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History

σ Orionis (lower right) and the Horsehead nebula. The brighter stars are Alnitak and Alnilam.

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 major stars of the σ Orionis cluster, described in the text, plus:
HD 294268, F6e, probable member
HD 294275, A0
HD 294297, G0
HD 294300, G5 T Tauri star
HD 294301, A5

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 70, 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.

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 every 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 main sequence stars with masses between 11 and 18 M_☉.

Components

An image of the Sigma Orionis star system by a small telescope. The components E, D, AB and C are visible from left to right.

The primary component Aa is the class O9.5 star, with a temperature of 35,000 K and a luminosity over 40,000 L_☉. Lines representing a B0.5 main sequence star have been shown to belong to its close companion Ab, which has 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 with conventional single mirror telescopes, their respective visual magnitudes have been calculated at 4.61 and 5.20. The two components of σ Orionis A have been resolved interferometrically using the CHARA array, and the combination of interferometric and visual observations yields a very accurate orbit.

The spectrum of component B, the outer star of the triple, cannot be detected. 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. The orbit of component B has been calculated precisely using the NPOI and CHARA arrays. The combined orbits of the three stars together give a parallax significantly more precise than the HIPPARCOS parallax.

The inclinations of the two orbits are known accurately enough to calculate their relative inclination. The two orbital planes are within 30° of being orthogonal, with the inner orbit being prograde and the outer retrograde. Although slightly surprising, this situation is not necessarily rare in triple systems.