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Location of σ Ori (circled) | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Orion |
| Right ascension | 05 38 42.0 |
| Declination | −2° 36′ 00″ |
| Apparent magnitude (V) | 4.07 (A) 5.27 (B) 8.79 (C) 6.62 (D) 6.66 (E) |
| Characteristics | |
| AB | |
| Spectral type | O9.5V + B0.5V |
| U−B color index | −1.02 |
| B−V color index | −0.31 |
| C | |
| Spectral type | A2 V |
| U−B color index | −0.25 |
| B−V color index | −0.02 |
| D | |
| Spectral type | B2 V |
| U−B color index | −0.87 |
| B−V color index | −0.17 |
| E | |
| Spectral type | B2 Vpe |
| U−B color index | −0.84 |
| B−V color index | −0.09 |
| Variable type | SX Ari |
| Astrometry | |
| Radial velocity (Rv) | −29.45 ± 0.45 km/s |
| Parallax (π) | AB: 3.04 ± 8.92 mas D: 6.38 ± 0.90 mas |
| Distance | 385 ± 19 pc |
| Absolute magnitude (MV) | −3.49 (Aa) −2.90 (Ab) −2.79 (B) |
| Orbit | |
| Primary | Aa/Ab |
| Period (P) | 143.198 ± 0.005 days |
| Semi-major axis (a) | 0.0043" (~360 R☉) |
| Eccentricity (e) | 0.7782 ± 0.0011 |
| Inclination (i) | ~56.3° |
| Semi-amplitude (K1) (primary) | 71.9 ± 0.3 km/s |
| Semi-amplitude (K2) (secondary) | 95.2 ± 0.3 km/s |
| Orbit | |
| Primary | A/B |
| Period (P) | 156.7 ± 3.0 yr |
| Semi-major axis (a) | 0.2662 ± 0.0021″ |
| Eccentricity (e) | 0.0515 ± 0.0080 |
| Inclination (i) | 159.7 ± 3.7° |
| Details | |
| Aa | |
| Mass | 18 M☉ |
| Radius | 5.6 R☉ |
| Luminosity | 41,700 L☉ |
| Surface gravity (log g) | 4.20 cgs |
| Temperature | 35,000 K |
| Rotational velocity (v sin i) | 135 km/s |
| Age | 300,000 years |
| Ab | |
| Mass | 13 M☉ |
| Radius | 4.8 R☉ |
| Luminosity | 18,600 L☉ |
| Surface gravity (log g) | 4.20 cgs |
| Temperature | 31,000 K |
| Rotational velocity (v sin i) | 35 km/s |
| Age | 900,000 years |
| B | |
| Mass | 14 M☉ |
| Radius | 5.0 R☉ |
| Luminosity | 15,800 L☉ |
| Surface gravity (log g) | 4.15 cgs |
| Temperature | 29,000 K |
| Rotational velocity (v sin i) | 250 km/s |
| Age | 1,900,000 years |
| Details | |
| C | |
| Mass | 2.7 M☉ |
| Details | |
| D | |
| Mass | 6.8 M☉ |
| Surface gravity (log g) | 4.3 cgs |
| Temperature | 21,500 K |
| Rotational velocity (v sin i) | 180 km/s |
| Details | |
| E | |
| Mass | 8.30 M☉ |
| Radius | 3.77 R☉ |
| Surface gravity (log g) | 3.95 cgs |
| Temperature | 22,500 K |
| Rotation | 1.190847 days |
| Other designations | |
| Sigma Orionis, Sigma Ori, σ Orionis, σ Ori, 48 Orionis, 48 Ori | |
| AB: HD 37468, HR 1931, HIP 26549, SAO 132406, BD−02°1326, 2MASS J05384476-0236001, Mayrit AB | |
| C: 2MASS J05384411-0236062, Mayrit 11238 | |
| D: HIP 26551, 2MASS J05384561-0235588, Mayrit 13084 | |
| E: V1030 Orionis, HR 1932, HD 37479, BD−02°1327, 2MASS J05384719-0235405, Mayrit 41062 | |
| Database references | |
| SIMBAD | σ Ori data |
| SIMBAD | σ Ori C data |
| SIMBAD | σ Ori D data |
| SIMBAD | σ Ori E data |
| SIMBAD | σ Ori Cluster data |
Sigma Orionis or Sigma Ori (σ Orionis, σ Ori) is a multiple star system in the constellation Orion, consisting of the brightest members of a young open cluster. It is found at the eastern end of the belt, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The total brightness of the component stars is magnitude 3.80.
