Omicron Cassiopeiae

Omicron Cassiopeiae (ο Cas, ο Cassiopeiae) is a triple star system in the constellation Cassiopeia. It is approximately 700 light-years from Earth, based on its parallax.^[1] It is visible to the naked eye with a slightly variable apparent magnitude of about 4.5.

A visual band light curve for Omicron Cassiopeiae, adapted from Koubský et al. (2010)^[4]

ο Cassiopeiae

Map of the Bayer-designated stars in Cassiopeia. Omicron Cassiopeiae is circled.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cassiopeia
Right ascension	00h 44m 43.51867s[1]
Declination	+48° 17′ 03.7136″[1]
Apparent magnitude (V)	4.30 - 4.62[2]
Characteristics
Spectral type	B2-5III-Ve(shell)[2]
U−B color index	−0.53[3]
B−V color index	−0.06[3]
Variable type	γ Cas[2]
Astrometry
Radial velocity (Rv)	−12.36±0.41[4] km/s
Proper motion (μ)	RA: +18.84[5] mas/yr Dec.: −7.18[5] mas/yr
Parallax (π)	4.7046±0.4306 mas[5]
Distance	690 ± 60 ly (210 ± 20 pc)
Absolute magnitude (MV)	−2.55 / 0.35[4]
Orbit[4]
Primary	ο Cas Aa
Companion	ο Cas Ab
Period (P)	1031.55 d
Semi-major axis (a)	0.0170 ± 0.0006″
Eccentricity (e)	0
Inclination (i)	115.0±2.6°
Longitude of the node (Ω)	267.3±0.8°
Periastron epoch (T)	JD 2452792.2±0.6
Argument of periastron (ω) (secondary)	0°
Semi-amplitude (K1) (primary)	21.593±0.071 km/s
Details[4]
ο Cas Aa
Mass	6.2 M☉
Radius	8.0 R☉
Temperature	14,000 K
Rotational velocity (v sin i)	220 km/s
ο Cas Ab
Mass	~5 M☉
Other designations
22 Cassiopeiae, BD+47°183, FK5 25, HD 4180, HIP 3504, HR 193, SAO 36620, ADS 622, CCDM J00447+4817
Database references
SIMBAD	ο Cas A
	ο Cas B

The primary component, ο Cassiopeiae A, is a spectroscopic binary, and its close companion completes one orbit every 2.83 years (1,031.55 days). The system has also been resolved with interferometry.^[4]

The primary of this spectroscopic binary is a blue-white B-type giant star.^[6] It is classified as a Gamma Cassiopeiae variable and its brightness varies from magnitude 4.30 to 4.62. It is rotating at a speed of 375 km/s at its equator (close to its theoretical break-up velocity of 390 km/s), although because the pole is inclined 36 degrees, its projected rotational velocity is only 220 km/s.^[4] The nature of the secondary is not well known. Despite the fact that the secondary is 2.9 magnitudes dimmer than the primary, the secondary appears to have a mass similar to, or even larger than primary.^[4] It is possible that the secondary is a pair of early A-type main-sequence stars.^[4]

A more distant companion, ο Cassiopeiae B, lies 33.6 arcseconds away. It is an eleventh-magnitude, F-type main-sequence star.^[6] Because it has a similar proper motion to the central system, it is assumed to be gravitationally bound.^[7]