Sigma Geminorum

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

A visual band light curve for Sigma Geminorum, adapted from Kővári et al. (2015)[9]

Sigma Geminorum is a single-lined spectroscopic binary,[9] which means that the spectrum of only one of its 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.

  1. ^ a b c d e f Cite error: The named reference vanLeeuwen2007 was invoked but never defined (see the help page).
  2. ^ a b c d Cite error: The named reference Mermilliod1986 was invoked but never defined (see the help page).
  3. ^ a b Cite error: The named reference Eggen1962 was invoked but never defined (see the help page).
  4. ^ a b Cite error: The named reference Makarov2003 was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference gcvs was invoked but never defined (see the help page).
  6. ^ a b c d e Cite error: The named reference Massarotti2008 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference Cardini2005 was invoked but never defined (see the help page).
  8. ^ a b c d e f g h i j k l Cite error: The named reference Roettenbacher2015 was invoked but never defined (see the help page).
  9. ^ a b c d e f g h i Cite error: The named reference Kovari2015 was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference SIMBAD was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference Huenemoerder2013 was invoked but never defined (see the help page).