The brightest components of Sigma Coronae Borealis form a visualbinary with an angular separation of 7 arcsecond first resolved in the 19th century,[24] and are designated σ Corona Borealis A and B. More recently, the designations σ2 and σ1 Corona Borealis have come into use. Somewhat confusingly, the brighter component A is referred to as σ2 because it has the higherright ascension. A third component, while being separated by635″ (translating to a minimum distance of 14,000 au), has a similar parallax andproper motion to the brighter stars and is physically associated. It is known in theWashington Double Star Catalog (WDS), a compilation of observations of double stars, as component E,[24] but it is usually called Sigma Coronae Borealis C.[16]
Sigma2 Corona Borealis itself is a close binary. Here, the two stars are extremely close and orbit fairly quickly, every 1.14 days.[16] This tiny separation of only 0.0279au[16] has allowed the two stars to exerttidal forces on each other, leading to synchronization of their rotation. They have also been classified asRS Canum Venaticorum variables (RS CVn)—young, active stars that show variability in their apparent magnitude due tostarspots on their surfaces.[7]
Despite Sigma2 Corona Borealis's two stars being separated only by about the diameter of each star, they wereresolved using theCHARAoptical interferometer at theMount Wilson Observatory. As of 2006, it is the shortest-period binary ever to be resolved. The primary is 13.7% more massive than the Sun, while the secondary is 9.0% more massive than the Sun, and both are 24.4% wider than the Sun.[16]
σ Coronae Borealis C, also known as HIP 79551, appears as ared dwarf with aspectral type of M2.5V.[8] It too is a binary star, with a companion in a 52-year orbit. The companion has a mass of 0.10 M☉ and has been detected throughastrometry.[17][16]
TheWashington Double Star Catalog (WDS), a compilation of observations of double stars, lists several components to the main system. Two of those are listed in the WDS as components C, and D. As of 1984, component C was separated from the primary by18″ along aposition angle of 103° and as of 1996, component D was separated from the primary by88″ along aposition angle of 82°. However, both of them have differentproper motions through space and are not related, just optical alignments.[24]
The spectroscopic binary σ2 CrB is anRS Canum Venaticorum variable. It varies in brightness by 0.05 magnitudes every 1.139789 days, the same as the orbital period. The brightness changes are caused by variations in surface brightness on the stars, effectively giant sunspots.Variable star designations are not given to stars withBayer designations, but in this case only one component of σ Coronae Borealis is identified as variable, so it has the designation TZ Coronae Borealis.[26]
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^abBoro Saikia, S.; Marvin, C. J.; Jeffers, S. V.; Reiners, A.; Cameron, R.; Marsden, S. C.; Petit, P.; Warnecke, J.; Yadav, A. P. (2018). "Chromospheric activity catalogue of 4454 cool stars. Questioning the active branch of stellar activity cycles".Astronomy and Astrophysics.616.arXiv:1803.11123.Bibcode:2018A&A...616A.108B.doi:10.1051/0004-6361/201629518.
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^abcdefghijklmnopqrstuvwxyzaaabRaghavan, Deepak; McAlister, Harold A.; Torres, Guillermo; Latham, David W.; Mason, Brian D.; Boyajian, Tabetha S.; Baines, Ellyn K.; Williams, Stephen J.; Brummelaar, Theo A. ten; Farrington, Chris D.; Ridgway, Stephen T.; Sturmann, Laszlo; Sturmann, Judit; Turner, Nils H. (2009). "The Visual Orbit of the 1.1-day Spectroscopic Binary σ2 Coronae Borealis from Interferometry at the CHARA Array".The Astrophysical Journal.690 (1):394–406.arXiv:0808.4015.Bibcode:2009ApJ...690..394R.doi:10.1088/0004-637X/690/1/394.S2CID14638405.
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