Theta2 Orionis

Theta2 Orionis (θ2 Ori) is a multiple star system in the constellation Orion. It is a few arc minutes from its more famous neighbour the Trapezium Cluster, also known as θ1 Orionis.

For other stars with this Bayer designation, see θ Orionis.
θ2 Orionis

The three components of θ2 Orionis, plus nearby stars
Credit: Ian Nartowicz (derived from ESA/Hubble image)
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
A
Right ascension 05h 35m 22.90115s[1]
Declination −05° 24′ 57.8207″[1]
Apparent magnitude (V) 5.02[2]
B
Right ascension 05h 35m 26.4018s[3]
Declination −05° 25′ 00.720″[3]
Apparent magnitude (V) 6.380[4]
C
Right ascension 05h 35m 31.436s[3]
Declination −05° 25′ 16.40″[3]
Apparent magnitude (V) 8.18[5]
Characteristics
Spectral type O9.5IVp[6] + B0.7V[7] + B5V[8]
Astrometry
Radial velocity (Rv)35.6[9] km/s
Proper motion (μ) RA: 2.29[1] mas/yr
Dec.: 0.96[1] mas/yr
Parallax (π)2.11 ± 0.41[1] mas
Distanceapprox. 1,500 ly
(approx. 470 pc)
Absolute magnitude (MV)−4.3[10] + −2.60[4] + −0.6[11]
Details
A
Mass39[12] M☉
Luminosity107,000[13] L☉
Temperature34,900[13] K
Rotational velocity (v sin i)133[7] km/s
B
Mass14.8[4] M☉
Radius4.3[4] R☉
Luminosity12,300[4] L☉
Surface gravity (log g)4.30[4] cgs
Temperature29,300[4] K
Rotational velocity (v sin i)33[7] km/s
Age<2[4] Myr
C
Mass4.86[13] M☉
Luminosity616[13] L☉
Temperature13,800[13] K
Rotational velocity (v sin i)78[14] km/s
Age<1[14] Myr
Other designations
θ2 Ori, STF 4016, WDS J05354-0525, ADS 4188
θ2 Ori A: 43 Ori, HR 1897, BD−05°1319, HD 37041, SAO 132321, HIP 26235, CCDM 05353-0524E
θ2 Ori B: BD−05°1320, HD 37042, SAO 132322, CCDM 05353-0524F
θ2 Ori C: V361 Ori, BD−05°1326, HD 37062, SAO 132329, S 490A, CCDM 05353-0524G
Database references
SIMBADdata
data2
data3

Components
The three stars of θ2 Orionis within the Orion Nebula

θ2 Orionis consists of three stars in a line, each about an arc-minute from the next. In addition to the well-known three stars, the Washington Double Star Catalog confusingly lists a component D which is actually θ1 Orionis C.[15]

There is one other star brighter than 10th magnitude in the region. V1073 Orionis is a B9.5 Orion variable that forms an equilateral triangle with θ2 Ori B and C.

Bizarrely, θ2 Orionis C has a second entry in the Washington Double Star Catalog under the name S490. The companion is 10th magnitude and actually lies between θ2 Ori B and V1073 Ori.[15]

θ1 Orionis, the well known Trapezium cluster, is only 2 arc minutes away from θ2 Orionis A. Despite the names, θ2 Orionis A is marginally brighter than the brightest star in the Trapezium. The Catalog of Components of Double and Multiple Stars includes the stars of θ1 and θ2 Ori within the same system of 13 components.

There are dozens of much fainter stars in the same field, many of them pre-main-sequence stars still forming from the Orion Molecular Cloud Complex.

Properties

The primary component A appears as a 5th magnitude O class subgiant over 100,000 times as luminous as the sun. The spectral type suggests it is evolving away from the main sequence, although it is thought to be less than 2 million years old. The spectral peculiarities may be related to close companions or could be caused by the extreme youth of the star.

The 6th magnitude component B is an early B main sequence star nearly 30,000 K and over 10,000 times the luminosity of the sun.

Component C is another B class main sequence star, but cooler and less than a thousand times the luminosity of the sun.

θ2 Orionis A system

θ2 Orionis A is itself a triple star system. Its spectral lines were seen to change position periodically, indicating orbital motion. The first orbit was derived in 1924, indicating a period of 21 days. and a rather eccentric orbit.[16]

Speckle interferometry has resolved a companion about 0.3" away, around 147 AU. High resolution spectroscopy shows that there is an even closer companion, only about 0.47 AU from the primary, for a total of three stars. Both companions are thought to be early A or late B with masses of 7-9 M☉. This helps to explain the high mass and visual luminosity for an O9.5 star at this distance. The three stars together have nearly the same mass as the O5.5 θ1 Orionis C and visually are even brighter.[17]

θ2 Orionis A also shows unexplained rapidly variable x-ray emission. The x-rays cannot be explained standard mechanisms such as colliding winds or coronal emissions from an unseen companion.[12]

