Theta Carinae

θ Carinae, Latinized as Theta Carinae, is a spectroscopic binary star in the southern constellation of Carina. With an apparent visual magnitude of 2.76, it is the brightest star in the open star cluster IC 2602. It marks the northeastern end of the Diamond Cross asterism. Parallax measurements from the Hipparcos mission place this star at a distance of about 460 light-years (140 parsecs) from Earth.

θ Carinae
Location of θ Carinae (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Carina
Right ascension 10h 42m 57.40197s[1]
Declination –64° 23′ 40.0208″[1]
Apparent magnitude (V) 2.76[2]
Characteristics
Spectral type B0.5 Vp[3]
U−B color index –1.00[2]
B−V color index –0.22[2]
Astrometry
Radial velocity (Rv)+24[4] km/s
Proper motion (μ) RA: –18.36[1] mas/yr
Dec.: +12.03[1] mas/yr
Parallax (π)7.16 ± 0.21 mas[1]
Distance460 ± 10 ly
(140 ± 4 pc)
Absolute magnitude (MV)−3.10[5]
Orbit[6]
Period (P)2.20288 ± 0.00001 d
Eccentricity (e)0.129 ± 0.002
Argument of periastron (ω)
(secondary)		81.8 ± 1.7°
Semi-amplitude (K1)
(primary)		18.93 ± 0.05 km/s
Details
θ Car A
Mass14.9 ± 0.4[7] M☉
Radius5.1[6] R☉
Luminosity25,673[8] L☉
Surface gravity (log g)4.20[9] cgs
Temperature31,000[9] K
Rotational velocity (v sin i)108 ± 3[9] km/s
Age4.0 ± 0.7[7] Myr
Other designations
θ Car, CPD−63° 1599, FK5 406, HD 93030, HIP 52419, HR 4199, SAO 251083.[10]
Database references
SIMBADdata

Properties
Simulated image of IC 2602 with θ Car at the centre

The MKK stellar classification of this star is B0.5 Vp,[3] which indicates this B-type main sequence star generates energy through the nuclear fusion of hydrogen in its core. The 'p' suffix designates peculiar spectral features, which have been observed in both optical and ultraviolet wavelengths.

Theta Carinae is a single-lined spectroscopic binary with a 2.2 day period; the shortest known orbital period among massive stars, suggesting earlier mass transfer between the two components, possibly explaining the spectral peculiarities.[6] In this spectroscopic system, the primary star is probably a blue straggler, which is an unusual type of star created by merging or the interaction between two or more stars. The source of the mass transfer is likely to be the less massive secondary companion, and what is now the primary star was probably originally the less massive component. The estimated age of the pair is 4 million years,[7] and it appears much younger than the surrounding IC 2602 cluster.[6]

The primary star is about 15 solar masses (M☉)[7] and five solar radii (R☉).[6] Theta Carinae has an intensely hot outer radiating envelope with an effective surface temperature of 31,000 K.[9] Once the primary reaches around 11 million years old, the star will expand and will begin to transferring its outer surface mass back to its companion. Little is known about the companion star, but it is likely an F-type star with a luminosity less than 1% of the primary.[6]

Etymology

In Chinese, 南船 (Nán Chuán), meaning Southern Boat, refers to an asterism consisting of θ Carinae, V337 Carinae, PP Carinae, β Carinae and ω Carinae.[11] Consequently, θ Carinae itself is known as 南船三 (Nán Chuán sān, English: the Third Star of Southern Boat).[12]

Apparent location

In most versions of its asterism, the neighbouring bright stars, thus plotted along the imaginary hull forming Carina are Omega Carinae which is approximately 120 light years closer and PP (also known as p) Carinae, of a similar distance to Theta.

References
  1. van Leeuwen, F. (November 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. Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
  3. Houk, Nancy (1978), Michigan catalogue of two-dimensional spectral types for the HD stars, vol. 1, Ann Arbor: Dept. of Astronomy, University of Michigan, Bibcode:1975mcts.book.....H
  4. Evans, D. S. (June 20–24, 1966), "The Revision of the General Catalogue of Radial Velocities", in Batten, Alan Henry; Heard, John Frederick (eds.), Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30, vol. 30, University of Toronto: International Astronomical Union, p. 57, Bibcode:1967IAUS...30...57E
  5. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  6. Hubrig, S.; et al. (September 2008), "New insights into the nature of the peculiar star θ Carinae", Astronomy and Astrophysics, 488 (1): 287–296, arXiv:0807.2067, Bibcode:2008A&A...488..287H, doi:10.1051/0004-6361:200809972, S2CID 59390038
  7. Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873
  8. Hohle, M. M.; Neuhäuser, R.; Schutz, B. F. (April 2010), "Masses and luminosities of O- and B-type stars and red supergiants", Astronomische Nachrichten, 331 (4): 349, arXiv:1003.2335, Bibcode:2010AN....331..349H, doi:10.1002/asna.200911355, S2CID 111387483
  9. Lefever, K.; et al. (June 2010), "Spectroscopic determination of the fundamental parameters of 66 B-type stars in the field-of-view of the CoRoT satellite", Astronomy and Astrophysics, 515: A74, arXiv:0910.2851, Bibcode:2010A&A...515A..74L, doi:10.1051/0004-6361/200911956, S2CID 54028925
  10. "* tet Car". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-02-16.
  11. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  12. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 28 日
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