View source for Astronomy:Sigma Geminorum

{{short description|Binary star system in the constellation Gemini}}
{{Starbox begin
 | name = σ Geminorum
}}
{{Starbox image
 | image=
{{Location mark
|image=Gemini constellation map.svg|alt=|float=center|width=280
|label=|position=right
|mark=Red circle.svg|mark_width=10|mark_link=σ Gem
|x=236|y=188
}}
|caption=Location of σ Geminorum (circled)
}}
{{Starbox observe
 | epoch = [[Astronomy:J2000.0|J2000.0]]
 | equinox = [[Astronomy:J2000.0|J2000.0]] ([[Astronomy:International Celestial Reference System|ICRS]])
 | constell = [[Astronomy:Gemini (constellation)|Gemini]]
 | ra = {{RA|07|43|18.72698}}<ref name=vanLeeuwen2007/>
 | dec = {{DEC|+28|53|00.6422}}<ref name=vanLeeuwen2007/>
 | appmag_v = 4.20<ref name=Mermilliod1986/>
}}
{{Starbox character
 | class = K1&nbsp;III<ref name=Eggen1962/> + dG/K<ref name=Makarov2003/>
 | b-v = +1.11<ref name=Mermilliod1986/>
 | u-b = +0.97<ref name=Mermilliod1986/>
 | r-i = 
 | variable = RS CVn<ref name=gcvs/>
}}
{{Starbox astrometry
 | radial_v = {{Val|+25.56|2.72}}<ref name=Massarotti2008/>
 | prop_mo_ra = +62.66<ref name=vanLeeuwen2007/>
 | prop_mo_dec = −230.32<ref name=vanLeeuwen2007/>
 | parallax = 26.08
 | p_error = 0.19
 | parallax_footnote = <ref name=vanLeeuwen2007/>
 | absmag_v = +1.36<ref name=Cardini2005/>
}}
{{Starbox orbit
 | reference = <ref name=Roettenbacher2015/>
 | period_unitless ={{Val|19.6027|0.0005|u=d}}
 | axis = <!--Semimajor axis (in arcseconds)-->
 | axis_unitless = {{Val|4.63|0.04|u=mas}}
 | eccentricity = 0
 | inclination = {{Val|107.7|0.8}}
 | node = <!--Longitude of node (in degrees)-->
 | periastron = <!--Periastron epoch-->
 | periarg = 0
 | periarg_primary = <!--Argument of periastron (in degrees), primary -->
 | k1 = <!-- Velocity semi-amplitude (SB1, or primary in SB2), in km/s -->
 | k2 = <!-- Velocity semi-amplitude (secondary in SB2), in km/s -->
}}
{{Starbox detail
 | component1 = σ Gem A
 | mass = {{Val|1.28|0.07}}<ref name=Roettenbacher2015/>
 | radius = {{Val|10.1|0.4}}<ref name=Roettenbacher2015/>
 | luminosity = {{Val|39|2}}<ref name=Roettenbacher2015/>
 | temperature = {{Val|4571|5}}<ref name=Massarotti2008/>
 | gravity = {{Val|2.54|0.02}}<ref name=Roettenbacher2015/>
 | metal_fe = 0.0<ref name=Roettenbacher2015/>
 | rotational_velocity = 26.2<ref name=Massarotti2008/>
 | rotation = 19.47<ref name=Kovari2015/>
 | age_gyr = {{Val|5|1}}<ref name=Roettenbacher2015/>
 | component2 = σ Gem B
 | mass2 = {{Val|0.73|0.03}}<ref name=Roettenbacher2015/>
}}
{{Starbox catalog
 | names = [[Astronomy:Bayer designation|σ Gem]], [[Astronomy:Flamsteed designation|75 Gem]], BD+29°1590, HD 62044, HIP 37629, [[Astronomy:Bright Star Catalogue|HR]] 2973, [[Astronomy:Smithsonian Astrophysical Observatory Star Catalog|SAO]] 79638.<ref name=SIMBAD/>
}}
{{Starbox reference
 | Simbad = sig+Gem
}}
{{starbox end}}

'''Sigma Geminorum''' ('''σ Gem''') is a [[Astronomy:Binary star|binary star]]<ref name=Kovari2015/> system in the [[Astronomy:Constellation|constellation]] [[Astronomy:Gemini (constellation)|Gemini]], just to the northeast of [[Astronomy:Pollux (star)|Pollux]]. It is visible to the naked eye with an [[Astronomy:Apparent visual magnitude|apparent visual magnitude]] of 4.20.<ref name=Mermilliod1986/> Its annual [[Astronomy:Stellar parallax|parallax shift]] of 26.08&nbsp;mas<ref name=vanLeeuwen2007/> indicates that it is located 125&nbsp;[[Light year|light year]]s from the [[Astronomy:Sun|Sun]].