References
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. Aldoretta, E. J.; Caballero-Nieves, S. M.; Gies, D. R.; Nelan, E. P.; Wallace, D. J.; Hartkopf, W. I.; Henry, T. J.; Jao, W.-C.; Maíz Apellániz, J.; Mason, B. D.; Moffat, A. F. J.; Norris, R. P.; Richardson, N. D.; Williams, S. J. (2015). "The Multiplicity of Massive Stars: A High Angular Resolution Survey with the Guidance Sensor". The Astronomical Journal. 149 (1): 26. arXiv:1410.0021. Bibcode:2015AJ....149...26A. doi:10.1088/0004-6256/149/1/26. S2CID 58911264.
  3. Hog, E.; Kuzmin, A.; Bastian, U.; Fabricius, C.; Kuimov, K.; Lindegren, L.; Makarov, V. V.; Roeser, S. (1998). "The TYCHO Reference Catalogue". Astronomy and Astrophysics. 335: L65. Bibcode:1998A&A...335L..65H.
  4. Nieva, María-Fernanda; Przybilla, Norbert (2014). "Fundamental properties of nearby single early B-type stars". Astronomy & Astrophysics. 566: A7. arXiv:1412.1418. Bibcode:2014A&A...566A...7N. doi:10.1051/0004-6361/201423373. S2CID 119227033.
  5. Pasinetti Fracassini, L. E.; Pastori, L.; Covino, S.; Pozzi, A. (2001). "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics". Astronomy and Astrophysics. 367 (2): 521. arXiv:astro-ph/0012289. Bibcode:2001A&A...367..521P. doi:10.1051/0004-6361:20000451. S2CID 425754.
  6. Sota, A.; Maíz Apellániz, J.; Walborn, N. R.; Alfaro, E. J.; Barbá, R. H.; Morrell, N. I.; Gamen, R. C.; Arias, J. I. (2011). "The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-violet at R ~ 2500". The Astrophysical Journal Supplement. 193 (2): 24. arXiv:1101.4002. Bibcode:2011ApJS..193...24S. doi:10.1088/0067-0049/193/2/24. S2CID 119248206.
  7. Simón-Díaz, S.; Herrero, A. (2014). "The IACOB project. I. Rotational velocities in northern Galactic O- and early B-type stars revisited. The impact of other sources of line-broadening". Astronomy & Astrophysics. 562: A135. arXiv:1311.3360. Bibcode:2014A&A...562A.135S. doi:10.1051/0004-6361/201322758. S2CID 119278062.
  8. Houk, N.; Swift, C. (1999). "Michigan catalogue of two-dimensional spectral types for the HD Stars, Vol. 5". Michigan Spectral Survey. 05: 0. Bibcode:1999MSS...C05....0H.
  9. Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington. Bibcode:1953GCRV..C......0W.
  10. Abbott, D. C. (1978). "The terminal velocities of stellar winds from early-type stars". Astrophysical Journal. 225: 893. Bibcode:1978ApJ...225..893A. doi:10.1086/156554.
  11. Abt, H. A. (1979). "The occurrence of abnormal stars in open clusters". Astrophysical Journal. 230: 485. Bibcode:1979ApJ...230..485A. doi:10.1086/157104.
  12. Mitschang, Arik W.; Schulz, Norbert S.; Huenemoerder, David P.; Nichols, Joy S.; Testa, Paola (2011). "Detailed X-Ray Line Properties of θ2 Ori a in Quiescence". The Astrophysical Journal. 734 (1): 14. arXiv:1009.1896. Bibcode:2011ApJ...734...14M. doi:10.1088/0004-637X/734/1/14. S2CID 15568141.
  13. Hillenbrand, Lynne A. (1997). "On the Stellar Population and Star-Forming History of the Orion Nebula Cluster". Astronomical Journal. 113: 1733. Bibcode:1997AJ....113.1733H. doi:10.1086/118389.
  14. Manoj, P.; Maheswar, G.; Bhatt, H. C. (2002). "Non-emission-line young stars of intermediate mass". Monthly Notices of the Royal Astronomical Society. 334 (2): 419. arXiv:astro-ph/0204491. Bibcode:2002MNRAS.334..419M. doi:10.1046/j.1365-8711.2002.05540.x. S2CID 14024844.
  15. Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  16. Struve, O. (1924). "The orbit of the spectroscopic binary 43 theta-2 Orionis". Astrophysical Journal. 60: 159. Bibcode:1924ApJ....60..159S. doi:10.1086/142845.
  17. Schulz, Norbert S.; Testa, Paola; Huenemoerder, David P.; Ishibashi, Kazunori; Canizares, Claude R. (2006). "X-Ray Variability in the Young Massive Triple θ2 Orionis A". The Astrophysical Journal. 653 (1): 636–646. arXiv:astro-ph/0608420. Bibcode:2006ApJ...653..636S. doi:10.1086/508625. S2CID 119414078.
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