Eta Orionis
Eta Orionis (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.
  | |
| 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[1] mas |
| Distance | approx. 1,000 ly (approx. 300 pc) |
| Orbit[6] | |
| Primary | Aa |
| Companion | Ab |
| 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] | |
| Companion | Ac |
| 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 | |
| Mass | 11.0±0.5[6] M☉ |
| Radius | 6.3±0.6[6] R☉ |
| Temperature | 26,600[6] K |
| Rotational velocity (v sin i) | 20[6] km/s |
| η Ori Ab | |
| Mass | 10.6±0.7[6] M☉ |
| Radius | 5.2±0.4[6] R☉ |
| Temperature | 26,600[6] K |
| Rotational velocity (v sin i) | 130[6] km/s |
| η Ori Ac | |
| Mass | 6.78[7] M☉ |
| η Ori B | |
| Mass | 8.7[7] M☉ |
| Other designations | |
| Database references | |
| SIMBAD | data |
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 1.8 thousand 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
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.[12]
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.[12]
See also
References
- 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.
- 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.
- 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.
- Watson; et al. (2006–2012). "AAVSO International Variable Star Index VSX". VizieR. Retrieved 2020-09-24.
- 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. 30: 57. Bibcode:1967IAUS...30...57E.
- 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.
- "HIP 25281". Multiple Star Catalog. Retrieved 2020-09-24.
- "CCDM J05245-0223AB -- Double or multiple star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2012-02-09.
- 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.
- 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.
- 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.
- 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.