Theta1 Orionis A
Star in the constellation Orion
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Theta1 Orionis A (θ1 Ori A) is a trinary star in the constellation Orion. The system consists of an close pair of eclipsing stars orbited by a visual companion farther away. Its apparent magnitude range is 6.72 to 7.65 with a period of 65.432 days. It is one of the main stars in The Trapezium in Orion, along with B, C, and D, as well as the fainter E.
| Observation data Epoch J2000 Equinox ICRS | |
|---|---|
| Constellation | Orion |
| Right ascension | 05h 35m 15.84743s[1] |
| Declination | −05° 23′ 14.3441″[1] |
| Apparent magnitude (V) | 6.72 - 7.65[2] |
| Characteristics | |
| Spectral type | B0.5V[3] + G[4] + F PMS[5] |
| Variable type | Algol[2] |
| Astrometry | |
| Radial velocity (Rv) | 32.354±0.062[4] km/s |
| Proper motion (μ) | RA: 4.81±0.10[6] mas/yr Dec.: −2.53±0.12[6] mas/yr |
| Parallax (π) | 2.626±0.100 mas[6] |
| Distance | 1,272.0 ± 6.5 ly (390±2 pc)[4] |
| Absolute magnitude (MV) | −2.80[3] |
| Orbit[4] | |
| Primary | A1 |
| Name | A3 |
| Period (P) | 65.433300±0.000019 days |
| Semi-major axis (a) | 178.56 ± 0.45 R☉ (0.8304 ± 0.0021 au) |
| Eccentricity (e) | 0.64383±0.00029 |
| Inclination (i) | 87.090±0.014° |
| Periastron epoch (T) | 2,459,441.257±0.016 HJD |
| Argument of periastron (ω) (secondary) | 177.88±0.06° |
| Semi-amplitude (K1) (primary) | 28.499±0.082 km/s |
| Semi-amplitude (K2) (secondary) | 151.43±0.76 km/s |
| Details[4] | |
| A1 | |
| Mass | 15.01±0.32 M☉ |
| Radius | 4.20±0.05 R☉ |
| Luminosity | 15,800+3,200 −2,700 L☉ |
| Surface gravity (log g) | 4.37±0.01 cgs |
| Temperature | 30,500±800 K |
| Age | 0.7 Myr |
| A3 | |
| Mass | 2.825±0.145 M☉ |
| Radius | 6.415±0.055 R☉ |
| Luminosity | 54+11 −9 L☉ |
| Surface gravity (log g) | 3.28±0.04 cgs |
| Temperature | 5,550±500 K |
| Age | 0.7 Myr |
| A2 | |
| Mass | 4[7][5] M☉ |
| Other designations | |
| 41 Ori A, V1016 Ori, BD−05 1315A, HD 37020, HIP 26220, HR 1893 | |
| Database references | |
| SIMBAD | data |
Variability
θ1 Orionis A varies in brightness, showing dips from magnitude 6.72 to 7.65 every 65.432 days. These are caused by eclipses of the central pair. θ1 Orionis A has been assigned the variable star designation V1016 Orionis.[2]
System
θ1 Orionis A1 and θ1 Orionis A3 form an eclipsing binary system, where only the primary is detected in the spectrum.[10][11] The components orbit around each other with an orbital period of roughly 65.4 days, following a highly-eccentric path with a semi-major axis of 0.8304 ± 0.0021 au and an eccentricity of 0.644. The primary is an B0.5V star with a temperature of 30,500 K, 15 times the mass of the Sun, 4.2 times the Sun's radius, and a luminosity 16,000 times solar.[4] The secondary has been proposed to be a T Tauri star, possibly of spectral class A,[12][11] but later confirmed to be of spectral class G with a temperature of 5,600±500 K. It has a mass 2.8 times solar and an inflated radius of 6.4 times that of the Sun, radiating a luminosity 54 times brighter than the Sun's.[4]
θ1 Orionis A2 is an F-type pre-main-sequence star, 1.63 fainter than the inner pair in the K band and with an estimated mass of 4 solar masses. It was proven to be gravitationally bound to the system in 2018 using long-term astrometric measurements. Its separation from the inner pair has changed from 0.227" in 1995 to 0.193" in late 2011. At the system's distance, the angular separations correspond to a projected separation of 90–100 AU. Its orbital period has been determined at 214 years.[5]