η Orionis
Orion constellation map.svg
Red circle.svg
Location of η Orionis (circled)
Observation data
Epoch J2000      Equinox 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]
Astrometry
Radial velocity (Rv)+19.8[5] km/s
Proper motion (μ) RA: −0.71[1] mas/yr
Dec.: −3.46[1] mas/yr
Parallax (π)3.34 ± 1.07 mas[1]
Distanceapprox. 1,000 ly
(approx. 300 pc)
Orbit[6]
PrimaryAa
CompanionAb
Period (P)7.989255±0.000005 days
Eccentricity (e)0.011±0.0002
Inclination (i)85°
Semi-amplitude (K1)
(primary)
145.5±0.03 km/s
Semi-amplitude (K2)
(secondary)
150±3 km/s
Orbit[3]
CompanionAc
Period (P)9.442±0.012 yr
Semi-major axis (a)0.0441±0.0015
Eccentricity (e)0.45±0.02
Inclination (i)102.8±1.8°
Details
η Ori Aa
Mass11.0±0.5[6] M
Radius6.3±0.6[6] R
Temperature26,600[6] K
Rotational velocity (v sin i)20[6] km/s
η Ori Ab
Mass10.6±0.7[6] M
Radius5.2±0.4[6] R
Temperature26,600[6] K
Rotational velocity (v sin i)130[6] km/s
η Ori Ac
Mass6.78[7] M
η Ori B
Mass8.7[7] M
Other designations
η Ori, 28 Orionis, BD−02°1235, HD 35411, HIP 25281, HR 1788, SAO 132071[8]
Database references
SIMBADdata

Eta Orionis, Latinized from η Orionis, is a multiple star in the constellation Orion. It lies a little to the west of Orion's Belt between Delta Orionis and Rigel, being closer to Delta Orionis than to Rigel. It lies at a distance of around 1,000 light-years from Earth and is part of the 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 spectroscopic triple star, known from multiple spectral lines with varying radial velocities.[6] The most distant component Ac, has been resolved using 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.
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. ^ a b c d e 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. ^ a b c Crawford, D. L.; Barnes, J. V.; Golson, J. C. (1971). "Four-color, H-beta, and UBV photometry for bright B-type stars in the northern hemisphere". The Astronomical Journal. 76: 1058. Bibcode:1971AJ.....76.1058C. doi:10.1086/111220.
  3. ^ a b c d Balega, I. I; Balega, Yu. Yu; Hofmann, K. -H; Tokovinin, A. A; Weigelt, G. P (1999). "Parameters of four multiple systems from speckle interferometry". Astronomy Letters. 25 (12): 797. Bibcode:1999AstL...25..797B.
  4. ^ a b Watson; et al. (2006–2012). "AAVSO International Variable Star Index VSX". VizieR. Retrieved 2020-09-24.
  5. ^ Evans, D. S. (June 20–24, 1966). Batten, Alan Henry; Heard, John Frederick (eds.). "The Revision of the General Catalogue of Radial Velocities". Determination of Radial Velocities and Their Applications, Proceedings from IAU Symposium No. 30. 30: 57. Bibcode:1967IAUS...30...57E.
  6. ^ a b c d e f g h i j k l De Mey, K.; Aerts, C.; Waelkens, C.; Van Winckel, H. (1996). "The early-type multiple system η Orionis. II. Line profile variations in component Ab". Astronomy and Astrophysics. 310: 164. Bibcode:1996A&A...310..164D.
  7. ^ a b c "HIP 25281". Multiple Star Catalog. Retrieved 2020-09-24.
  8. ^ "CCDM J05245-0223AB -- Double or multiple star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2012-02-09.
  9. ^ 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.
  10. ^ Abt, H. A; Levato, H (1977). "Spectral types in the Orion OB1 association". Publications of the Astronomical Society of the Pacific. 89: 797. Bibcode:1977PASP...89..797A. doi:10.1086/130230.
  11. ^ Warren, W. H., Jr; Hesser, J. E (1978). "A photometric study of the Orion OB 1 association. III - Subgroup analyses". The Astrophysical Journal Supplement Series. 36: 497. Bibcode:1978ApJS...36..497W. doi:10.1086/190510.
  12. ^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  13. ^ a b Waelkens, C.; Lampens, P. (1988). "The early-type multiple system eta Orionis. I. Photometric variability and rediscussion of the physical parameters of the components". Astronomy and Astrophysics. 194: 143. Bibcode:1988A&A...194..143W.