[[File:SigmaGemLightCurve.png|thumb|left|A visual band light curve for Sigma Geminorum, adapted from Kővári ''et al.'' (2015)<ref name=Kovari2015/>]]
Sigma Geminorum is a single-lined [[Astronomy:Spectroscopic binary|spectroscopic binary]],<ref name=Kovari2015/> which means that the spectrum of only one of its components can be discerned. It is an [[Astronomy:RS Canum Venaticorum variable|RS Canum Venaticorum variable]] with a period of 19.6&nbsp;days,<ref name=Kovari2015/> matching the [[Astronomy:Orbital period|orbital period]]. The stellar luminosity shows indications of [[Astronomy:Rotating ellipsoidal variable|ellipsoidal variation]], as the primary component is partly filling its [[Physics:Roche lobe|Roche lobe]] due to gravitational interaction between the two stars.<ref name=Roettenbacher2015/>

The primary component is an [[Astronomy:Stellar evolution|evolved]] [[Astronomy:K-type star|K-type]] [[Astronomy:Giant star|giant star]] with a [[Astronomy:Stellar classification|stellar classification]] of K1&nbsp;III.<ref name=Eggen1962/> It has a relatively high rate of spin for a giant star, showing a projected rotational velocity of 26.2&nbsp;km/s<ref name=Massarotti2008/> and a rotation period of 19.47 days.<ref name=Kovari2015/> 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.<ref name=Kovari2015/> These spots appear to migrate poleward at an average velocity of {{Val|0.12|0.03|u=km/s}}.<ref name=Kovari2015/> The surface activity makes the star a bright [[Astronomy:X-ray astronomy|X-ray emission]] source<ref name=Huenemoerder2013/> with a luminosity of {{Val|119.41e29|u=ergs s<sup>−1</sup>}}.<ref name=Makarov2003/> It displays indications of anti-solar [[Astronomy:Differential rotation|differential rotation]].<ref name=Kovari2015/>

The primary has 1.28 times the [[Astronomy:Solar mass|mass of the Sun]], but has expanded to 10.1 times the [[Astronomy:Solar radius|Sun's radius]].<ref name=Roettenbacher2015/> It shines with 39<ref name=Roettenbacher2015/> times the [[Astronomy:Solar luminosity|solar luminosity]] from its [[Astronomy:Stellar atmosphere|outer atmosphere]] at an [[Astronomy:Effective temperature|effective temperature]] of 4571&nbsp;K.<ref name=Massarotti2008/> It is roughly 5<ref name=Roettenbacher2015/> billion years old.

==References==
{{reflist|refs=

<ref name=vanLeeuwen2007>{{citation
 | title=Validation of the new Hipparcos reduction
 | last1=van Leeuwen | first1=F.
 | journal=Astronomy and Astrophysics
 | volume=474 | issue=2 | pages=653–664 | date=2007
 | arxiv=0708.1752 | bibcode=2007A&A...474..653V
 | doi=10.1051/0004-6361:20078357 | s2cid=18759600 | postscript=. }}</ref>

<ref name=Cardini2005>{{citation
 | title=Mg II chromospheric radiative loss rates in cool active and quiet stars
 | last1=Cardini | first1=D. | postscript=.
 | journal=Astronomy and Astrophysics
 | volume=430 | pages=303–311 | date=January 2005
 | doi=10.1051/0004-6361:20041440 | bibcode=2005A&A...430..303C |arxiv = astro-ph/0409683 | s2cid=12136256 }}</ref>

<ref name=Eggen1962>{{citation
 | last1=Eggen | first1=O. J.
 | title=Space-velocity vectors for 3483 stars with proper motion and radial velocity
 | journal=Royal Observatory Bulletin
 | volume=51 | year=1962
 | page=79
 | bibcode=1962RGOB...51...79E | postscript=. }}</ref>

<ref name=Huenemoerder2013>{{citation
 | title=Stellar Coronae, Solar Flares: A Detailed Comparison of σ GEM, HR 1099, and the Sun in High-resolution X-Rays
 | last1=Huenemoerder | first1=David P. | last2=Phillips | first2=Kenneth J. H.
 | last3=Sylwester | first3=Janusz | last4=Sylwester | first4=Barbara
 | journal=The Astrophysical Journal | display-authors=1
 | volume=768 | issue=2 | id=135 | pages=15 | date=May 2013
 | doi=10.1088/0004-637X/768/2/135 | bibcode=2013ApJ...768..135H
 | arxiv=1304.0408 | s2cid=119255876 | postscript=. }}</ref>

