Sigma Geminorum

Sigma Geminorum (σ Gem) is a binary star[9] system in the constellation Gemini, just to the northeast of Pollux. It is visible to the naked eye with an apparent visual magnitude of 4.20.[2] Based upon an annual parallax shift of 26.08 mas,[1] it is located 125 light years from the Sun.

σ Geminorum
Location of σ Geminorum (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Gemini
Right ascension  07h 43m 18.72698s[1]
Declination +28° 53′ 00.6422″[1]
Apparent magnitude (V) 4.20[2]
Characteristics
Spectral type K1 III[3] + dG/K[4]
U−B color index +0.97[2]
B−V color index +1.11[2]
Variable type RS CVn[5]
Astrometry
Radial velocity (Rv)+25.56±2.72[6] km/s
Proper motion (μ) RA: +62.66[1] mas/yr
Dec.: -230.32[1] mas/yr
Parallax (π)26.08 ± 0.19[1] mas
Distance125.1 ± 0.9 ly
(38.3 ± 0.3 pc)
Absolute magnitude (MV)+1.36[7]
Orbit[8]
Period (P)19.6027±0.0005 d
Semi-major axis (a)4.63±0.04 mas
Eccentricity (e)0
Inclination (i)107.7±0.8°
Argument of periastron (ω)
(secondary)		0°
Details
σ Gem A
Mass1.28±0.07[8] M☉
Radius10.1±0.4[8] R☉
Luminosity39±2[8] L☉
Surface gravity (log g)2.54±0.02[8] cgs
Temperature4571±5[6] K
Metallicity [Fe/H]0.0[8] dex
Rotation19.47[9]
Rotational velocity (v sin i)26.2[6] km/s
Age5±1[8] Gyr
σ Gem B
Mass0.73±0.03[8] M☉
Other designations
σ Gem, 75 Gem, BD+29° 1590, HD 62044, HIP 37629, HR 2973, SAO 79638.[10]
Database references
SIMBADdata

Sigma Geminorum is a single-lined spectroscopic binary,[9] which means that the spectrum of only one of the components can be discerned. It is an RS Canum Venaticorum variable with a period of 19.6 days,[9] matching the orbital period. The stellar luminosity shows indications of ellipsoidal variation, as the primary component is partly filling its Roche lobe due to gravitational interaction between the two stars.[8]

The primary component is an evolved K-type giant star with a stellar classification of K1 III.[3] It has a relatively high rate of spin for a giant star, showing a projected rotational velocity of 26.2 km/s[6] and a rotation period of 19.47 days.[9] This rate is being maintained by the tidal interaction between the two stars. The surface of the primary has large star spots that are locked onto the face oriented toward the secondary component.[9] These spots appear to migrate poleward at an average velocity of 0.12±0.03 km/s.[9] The surface activity makes the star a bright X-ray emission source[11] with a luminosity of 119.41×1029 ergs s−1.[4] It displays indications of anti-solar differential rotation.[9]

The primary has 1.28 times the mass of the Sun, but has expanded to 10.1 times the Sun's radius.[8] It shines with 39[8] times the solar luminosity from its outer atmosphere at an effective temperature of 4571 K.[6] It is roughly 5[8] billion years old.

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. Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
  3. Eggen, O. J. (1962), "Space-velocity vectors for 3483 stars with proper motion and radial velocity", Royal Observatory Bulletin, 51, Bibcode:1962RGOB...51...79E.
  4. Makarov, Valeri V. (October 2003), "The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun", The Astronomical Journal, 126 (4): 1996–2008, Bibcode:2003AJ....126.1996M, doi:10.1086/378164.
  5. 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. Bibcode:2009yCat....102025S.
  6. Massarotti, Alessandro; et al. (January 2008), "Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209.
  7. Cardini, D. (January 2005), "Mg II chromospheric radiative loss rates in cool active and quiet stars", Astronomy and Astrophysics, 430: 303–311, arXiv:astro-ph/0409683, Bibcode:2005A&A...430..303C, doi:10.1051/0004-6361:20041440.
  8. Roettenbacher, Rachael M.; et al. (July 2015), "Detecting the Companions and Ellipsoidal Variations of RS CVn Primaries. I. σ Geminorum", The Astrophysical Journal, 807 (1): 10, arXiv:1504.06628, Bibcode:2015ApJ...807...23R, doi:10.1088/0004-637X/807/1/23, 23.
  9. Kővári, Zs.; et al. (January 2015), "Antisolar differential rotation of the K1-giant σ Geminorum revisited", Astronomy & Astrophysics, 573: 9, arXiv:1411.1774, Bibcode:2015A&A...573A..98K, doi:10.1051/0004-6361/201424138, A98.
  10. "sig Gem". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2016-12-08.
  11. Huenemoerder, David P.; et al. (May 2013), "Stellar Coronae, Solar Flares: A Detailed Comparison of σ GEM, HR 1099, and the Sun in High-resolution X-Rays", The Astrophysical Journal, 768 (2): 15, arXiv:1304.0408, Bibcode:2013ApJ...768..135H, doi:10.1088/0004-637X/768/2/135, 135.
  • Kaler, James B. (September 9, 2015), "Sigma Geminorum", Stars, University of Illinois, retrieved 2016-12-08.
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