cas Ab
Period (P)	48.72±0.45 yr
Semi-major axis (a)	0.423±0.004″
Eccentricity (e)	0.637±0.004
Inclination (i)	148.2±1.3°
Longitude of the node (Ω)	176.6±1.8°
Periastron epoch (T)	B 1993.21±0.05
Argument of periastron (ω) (secondary)	328.2±1.9°
Orbit[9]
Primary	ι Cas A
Companion	ι Cas B
Period (P)	2400 yr
Semi-major axis (a)	6.50″
Eccentricity (e)	0.40
Inclination (i)	102.9±0.3°
Longitude of the node (Ω)	188.0±0.9°
Periastron epoch (T)	B 940±47
Argument of periastron (ω) (secondary)	113.3±3.4°
Details
ι Cas Aa
Mass	1.98[9] M☉
Radius	2.3±0.4[11] R☉
Luminosity	24[11] L☉
Temperature	8,360±275[11] K
Rotation	1.74033 d[11]
Rotational velocity (v sin i)	48[11] km/s
Age	100[10] Myr
ι Cas Ab
Mass	0.98[9] M☉
ι Cas B
Mass	1.28[9] M☉
Temperature	6540[4] K
ι Cas Ca
Temperature	4520 ± 20[4] K
ι Cas Cb
Temperature	3,590±45[4] K
Other designations
BD+66°213, HD 15089, HIP 11569, HR 707, SAO 12298
ι Cas A: TYC 4058-1504-1
ι Cas B: TYC 4058-1504-2
ι Cas C: TYC 4058-1505-1
Database references
	ι Cas AB
	ι Cas A
	ι Cas B
	ι Cas C

Components

A light curve for Iota Cassiopeiae, plotted from TESS data^[12]

Iota Cassiopeiae is known to be a quintuple star system. The brightest star system, ι Cassiopeiae A, contains a white-colored A-type main-sequence star with a mean apparent magnitude of +4.61.^[2] The primary is itself a tighter binary star system. The two stars were resolved by adaptive optics.^[3] These are designated Aa and Ab (although confusingly they may also be labeled as A and Aa, respectively).^[4] The primary is classified as an Alpha² Canum Venaticorum-type variable star and the brightness of the system varies from magnitude +4.45 to +4.53 with a period of 1.74 days,^[6] because of its magnetic field.^[11] The variability in brightness was first detected by Karl D. Rakos in 1962, although its spectrum was known to be variable from earlier observations.^[13] The fainter companion is a G-type star with a mass of 0.98 M_☉.^[9] The orbital period of the system is about 49 years.^[9]

ι Cassiopeiae B is a yellow-white F-type main sequence dwarf with an apparent magnitude of +6.87.^[2] It orbits around ι Cassiopeiae A approximately every 2,400 years with a semi-major axis of around 6.5 arcseconds, but the orbit is not very well constrained.^[9] This object may be causing Kozai–Lidov cycles in the inner orbital pair.^[9]

ι Cassiopeiae C is itself another binary, designated Ca and Cb,^[9] or just C and c.^[4] It comprises two stars, a K-type star and an M-type star.^[4] It is currently at an angular distance of about 7 arcseconds from the AB pair.^[14] Since the semimajor axis of the AB orbit is about 6.5 arcseconds, the true semimajor axis of C's orbit around them is thought to be significantly larger than 7 arcseconds.^[9]