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θ Aurigae
	Location of θ Aurigae (circled)
Observation data; Epoch J2000 Equinox J2000
Constellation	Auriga
Right ascension	05h 59m 43.27012s
Declination	+37° 12′ 45.3047″
Apparent magnitude (V)	2.62 - 2.70
Characteristics
Spectral type	A0pSi + F2-5V
U−B color index	−0.18
B−V color index	−0.08
R−I color index	−0.06
Variable type	α2 CVn
Astrometry
Radial velocity (Rv)	+29.5 km/s
Proper motion (μ)	RA: +43.63 mas/yr ; Dec.: −73.79 mas/yr
Parallax (π)	19.70 ± 0.16 mas
Distance	166 ± 1 ly ; (50.8 ± 0.4 pc)
Absolute magnitude (MV)	−1.05
Details
Mass	3.24 M☉
Radius	4.68 R☉
Luminosity	214 L☉
Surface gravity (log g)	3.61 cgs
Temperature	10,220 K
Rotation	3.6187 days
Rotational velocity (v sin i)	55 km/s
Age	288 Myr
Other designations
	Mahasim, 37 Aurigae, BD+37°1380, HD 40312, HIP 28380, HR 2095, SAO 58636, WDS 05597+3713.
Database references
SIMBAD	data

Theta Aurigae (Latinized from θ Aurigae, abbreviated Theta Aur, θ Aur) is a binary star in the constellation of Auriga. Based upon parallax measurements, the distance to this system is about 166 light-years (51 parsecs).^[1]

The two components are designated Theta Aurigae A (also named Mahasim^[10]) and B.

Nomenclature

θ Aurigae (Latinised to Theta Aurigae) is the system's Bayer designation. The designations of the two components as Theta Aurigae A and B derives from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[11]

Some authors state that Theta Aurigae had no traditional name,^[12]although Richard Hinckley Allen makes a passing reference about the name Mahasim, as a name also used, with various spellings, for Eta Aurigae and Lambda Herculis,^[13] from the Arabic المِعْصَم al-miʽşam "the wrist" (of the charioteer). In 2016, the IAU organized a Working Group on Star Names (WGSN)^[14] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[15] It approved the name Mahasim for the component Theta Aurigae A on 30 June 2017 and it is now so included in the List of IAU-approved Star Names.^[10]

It is known as 五車四 (the Fourth Star of the Five Chariots) in Chinese^[16]

Properties

The primary component, Theta Aurigae A, is a large star with more than three times the mass of the Sun and nearly five times the Sun's radius. It is radiating 214 times the Sun's luminosity from its outer atmosphere at an effective temperature of 10,220 K, giving it the white hue of an A-type star. The star has a stellar classification of A0pSi,^[3] with the 'pSi' suffix indicating it is a chemically peculiar star with an abnormal abundance of silicon.

A light curve for Theta Aurigae, plotted from *TESS* data^[17]

The primary is classified an Alpha2 Canum Venaticorum type variable star and has a surface magnetic field of about 1 kG.^[9] Its projected rotational velocity is 55 km s⁻¹,^[9] with the star completing a rotation in only 3.6 days.^[8] The axis of rotation is inclined by an angle of 51°±6° to the line of sight from the Earth.^[9]

The secondary, Theta Aurigae B, is a +7.2 magnitude companion, 4.5^[3] magnitudes fainter than the primary, located at an angular separation of 3.91 arcseconds along a position angle of 304.9° as of 2002.^[18] This is an F-type main sequence star with a stellar classification in the range F2-5 V.^[3]

The mean combined apparent magnitude of the system is +2.65 but the variation of the primary causes the system's brightness to range from magnitude +2.62 to +2.70 with a period of 1.37 days. The system is an X-ray source with a luminosity of 9.49 × 10²⁶ erg s⁻¹.^[3]

References

^ ^a ^b ^c ^d ^e ^f 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
^ ^a ^b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
^ ^a ^b ^c ^d ^e Schröder, C.; Schmitt, J. H. M. M. (November 2007), "X-ray emission from A-type stars", Astronomy and Astrophysics, 475 (2): 677–684, Bibcode:2007A&A...475..677S, doi:10.1051/0004-6361:20077429
^ ^a ^b 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.
^ 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
^ North, P. (June 1998), "Do SI stars undergo any rotational braking?", Astronomy and Astrophysics, 334: 181–187, arXiv:astro-ph/9802286, Bibcode:1998A&A...334..181N
^ ^a ^b ^c ^d ^e ^f Sikora, J.; et al. (February 2019). "A volume-limited survey of mCP stars within 100 pc - I. Fundamental parameters and chemical abundances". Monthly Notices of the Royal Astronomical Society. 483 (2): 2300–2324. arXiv:1811.05633. Bibcode:2019MNRAS.483.2300S. doi:10.1093/mnras/sty3105.
^ ^a ^b Rice, J. B.; Holmgren, D. E.; Bohlender, D. A. (September 2004), "The distribution of oxygen on the surface of the Ap star θ Aur. An abundance Doppler image to compare with ɛ UMa", Astronomy and Astrophysics, 424: 237–244, Bibcode:2004A&A...424..237R, doi:10.1051/0004-6361:20035639
^ ^a ^b ^c ^d Shulyak, D.; et al. (March 2007), "The Lorentz force in atmospheres of CP stars: θ Aurigae", Astronomy and Astrophysics, 464 (3): 1089–1099, arXiv:astro-ph/0612301, Bibcode:2007A&A...464.1089S, doi:10.1051/0004-6361:20064998, S2CID 119333293
^ ^a ^b "Naming Stars". IAU.org. Retrieved 16 December 2017.
^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
^ Kaler, Jim. "Theta Aur". Retrieved 2017-01-05.
^ Allen, Richard Hinckley (1963) [1899]. Star Names: Their Lore and Meaning. New York: Dover Publications, Inc. p. 245. ISBN 0-486-21079-0.
^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
^ "香港太空館 – 研究資源 – 亮星中英對照表 (Chinese/English Star names)" (in Chinese). Hong Kong Space Museum. Archived from the original on 29 September 2009. Retrieved 31 December 2008.
^ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
^ Roberts, Lewis C. Jr. (May 2011), "Astrometric and photometric measurements of binary stars with adaptive optics: observations from 2002", Monthly Notices of the Royal Astronomical Society, 413 (2): 1200–1205, arXiv:1012.3383, Bibcode:2011MNRAS.413.1200R, doi:10.1111/j.1365-2966.2011.18205.x, S2CID 118398949