Xi Ursae Majoris is a star system in the constellation of Ursa Major. It has the traditional name Alula Australis;[1][11]Xi Ursae Majoris is the Bayer designation, which is Latinised from ξUrsae Majoris and abbreviated XiUMa or ξUMa. It was the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828.[citation needed] It is also a variable star with a small amplitude. Xi Ursae Majoris is found in the left hind paw of the Great Bear.[12]
Star system in the constellation Ursa Major
Xi Ursae Majoris
The red circle shows the location of Xi Ursae Majoris in Ursa Major.
The two components are yellow main-sequence stars. The brighter component (designated Xi Ursae Majoris A), has a mean apparent magnitude of +4.41. The companion star (Xi Ursae Majoris B) has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years. They are currently (2022) separated by 2.3 arcseconds, and will widen to a maximum 3.0 arcseconds in 2035.
Double star Xi Ursae Majoris.(θ):156.6° (ρ):2.1 arc"
Each component of this double star is itself a single-lined spectroscopic binary. The orbit of the A pair has been determined from spectroscopy and speckle interferometry, giving a period of 669 days and an eccentricity of 0.53.[10] B's binary companion (Xi Ursae Majoris Bb) has not been detected visually or spectroscopically, but the radial velocity variations of the spectral lines show a circular orbit with a period of 3.98 days.[13] The masses of both A and B's companions (Ab and Bb) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) indicate that they are probably red dwarfs, Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.[14]
In 2012 Wright et al. discovered the fifth component and the second brown dwarf (if Bb is also a brown dwarf) of the system using Wide-field Infrared Survey Explorer (WISE) data—a T8.5 brown dwarf WISE J111838.70+312537.9 with angular separation 8.5 arcmin, and the projected physical separation about 4,000 AU.[15]
Variable star
A light curve for Xi Ursae Majoris, plotted from TESS data[16]
ξUrsae Majoris is classified as an RS Canum Venaticorum variable and its brightness varies by 0.01 magnitude.[17] Component B is believed to be the variable star, showing characteristic emission lines in its spectrum that are not present for component A.[18]
Nomenclature
ξUrsae Majoris (Latinised to Xi Ursae Majoris) is the star's Bayer designation.
It also bore the traditional names Alula Australis[19] (and erroneously Alula Australe[20]). Alula (shared with Nu Ursae Majoris) comes from the Arabic phrase Al Ḳafzah al Ūla 'the First Spring'[21] and Australis is Latin for 'southern'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[22] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[23] included a table of the first two batches of names approved by the WGSN; which included Alula Australis for this star.
Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev.ed.). Cambridge, Massachusetts: Sky Pub. ISBN978-1-931559-44-7.
Høg, E.; etal. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H.
Keenan, Philip C; McNeil, Raymond C (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
Nicolet, B (1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics. 34: 1. Bibcode:1978A&AS...34....1N.
Dempsey, Robert C; Linsky, Jeffrey L; Fleming, Thomas A; Schmitt, J. H. M. M (1993). "The ROSAT All-Sky Survey of active binary coronae. I - Quiescent fluxes for the RS Canum Venaticorum systems". Astrophysical Journal Supplement Series. 86: 599. Bibcode:1993ApJS...86..599D. doi:10.1086/191791.
Nordström, B.; etal. (2004). "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ~14000 F and G dwarfs". Astronomy and Astrophysics. 418 (3): 989–1019. arXiv:astro-ph/0405198. Bibcode:2004A&A...418..989N. doi:10.1051/0004-6361:20035959. S2CID11027621.
Karataș, Yüksel; Bilir, Selçuk; Eker, Zeki; Demircan, Osman; Liebert, James; Hawley, Suzanne L.; Fraser, Oliver J.; Covey, Kevin R.; Lowrance, Patrick; Kirkpatrick, J. Davy; Burgasser, Adam J. (2004). "Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution". Monthly Notices of the Royal Astronomical Society. 349 (3): 1069–1092. arXiv:astro-ph/0404219. Bibcode:2004MNRAS.349.1069K. doi:10.1111/j.1365-2966.2004.07588.x. S2CID15290475.
Mason, Brian D.; McAlister, Harold A.; Hartkopf, William I.; Shara, M. M.; Shara, M. M. (January 1995). "Binary star orbits from speckle interferometry. 7: The multiple system XI Ursae Majoris". The Astronomical Journal. 109 (1669): 332–340. Bibcode:1995AJ....109..332M. doi:10.1086/117277.
Griffin, R. F. (1998). "Spectroscopic binary orbits from photoelectric radial velocities. Paper 142: Xi Ursae Majoris". The Observatory. 118: 273. Bibcode:1998Obs...118..273G.
Wright, Edward L.; Skrutskie, M. F.; Kirkpatrick, J. Davy; Gelino, Christopher R.; Griffith, Roger L.; Marsh, Kenneth A.; Jarrett, Tom; Nelson, M. J.; Borish, H. J.; Mace, Gregory; Mainzer, Amanda K.; Eisenhardt, Peter R.; McLean, Ian S.; Tobin, John J.; Cushing, Michael C. (2012). "A T8.5 Brown Dwarf Member of the Xi Ursae Majoris System". The Astronomical Journal. 145 (3): 84. arXiv:1203.5764v1. Bibcode:2013AJ....145...84W. doi:10.1088/0004-6256/145/3/84. S2CID8588271.
Samus, N. N.; Durlevich, O. V.; etal. (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.
Strassmeier, Klaus G; Hall, Douglas S; Boyd, Louis J; Genet, Russell M (1989). "Photometric variability in chromospherically active stars. III - the binary stars". Astrophysical Journal Supplement Series. 69: 141. Bibcode:1989ApJS...69..141S. doi:10.1086/191310.
Piazzi, G. (1814). The Palermo Catalogue. Palermo.
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