Eta Orionis

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η Orionis
Location of η Orionis (circled)
Observation data
J2000
Constellation Orion
Right ascension 05h 24m 28.61672s[1]
Declination –02° 23′ 49.7311″[1]
Apparent magnitude (V) 3.42[2] (4.50 + 5.90 + 5.65 + 4.95)[3]
Characteristics
Spectral type Aa: B1 V
Ab: B3 V
Ac: B3 V
B: B2 V[3]
U−B color index –0.90[2]
B−V color index −0.17[2]
Variable type Algol[4]
Distance
approx. 1,000 ly
(approx. 300 pc)
Semi-amplitude
(K1)
(primary)		145.5±0.03 km/s
Semi-amplitude (K2)
(secondary)		150±3 km/s
Inclination
(i)		102.8±1.8°
Details
η Ori Aa
Rotational velocity (v sin i)
130[6] km/s
η Ori Ac
Mass6.78[7] M☉
η Ori B
Mass8.7[7] M☉
HR 1788, SAO 132071[8]
Database references
SIMBADdata

Eta Orionis or Algjebbah,

Orion OB1 association
.

System

Eta Orionis is listed in multiple star catalogues as having two companions: a bright component B less than 2″ away; and a faint component C nearly 2′ away.[9] The two are estimated to orbit every 1,800 years.[7]

The primary star, Eta Orionis A, is itself a

speckle interferometry, at a separation of about 0.04″. It orbits the other two in 9.4 years.[3] The two closest stars, Aa and Ab, are separated by only about a tenth of an astronomical unit and orbit in just under eight days.[6]

The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion.[10] It is assigned to the oldest and closest part of the association, known as OB1a.[11]

Variability

A light curve for Eta Orionis, plotted from TESS data,[12] showing both eclipses and the 0.432 day variability.

Eta Orionis drops in brightness every four days from a combined apparent magnitude of 3.31 to about magnitude 3.6. This is due to eclipses between the two closest components, Aa and Ab.[4] The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep, respectively.[13]

It has also been suggested that component Ab is intrinsically variable with a period of 0.3 days and a very small amplitude. This star has unusual variable spectral lines and lies with the

β Cephei variable instability strip.[6] However, it is now thought that the variable component is either B and Ac, possibly due to an unseen companion or rotational modulation. The actual period is 0.432 days and the 0.3-day period was an alias.[13]

See also

References

  1. ^
    S2CID 18759600
    .
  2. ^
    doi:10.1086/111220
    .
  3. ^
    Bibcode:1999AstL...25..797B
    .
  4. ^ a b Watson; et al. (2006–2012). "AAVSO International Variable Star Index VSX". VizieR. Retrieved 2020-09-24.
  5. Bibcode:1967IAUS...30...57E
    .
  6. ^
    Bibcode:1996A&A...310..164D
    .
  7. ^ a b c "HIP 25281". Multiple Star Catalog. Archived from the original on 2020-10-03. Retrieved 2020-09-24.
  8. Centre de Données astronomiques de Strasbourg
    . Retrieved 2012-02-09.
  9. doi:10.1086/323920
    .
  10. doi:10.1086/130230
    .
  11. doi:10.1086/190510
    .
  12. ^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  13. ^
    Bibcode:1988A&A...194..143W
    .
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