<ref name=Roettenbacher2015>{{citation
 | title=Detecting the Companions and Ellipsoidal Variations of RS CVn Primaries. I. σ Geminorum
 | last1=Roettenbacher | first1=Rachael M. | last2=Monnier | first2=John D.
 | last3=Henry | first3=Gregory W. | last4=Fekel | first4=Francis C.
 | last5=Williamson | first5=Michael H. | last6=Pourbaix | first6=Dimitri
 | last7=Latham | first7=David W. | last8=Latham | first8=Christian A.
 | last9=Torres | first9=Guillermo | last10=Baron | first10=Fabien
 | last11=Che | first11=Xiao | last12=Kraus | first12=Stefan
 | last13=Schaefer | first13=Gail H. | last14=Aarnio | first14=Alicia N.
 | last15=Korhonen | first15=Heidi | last16=Harmon | first16=Robert O.
 | last17=ten Brummelaar | first17=Theo A. | last18=Sturmann | first18=Judit
 | last19=Sturmann | first19=Laszlo | last20=Turner | first20=Nils H.
 | journal=The Astrophysical Journal | display-authors=1
 | volume=807 | issue=1 | id=23 | pages=10 | date=July 2015
 | doi=10.1088/0004-637X/807/1/23 | bibcode=2015ApJ...807...23R
 | arxiv=1504.06628 | s2cid=16705461 | postscript=. }}</ref>

<ref name=Kovari2015>{{citation
 | title=Antisolar differential rotation of the K1-giant σ Geminorum revisited
 | display-authors=1 | last1=Kővári | first1=Zs.
 | last2=Kriskovics | first2=L. | last3=Künstler | first3=A.
 | last4=Carroll | first4=T. A. | last5=Strassmeier | first5=K. G.
 | last6=Vida | first6=K. | last7=Oláh | first7=K.
 | last8=Bartus | first8=J. | last9=Weber | first9=M.
 | journal=Astronomy & Astrophysics
 | volume=573 | id=A98 | pages=9 | date=January 2015
 | doi=10.1051/0004-6361/201424138 | bibcode=2015A&A...573A..98K
 | arxiv=1411.1774 | s2cid=119177256 | postscript=. }}</ref>

<ref name=Massarotti2008>{{citation
 | display-authors=1
 | last1=Massarotti | first1=Alessandro | last2=Latham | first2=David W.
 | last3=Stefanik | first3=Robert P. | last4=Fogel | first4=Jeffrey
 | title=Rotational and radial velocities for a sample of 761 HIPPARCOS giants and the role of binarity
 | journal=The Astronomical Journal
 | volume=135 | issue=1 | pages=209–231 | date=January 2008
 | doi=10.1088/0004-6256/135/1/209 | bibcode=2008AJ....135..209M
 | s2cid=121883397 | postscript=. | doi-access=free }}</ref>

<ref name=Mermilliod1986>{{citation
 | last1=Mermilliod | first1=J.-C.
 | title=Compilation of Eggen's UBV data, transformed to UBV (unpublished)
 | journal=Catalogue of Eggen's UBV Data
 | year=1986 | publisher=[[Astronomy:SIMBAD|SIMBAD]]
 | bibcode=1986EgUBV........0M | postscript=. }}</ref>

<ref name=SIMBAD>{{cite simbad
 | title= sig Gem
 | access-date=2016-12-08 | postscript=. }}</ref>

<ref name=Makarov2003>{{citation
 | last1=Makarov | first1=Valeri V.
 | title=The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
 | journal=The Astronomical Journal | postscript=.
 | volume=126 | issue=4 | pages=1996–2008 | date=October 2003
 | doi=10.1086/378164 | bibcode=2003AJ....126.1996M | doi-access=free }}</ref>

<ref name=gcvs>{{cite journal|bibcode=2009yCat....102025S|title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)|journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S|volume=1|display-authors=0|last1=Samus|first1=N. N.|last2=Durlevich|first2=O. V.|year=2009}}</ref>

}}

==External links==
*{{citation
  | first1=James B. | last1=Kaler | date=September 9, 2015
  | title=Sigma Geminorum | work=Stars | publisher=University of Illinois
  | url=http://stars.astro.illinois.edu/sow/sigmagem.html
  | accessdate=2016-12-08 | postscript=. }}

{{Stars of Gemini|state=collapsed}}

{{DEFAULTSORT:Sigma Geminorum}}
[[Category:K-type giants]]
[[Category:RS Canum Venaticorum variables]]
[[Category:Gemini (constellation)]]
[[Category:Bayer objects|Geminorum, Sigma]]
[[Category:Durchmusterung objects]]
[[Category:Flamsteed objects|Geminorum, 75]]
[[Category:Henry Draper Catalogue objects|062044]]
[[Category:Hipparcos objects|037629]]
[[Category:Bright Star Catalogue objects|02973]]
[[Category:Spectroscopic binaries]]

{{Sourceattribution|Sigma Geminorum}}