Theta1 Orionis C

For other stars with this Bayer designation, see θ Orionis.

Theta¹ Orionis C (θ¹ Orionis C) is a member of the Trapezium open cluster that lies within the Orion Nebula. The star C is the most massive of the four bright stars at the heart of the cluster. It is an O class blue main sequence star with a B-type main sequence companion. Its high luminosity and large distance (about 1,500 light years) give it an apparent visible magnitude of 5.1.

Theta¹ Orionis C

Theta1 Orionis C and companions in the Trapezium cluster
Observation data Epoch J2000      Equinox J2000
Constellation	Orion
Right ascension	05h 35m 16.46375s[1]
Declination	−05° 23′ 22.8486″[1]
Apparent magnitude (V)	5.13[2]
Characteristics
Spectral type	θ1 Orionis C1: O6Vp[3] θ1 Orionis C2: B0V[4]
U−B color index	−0.95[2]
B−V color index	+0.02[2]
Variable type	suspected[5]
Astrometry
Radial velocity (Rv)	+23.6 ± 2.0[6] km/s
Proper motion (μ)	RA: −4.13[1] mas/yr Dec.: 6.82[1] mas/yr
Parallax (π)	2.11±0.41 mas[1]
Distance	410 ± 20[7] pc
Absolute magnitude (MV)	−4.9[8]
Details
C1
Mass	33 ± 5[4] M☉
Radius	10.6 ± 1.5[8] R☉
Luminosity	204,000[8] L☉
Surface gravity (log g)	4.1[8] cgs
Temperature	39,000 ± 1,000[8] K
Rotational velocity (v sin i)	24 ± 3[8] km/s
Age	2.5 ± 0.5[8] Myr
C2
Mass	11 ± 5[4] M☉
Orbit[7]
Primary	C1
Companion	C2
Period (P)	11.05 ± 0.03 yr
Semi-major axis (a)	40.00 ± 3.00″
Eccentricity (e)	0.534 ± 0.050
Inclination (i)	100.7 ± 1.0°
Longitude of the node (Ω)	25.3 ± 1.5°
Argument of periastron (ω) (secondary)	290.9 ± 2.5°
Other designations
41 Ori C, HR 1895, HD 37022, SAO 132314, HIP 26221, BD–05° 1315C
Database references
SIMBAD	data

θ¹ Orionis C is seen as a double star (right inset) with VLTI near-infrared interferometry

Theta¹ Orionis consists of multiple components, primarily the four stars of the Trapezium cluster, all within one arc-minute of each other. Theta² Orionis is a more distant grouping of three main stars plus several fainter companions, 1-2 arc-minutes from Theta¹.

Theta¹ C is itself a binary of two massive stars, C1 and C2, plus a very close fainter companion apparently escaping the system.^[9]

Theta¹ Orionis C1 is responsible for generating most of the ultraviolet light that is slowly ionizing (and perhaps photoevaporating) the Orion Nebula. This UV light is also the primary cause of the glow that illuminates the Orion Nebula. The star emits a powerful stellar wind that is a hundred thousand times stronger than the Sun's, and the outpouring gas moves at 1,000 km/s.

The primary has a strong dipolar magnetic field with an intensity of 1.1±0.1 kG. The field is inclined at an angle of 42°±6° to the rotation axis. Detected in 2002, this was the first O-type star found to have a magnetic field, and is most likely a fossil field. This field is confining the motion of the strong stellar wind, which should reduce the mass loss rate by 80%.^[10]