Xi Ursae Majoris

Xi Ursae Majoris is a quintuple star system 28.5 light-years (8.7 parsecs) away in the constellation of Ursa Major. It has the traditional name Alula Australis;^[1][10] Xi Ursae Majoris is the Bayer designation, which is Latinised from ξ Ursae Majoris and abbreviated Xi UMa or ξ UMa. 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.^[11]

Xi Ursae Majoris

The red circle shows the location of Xi Ursae Majoris in Ursa Major.
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Pronunciation	/æˈluːlə ɔːˈstreɪlɪs/[1]
ξ UMa A
Right ascension	11h 18m 10.902s[2]
Declination	+31° 31′ 44.98″[2]
Apparent magnitude (V)	4.264[2]
ξ UMa B
Right ascension	11h 18m 10.950s[2]
Declination	+31° 31′ 45.74″[2]
Apparent magnitude (V)	4.729[2]
Characteristics
Spectral type	F8.5:V / G2V[3]
U−B color index	0.04[4]
B−V color index	0.59[4]
Variable type	RS CVn[5]
Astrometry
Radial velocity (Rv)	−18.2±2.7[6] km/s
Proper motion (μ)	RA: −339.398 mas/yr[7] Dec.: −607.892 mas/yr[7]
Parallax (π)	114.4867 ± 0.4316 mas[7]
Distance	28.5 ± 0.1 ly (8.73 ± 0.03 pc)
ξ UMa Aa
Absolute magnitude (MV)	4.66[note 1]
Absolute bolometric magnitude (Mbol)	4.54±0.06[8]
ξ UMa Ba
Absolute magnitude (MV)	5.16[note 2]
Absolute bolometric magnitude (Mbol)	5.00±0.06[8]
Orbit[9]
Primary	ξ UMa A
Companion	ξ UMa B
Period (P)	59.878 yr
Semi-major axis (a)	2.536″
Eccentricity (e)	0.398
Inclination (i)	127.94°
Longitude of the node (Ω)	101.85 (ascending)°
Periastron epoch (T)	1935.195
Orbit[9]
Primary	ξ UMa Aa
Companion	ξ UMa Ab
Period (P)	1.832 yr
Semi-major axis (a)	0.057″
Eccentricity (e)	0.53
Inclination (i)	94.9°
Details[8]
ξ UMa Aa
Mass	0.97 M☉
Radius	1.02±0.04 R☉
Luminosity	1.21 L☉
Surface gravity (log g)	4.39±0.10 cgs
Temperature	6,005±80 K
Rotational velocity (v sin i)	1.0±1.0 km/s
ξ UMa Ab
Mass	0.38±0.02 M☉
Radius	0.32 R☉
Temperature	~3,700[note 3] K
ξ UMa Ba
Mass	0.86 M☉
Radius	0.92±0.04 R☉
Luminosity	0.79 L☉
Surface gravity (log g)	4.46±0.10 cgs
Temperature	5,692±90 K
Metallicity [Fe/H]	−0.35±0.08 dex
Rotational velocity (v sin i)	3.0±1.0 km/s
ξ UMa Bb
Mass	0.14+0.05 −0.09 M☉
Other designations
Alula Australis, ξ Ursae Majoris, ξ UMa, Xi UMa, 53 Ursae Majoris, BD+32°2132, GC 15537, GJ 423, HIP 55203, SAO 62484, CCDM J11182+3132, WDS J11182+3132
A: HD 98231, HR 4375
B: HD 98230, HR 4374
E: WISE J111838.70+312537.9
Database references
SIMBAD	ξ UMa
	ξ UMa AB
	ξ UMa A
	ξ UMa B
	ξ UMa Bb
	WISE J1118+3125

Stellar system

Orbit of Xi Ursae Majoris

The two main 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.^[9] B's binary companion (Xi Ursae Majoris Bb) has not been detected visually, but the radial velocity variations of the spectral lines show a circular orbit with a period of 3.98 days.^[12] 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.^[13] However, component Ba has been found to be enriched in barium relative to component Aa, suggesting that its companion Bb may be a white dwarf.^[14]

In 2012 Wright et al. discovered the fifth component and the first 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]

ξ Ursae 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

ξ Ursae 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.

In Chinese, 三台 (Sān Tái), meaning Three Steps, refers to an asterism consisting of Xi Ursae Majoris, Iota Ursae Majoris, Kappa Ursae Majoris, Lambda Ursae Majoris, Mu Ursae Majoris and Nu Ursae Majoris. Consequently, the Chinese name for Xi Ursae Majoris itself is 下台二 (Xià Tái èr, English: Star of Second Lower Step).^[24]

See also

• List of star systems within 25–30 light-years

References

1. 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. ISBN 978-1-931559-44-7.
2. Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H.
3. 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.
4. Nicolet, B (1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics. 34: 1. Bibcode:1978A&AS...34....1N.
5. 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.
6. Nordström, B.; et al. (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. S2CID 11027621.
7. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
8. Fuhrmann, Klaus (2008). "Nearby stars of the Galactic disc and halo - IV". Monthly Notices of the Royal Astronomical Society. 384 (1): 173–224. Bibcode:2008MNRAS.384..173F. doi:10.1111/j.1365-2966.2007.12671.x.
9. 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.
10. "IAU Catalog of Star Names". Retrieved 28 July 2016.
11. Griffin, R. F. (1998). "Spectroscopic binary orbits from photoelectric radial velocities. Paper 142: Xi Ursae Majoris". The Observatory. 118: 273. Bibcode:1998Obs...118..273G.
12. Berman, Louis (1931). "The spectroscopic orbit of the fainter component in the system [xi] Ursae Majoris". Lick Observatory Bulletin. 15: 109–116. Bibcode:1931LicOB..15..109B. doi:10.5479/ADS/bib/1931LicOB.15.109B.
13. Jim Kaler. "Alula Australis". Retrieved 2019-05-20.
14. Fuhrmann, K.; Chini, R.; et al. (June 2016). "Evidence for very nearby hidden white dwarfs". Monthly Notices of the Royal Astronomical Society. 459 (2): 1682–1686. Bibcode:2016MNRAS.459.1682F. doi:10.1093/mnras/stw760.
15. 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. S2CID 8588271.
16. "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 30 October 2022.
17. 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.
18. 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.
19. Piazzi, G. (1814). The Palermo Catalogue. Palermo.
20. Antonín Bečvář (1951). Atlas of the heavens: Atlas coeli. Skalnaté pleso II. Katalog 1950.0. Prírodovedecké vydavatelstvi.
21. Richard Hinckley Allen. "Star Names — Their Lore and Meaning – Ursa Major, the Greater Bear". Retrieved 2019-05-20.
22. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
23. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
24. (in Chinese) (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 21 日 Archived 2016-03-03 at the Wayback Machine

Note

1. Calculated from the bolometric magnitude 4.54 ± 0.06 and the bolometric correction −0.12 ± 0.05 using the formula: BC = M_bol − M_V
2. Calculated from the bolometric magnitude 5.00 ± 0.06 and the bolometric correction −0.16 ± 0.05 using the formula: BC = M_bol − M_V
3. This estimate assumes that ξ UMa Ab is a red dwarf.

External links

• Animation of the orbits of the stars in the Alula Australis System at SolStation.com
• Main Article on Alula Australis at SolStation.com
• Alula Australis by Dr. Jim Kaler.