Lambda Eridani

Lambda Eridani (λ Eri) is a star in the constellation Eridanus. It is visible to the naked eye on a dark night with an apparent visual magnitude of 4.25.[2] The distance to this star, based upon an annual parallax shift of 0.00402 arcseconds,[1] is roughly 810 light years.

λ Eridani
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Eridanus
Right ascension  05h 09m 08.78315s[1]
Declination −08° 45′ 14.6908″[1]
Apparent magnitude (V) 4.25[2]
Characteristics
Spectral type B2 IVne[3] or B2 III(e)p[4]
U−B color index −0.88
B−V color index −0.19
Variable type λ Eri
Astrometry
Radial velocity (Rv)−2.51±5.43[5] km/s
Proper motion (μ) RA: +0.25[1] mas/yr
Dec.: −1.97[1] mas/yr
Parallax (π)4.02 ± 0.18[1] mas
Distance810 ± 40 ly
(250 ± 10 pc)
Absolute magnitude (MV)−3.75[3]
Details
Mass8.99±0.69[6] M☉
Radius8.6[3] R☉
Luminosity7,510[6] L☉
Surface gravity (log g)3.6[7] cgs
Temperature21,150[6] K
Rotation1.40346 d[3]
Rotational velocity (v sin i)327±24[5] km/s
Other designations
λ Eridani, 69 Eridani, CD-08° 1040, FK5 190, HD 33328, HIP 23972, HR 1679, SAO 131824.[8]
Database references
SIMBADdata

λ Eri is classified in the General Catalogue of Variable Stars as a Beta Cephei variable.[9] The AAVSO International Variable Star Index defines a LERI type of variation. λ Eri is classified as LERI + GCAS since it shows both short term periodic variations and longer timescale eruptive variation.[10] It was one of the first stars where short-period variations were found.[3] The line profile variability periods are 0.702d and 0.269d, with intermittently present periods of 0.6d and 0.75d. The photometric amplitude of the variation is 0.010 magnitude.[4]

This is a giant or subgiant Be star with a stellar classification of B2 IVne[3] or B2 III(e)p,[4] depending on the source. It is spinning rapidly with a projected rotational velocity of 327 km/s.[5] Compare this to the estimated break-up velocity of 440 km/s.[7] This rotation is giving the star an oblate shape with an equatorial bulge that is 25% larger than the polar radius.[11] The most likely rotation period is deemed to be twice the period of variation, or 1.4 days.[3] The star also possesses a rotating circumstellar disc, seen edge on, which makes Lambda Eri a "shell star", where the disk appears more opaque than usual. [12]

Like most Be stars, Lambda Eridani emits soft X-rays. In 1993, a giant X-ray flare was observed in which the X-ray luminosity increased by a factor of six over a 39-hour period.[3] Lambda Eridani has about nine times the mass of the Sun,[6] and 8.6[3] times the Sun's radius.[3] It radiates 7,510[6] times the solar luminosity from its outer atmosphere at an effective temperature of 21,150 K.[6]

References
  1. 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.
  2. Crawford, D. L.; et al. (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. Balona, L. A.; James, D. J. (May 2002), "Short-period line profile and light variations in the Be star λ Eridani", Monthly Notices of the Royal Astronomical Society, 332 (3): 714–722, Bibcode:2002MNRAS.332..714B, doi:10.1046/j.1365-8711.2002.05336.x.
  4. Rivinius, Th.; et al. (November 2003), "Non-radially pulsating Be stars", Astronomy and Astrophysics, 411: 229–247, Bibcode:2003A&A...411..229R, doi:10.1051/0004-6361:20031285.
  5. Becker, Juliette C.; et al. (April 2015), "Extracting Radial Velocities of A- and B-type Stars from Echelle Spectrograph Calibration Spectra", The Astrophysical Journal Supplement Series, 217 (2): 13, arXiv:1503.03874, Bibcode:2015ApJS..217...29B, doi:10.1088/0067-0049/217/2/29, 29.
  6. Hohle, M. M.; et al. (April 2010), "Masses and luminosities of O- and B-type stars and red supergiants", Astronomische Nachrichten, 331 (4): 349–360, arXiv:1003.2335, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355.
  7. Frémat, Y.; et al. (2005), "Effects of gravitational darkening on the determination of fundamental parameters in fast-rotating B-type stars", Astronomy and Astrophysics, 440 (1): 305–320, arXiv:astro-ph/0503381, Bibcode:2005A&A...440..305F, doi:10.1051/0004-6361:20042229.
  8. "lam Eri". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2016-11-01.
  9. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR. 1. Bibcode:2009yCat....102025S.
  10. Otero, S. A.; et al. "Variable Star Type Designations in the VSX". AAVSO Website. American Association of Variable Star Observers. Retrieved 19 July 2014.
  11. van Belle, Gerard T. (March 2012), "Interferometric observations of rapidly rotating stars", The Astronomy and Astrophysics Review, 20 (1): 51, arXiv:1204.2572, Bibcode:2012A&ARv..20...51V, doi:10.1007/s00159-012-0051-2.
  12. "Lambda Eridani". stars.astro.illinois.edu. Retrieved 2018-08-20.
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