Theta Circini

Theta Circini (θ Cir), is a binary star located in the southern constellation of Circinus, to the northwest of Alpha Circini.^[6] It is faintly visible to the naked eye with an apparent visual magnitude of 5.110.^[3] Based upon an annual parallax shift of 11.82 mas,^[2] it is located at a distance of about 276 light years from the Sun.

θ Circini

A light curve for Theta Circini, plotted from Hipparcos data[1]
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Circinus
Right ascension	14h 56m 43.987s[2]
Declination	−62° 46′ 51.66″[2]
Apparent magnitude (V)	5.110[3] (5.90 + 5.90)[4]
Characteristics
Spectral type	B3 Ve[5] (B2 + B2)[4]
B−V color index	+0.00[3]
Variable type	GCAS[6]
Astrometry
Proper motion (μ)	RA: +102.65[2] mas/yr Dec.: +9.35[2] mas/yr
Parallax (π)	11.82±0.30 mas[2]
Distance	276 ± 7 ly (85 ± 2 pc)
Absolute magnitude (MV)	−2.43/−2.43[4]
Orbit[5]
Period (P)	39.62±0.78 yr
Semi-major axis (a)	0.08564±0.00056″
Eccentricity (e)	0.3014±0.0081
Inclination (i)	153.3±2.0°
Longitude of the node (Ω)	228.0±5.2°
Periastron epoch (T)	1993.81±0.12
Argument of periastron (ω) (secondary)	68.6±6.5°
Details
θ Cir A
Mass	9.3±0.6[7] M☉
Surface gravity (log g)	3.38[3] cgs
Temperature	19,099[3] K
Rotational velocity (v sin i)	195±13[8] km/s
Age	27.1±6.1[7] Myr
Other designations
θ Cir, CD−62°891, HD 131492, HIP 73129, HR 5551, SAO 252965.[9]
Database references
SIMBAD	data

This is an astrometric binary^[10] star system with an orbital period of about 39.6 years, an eccentricity of 0.3, and a semimajor axis of 85.64 mas.^[5] The pair show a combined stellar classification of B3 Ve,^[5] which matches a B-type main sequence star.^[5] The 'e' suffix on the class indicates this is a Be star. Alternate classifications include B4 Vnp^[7] and B4npe,^[5] with the 'n' indicating broad ("nebulous") absorption lines due to rotation and the 'p' meaning a chemically peculiar star. The two components appear to have similar visual magnitude, mass, and classification.^[5] The system behaves as a Gamma Cassiopeiae variable^[6] showing occasional outbursts of up to 0.27 in magnitude.^[11]