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 combined 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, but it was not included in Ptolemy's Almagest. It was referred to by Al Sufi, but not formally listed in his catalogue. In more modern times, it was measured by Tycho Brahe and included in his catalogue. In Kepler's extension it is described as "Quae ultimam baltei praecedit ad austr." (preceding the outermost of the belt, to the south). It was then recorded by Johann Bayer in his Uranometria as a single star with the Greek letter σ (sigma). He described it as "in enſe, prima" (in the sword, first). 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 FGW Struve who also added a fourth (C), published 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. There 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 (myr) 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. An infrared and radio source, IRS1, 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. The cluster is considered to include a number of other stars of spectral class A or B:
HD 37699, an outlying B5 giant very close to the Horsehead Nebula HD 37525, a B5 main sequence star and spectroscopic binary HD 294271, a B5 young stellar object with two low mass companions 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 V1147 Ori, a B9.5 giant and α2 CVn variable HD 37686, a B9.5 main sequence star close to HD 37699 HD 37545, an outlying B9 main sequence HD 294273, an A8 young stellar object 2MASS J05374178-0229081, an A9 young stellar object HD 294271 and HD 294272 make up the "double" star Struve 761 (or STF 761). It is three arc minutes from σ Orionis, which is also known as Struve 762.
Over 30 other probable cluster members have been detected within an arc minute of the central star, mostly brown dwarfs and planetary mass objects such as S Ori 60, but including the early M red dwarfs 2MASS J05384746-0235252 and 2MASS J05384301-0236145. In total, several hundred low mass objects are thought to be cluster members, including around a hundred spectroscopically measured class M stars, around 40 K class stars, and a handful of G and F class objects. Many are grouped in a central core, but there is a halo of associated objects scattered across more than 10 arc-minutes. The cluster includes a few L-dwarfs, which are determined to be planetary mass objects. In the past a few T-dwarfs were thought to be part of the cluster, but so far most of these T-dwarfs turned out to be brown dwarfs in the foreground. Some of these L-dwarfs (around 29%) are surrounded by a dusty disk. The cluster also contains a pair consisting out of the brown dwarf SE 70 and the planetary-mass object S Ori 68, which are separated by 1700 astronomical units. In 2024 high-resolution imaging with ALMA of K-stars and early M-stars showed gaps and rings in the disks around these stars. One star called Haro 5-34 (SO 1274, K7-type star) showed five gaps, seemingly arranged in a resonant chain. The disks in the cluster are small, either due to external photoevaporation by σ Orionis or the intermediate age of the region.
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