History
σ Orionis is a naked eye star at the eastern end of Orion's Belt, and has been known since antiquity. It was included in Ptolemy's Almagest. In modern times, it was catalogued by Johann Bayer as a single star with the Greek letter σ (sigma). It was also given the Flamsteed designation 48.
In 1776, Christian Mayer described σ Ori as a triple star, having seen components AB and E, and suspected another between the two. Component D was confirmed by Otto Struve who also added a fourth (C) in 1876. In 1892 Sherburne Wesley Burnham reported that σ Ori A was itself a very close double, although a number of later observers failed to confirm it. In the second half of the twentieth century, the orbit of σ Ori A/B was solved and at the time was one of the most massive binaries known.
σ Ori A was discovered to have a variable radial velocity in 1904, considered to indicate a single-lined spectroscopic binary. The spectral lines of the secondary were elusive and often not seen at all, possibly because they are broadened by rapid rotation. The was confusion over whether the reported spectroscopic binary status actually referred to the known visual companion B. Finally in 2011, it was confirmed that the system is triple, with an inner spectroscopic pair and a wider visual companion. The inner pair was resolved interferometrically in 2013.
σ Ori E was identified as helium-rich in 1956, having variable radial velocity in 1959, having variable emission features in 1974, having an abnormally strong magnetic field in 1978,, being photometrically variable in 1977, and formally classified as a variable star in 1979.
In 1996, a large number of low mass pre-main sequence stars were identified in the region of Orion's Belt. A particular close grouping was discovered to lie around σ Orionis. A large number of brown dwarfs were found in the same area and at the same distance as the bright σ Orionis stars. Optical, infrared, and x-ray objects in the cluster, including 115 non-members lying in the same direction, were listed in the Mayrit Catalogue with a running number, except for the central star which was listed simply as Mayrit AB.
Cluster
The σ Orionis cluster is part of the Ori OB1b stellar association, commonly referred to as Orion's Belt. The cluster was not recognised until 1996 when a population of pre-main sequence stars was discovered around σ Ori. Since then it has been extensively studied because of its closeness and the lack of interstellar extinction. It has been calculated that star formation in the cluster began 3 million years ago and it is approximately 360 pc away.
In the central arc-minute of the cluster five particularly bright stars are visible, labelled A to E in order of distance from the brightest component σ Ori A. The closest pair AB are only separated by 0.2" - 0.3" but were discovered with a 12" telescope.
Several additional companions have been listed since. In 2011, σ Ori A was discovered to be a spectroscopic binary with a period of 143 days. σ Ori C has a companion 2" away and σ Ori E has one about a third of an arc-second away, both faint low mass stars.
An infrared and radio source 3.3" from σ Ori A that was considered to be a patch of nebulosity has been resolved into two subsolar stars. There is an associated variable x-ray source that is assumed to be a T Tauri star.
In addition to the many low mass and pre-main sequence objects, the cluster is considered to include a number of other stars brighter than 9th magnitude:
- V1147 Ori, a B9.5 giant and α CVn variable
- HD 37699, a B5 giant
- HD 294271, a B5 young stellar object
- HD 37525, a B5 main sequence star and spectroscopic binary
- HD 294272, a binary containing two B class young stellar objects
- HD 37333, a peculiar A1 main sequence star
- HD 37564, an A8 young stellar object
σ Orionis AB
The brightest member of the σ Orionis system appears as a late O class star, but is actually made up of three stars. The inner pair complete a highly eccentric orbit ever 143 days, while the outer star completes its near-circular orbit once every 157 years. It has not yet completed a full orbit since it was first discovered to be a double star. All three are very young massive main sequence stars, 13-18 M☉.
The primary component Aa is the class O9.5 star, with a temperature of 35,000 K and a luminosity over 40,000 L☉. Its close companion Ab has spectral type B0.5, a temperature of 31,000 K, and a luminosity of 18,600 L☉. Their separation varies from less than half an astronomical unit to around two AU. Although they cannot be directly imaged, their respective visual magnitudes have been calculated at 4.61 and 5.20.
The spectrum of component B, the outer star of the triple, cannot be detected. Lines representing a B0.5 main sequence star have been shown to belong to the inner secondary component Ab. The luminosity contribution from σ Ori B can be measured and it is likely to be a B0-2 main sequence star. Its visual magnitude of 5.31 is similar to σ Ori Ab and so it should be easily visible, but it is speculated that its spectral lines are highly broadened and invisible against the backdrop of the other two stars.
σ Orionis C
The faintest member of the main σ Orionis stars is component C. It is also the closest to σ Ori AB at 11", corresponding to 3,960 astronomical units. It is an A-type main sequence star.
σ Orionis D
Component D is a standard B2 main sequence star of magnitude 6.56. It is 13" from σ Ori AB, corresponding to 4,680 AU.
σ Orionis E
Component E is an unusual variable star, classified as an SX Arietis variable and also known as V1030 Orionis. It is helium-rich, has a strong magnetic field, and varies between magnitudes 6.61 and 6.77 during a 1.19 day period of rotation. It has a spectral type of B2 Vpe. The variability is believed to be due to large-scale variations in surface brightness caused by the magnetic field. The rotational period is slowing due to magnetic braking. σ Ori E is 41" from σ Ori AB, approximately 15,000 AU.
The magnetic field is highly variable from −2,300 to +3,100 gauss, matching the brightness variations and the likely rotational period. This requires a magnetic dipole of at least 10,000 G. Around minimum brightness, a shell type spectrum appears, attributed to plasma clouds rotating above the photosphere. The helium enhancement in the spectrum may be due to hydrogen being preferentially trapped towards the magnetic poles leaving excess helium near the equator. It has been suggested that σ Ori E could be further away and older than the other members of the cluster, from modelling its evolutionary age and size.
σ Orionis IRS1
The infrared source IRS1 is close to σ Ori A. It has been resolved to a pair of low mass objects, a proplyd, and a possible third object. The brighter object has an M1 spectral class, a mass around a half M☉, and appears to be a relatively normal low mass star. The fainter object is very unusual, showing a diluted M7 or M8 absorption spectrum with emission lines of hydrogen and helium. The interpretation is that it is a brown dwarf embedded within a proplyd that is being photoevaporated by σ Ori A. X-ray emission from IRS1 suggests the presence of an accretion disc around a T Tauri star, but it is unclear how this can fit with the proplyd scenario.
References
- ^ Wu, Zhen-Yu; Zhou, Xu; Ma, Jun; Du, Cui-Hua (2009). "The orbits of open clusters in the Galaxy". Monthly Notices of the Royal Astronomical Society. 399 (4): 2146. Bibcode:2009MNRAS.399.2146W. doi:10.1111/j.1365-2966.2009.15416.x.
- ^ 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.
- Caballero, J. A. (2014). "Stellar multiplicity in the sigma Orionis cluster: A review". The Observatory. 134: 273. arXiv:1408.2231. Bibcode:2014Obs...134..273C.
- ^ Echevarria, J.; Roth, M.; Warman, J. (1979). "Photometric Study of Trapezium-Type Systems". Revista Mexicana de Astronomia y Astrofisica. 4: 287. Bibcode:1979RMxAA...4..287E.
- ^ Caballero, J. A. (2007). "The brightest stars of the σ Orionis cluster". Astronomy and Astrophysics. 466 (3): 917. arXiv:astro-ph/0701067. Bibcode:2007A&A...466..917C. doi:10.1051/0004-6361:20066652.
- ^ Greenstein, Jesse L.; Wallerstein, George (1958). "The Helium-Rich Star, Sigma Orionis E". Astrophysical Journal. 127: 237. Bibcode:1958ApJ...127..237G. doi:10.1086/146456.
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- ^ Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
- ^ Simón-Díaz, S.; Caballero, J. A.; Lorenzo, J.; Maíz Apellániz, J.; Schneider, F. R. N.; Negueruela, I.; Barbá, R. H.; Dorda, R.; Marco, A.; Montes, D.; Pellerin, A.; Sanchez-Bermudez, J.; Sódor, Á.; Sota, A. (2015). "Orbital and Physical Properties of the σ Ori Aa, Ab, B Triple System". The Astrophysical Journal. 799 (2): 169. arXiv:1412.3469. Bibcode:2015ApJ...799..169S. doi:10.1088/0004-637X/799/2/169.
- ^ Simón-Díaz, S.; Caballero, J. A.; Lorenzo, J. (2011). "A Third Massive Star Component in the σ Orionis AB System". The Astrophysical Journal. 742: 55. arXiv:1108.4622. Bibcode:2011ApJ...742...55S. doi:10.1088/0004-637X/742/1/55.
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{{cite journal}}: Cite journal requires|journal=(help) - Walter, F. M.; Wolk, S. J.; Freyberg, M.; Schmitt, J. H. M. M. (1997). "Discovery of the σ Orionis Cluster". Memorie della Società Astronomia Italiana. 68: 1081. Bibcode:1997MmSAI..68.1081W.
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- Hodapp, Klaus W.; Iserlohe, Christof; Stecklum, Bringfried; Krabbe, Alfred (2009). "Σ Orionis IRS1 a and B: A Binary Containing a Proplyd". The Astrophysical Journal Letters. 701 (2): L100. arXiv:0907.3327. Bibcode:2009ApJ...701L.100H. doi:10.1088/0004-637X/701/2/L100.
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