# Omicron Ursae Majoris

Observation data Constellation Epoch J2000.0      Equinox J2000.0 Location of ο Ursa Major (circled) Ursa Major 08h 30m 15.87064s[1] +60° 43′ 05.4115″[1] +3.35[2] G4 II–III[3] +0.52[4] +0.85[4] Radial velocity (Rv) +19.8[5] km/s Proper motion (μ) RA: –133.76[1] mas/yr Dec.: –107.45[1] mas/yr Parallax (π) 18.21 ± 0.16[1] mas Distance 179 ± 2 ly (54.9 ± 0.5 pc) Absolute magnitude (MV) –0.40[3] Mass 3.09[3] M☉ Radius 14[6] R☉ Luminosity 138[3] L☉ Surface gravity (log g) 2.64[3] cgs Temperature 5,242[3] K Metallicity [Fe/H] –0.09[3] dex Rotational velocity (v sin i) 15[7] km/s Age 360 ± 30[8] Myr Muscida, ο Ursae Majoris, ο UMa, Omicron UMa, 1 Ursae Majoris, BD+61 1054, CCDM J08303+6043A, FK5 317, GC 11593, HD 71369, HIP 41704, HR 3323, PPM 16654, SAO 14573, WDS J08303+6043A.[9] SIMBAD data

Omicron Ursae Majoris (ο Ursae Majoris, abbreviated Omicron UMa, ο UMa), also named Muscida,[10] is a star system in the northern circumpolar constellation of Ursa Major. It has an apparent visual magnitude of +3.35[2] and is located at a distance of around 179 light-years (55 parsecs) from the Sun.[1] In 2012, an extrasolar planet, designated Omicron Ursae Majoris Ab was found to be orbiting the primary.[11]

## Nomenclature

ο Ursae Majoris (Latinised to Omicron Ursae Majorise) is the star's Bayer designation.

The traditional name Muscida was shared with the optical double star Pi Ursae Majoris. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[12] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[13] included a table of the first two batches of names approved by the WGSN; which included Muscida for this star.

In Chinese, 內階 (Nèi Jiē), meaning Inner Steps, refers to an asterism consisting of ο Ursae Majoris, 16 Ursae Majoris, 6 Ursae Majoris, 23 Ursae Majoris, 5 Ursae Majoris and 17 Ursae Majoris. Consequently, ο Ursae Majoris itself is known as 內階一 (Nèi Jiē yī, English: the First Star of Inner Steps.).[14]

## Properties and observations

The stellar classification of this star, G4 II–III,[3] places it midway between the giant and bright giant stages of its evolution. The interferometry-measured angular diameter of this star is about 2.42 mas,[15] which, at its estimated distance, equates to a physical radius of about 14 times the radius of the Sun.[6] It has about three[3] times the mass of the Sun and radiates 138[3] times the Sun's luminosity from its outer atmosphere at an effective temperature of 5,282 K,[2] giving it the yellowish hue of a G-type star.[16]

In 1963, East German astronomer Gerhard Jakisch reported this star as a variable with a period of 358 days and an amplitude of 0.08 magnitude. The 1982 edition of the New Catalogue of Suspected Variable Stars listed it with a variability from 3.30 to 3.36 in the visual band. However, in 1992 American astronomer Dorrit Hoffleit noted that the two comparison stars used to determine the variability may themselves be variable. Hence the actual variability of this star may be suspect.[17]

Muscida has a magnitude 15.2 common proper motion companion at an angular separation of 7.1 arcseconds.[18] With a probability of 99.4%, this companion is the source for the X-ray emission from the system.[19] Omicron Ursae Majoris is sometimes listed with two more companions, but, based on proper motion data, these appear to be optical companions.

This system is a member of the thin disk population and is following an orbit through the Milky Way galaxy with an eccentricity of 0.12. It comes as close to the Galactic Center as 23.5 kly (7.2 kpc) and as distant as 30.2 kly (9.3 kpc). This orbit carries it no more than about 330 ly (100 pc) above the galactic plane.[8] It is considered a runaway star because it has a high peculiar velocity of 35.5 km s−1 relative to the typical motion of stars in its vicinity.[20]

## Planetary system

In 2012, an extrasolar planet designated Omicron Ursae Majoris Ab and orbiting the primary at 3.9 astronomical units, was found. This gas giant (4.1 times as massive as Jupiter) completes an orbit in 1630 days.[11]

The Omicron Ursae Majoris planetary system[11]
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
b >4.1 MJ 3.9 1630±35 0.130 ± 0.065

## References

1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
2. ^ a b c Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics, 352: 495–507, Bibcode:1999A&A...352..495M
3. Takeda, Yoichi; Sato, Bun'ei; Murata, Daisuke (August 2008), "Stellar parameters and elemental abundances of late-G giants", Publications of the Astronomical Society of Japan, 60 (4): 781–802, arXiv:, Bibcode:2008PASJ...60..781T, doi:10.1093/pasj/60.4.781
4. ^ a b Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99). Bibcode:1966CoLPL...4...99J.
5. ^ Wilson, Ralph Elmer (1953), General Catalogue of Stellar Radial Velocities, Washington: Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W
6. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
{\displaystyle {\begin{aligned}2\cdot R_{*}&={\frac {(10^{-3}\cdot 54.9\cdot 2.42)\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 28.6\cdot R_{\bigodot }\end{aligned}}}
7. ^ Bernacca, P. L.; Perinotto, M. (1970), "A catalogue of stellar rotational velocities", Contributi Osservatorio Astronomico di Padova in Asiago, 239 (1), Bibcode:1970CoAsi.239....1B
8. ^ a b Soubiran, C.; et al. (2008), "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants", Astronomy and Astrophysics, 480 (1): 91–101, arXiv:, Bibcode:2008A&A...480...91S, doi:10.1051/0004-6361:20078788
9. ^ "bet CMi -- Be Star", SIMBAD Astronomical Object Database, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-09
10. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
11. ^ a b c Sato, Bun'ei; et al. (2012). "Substellar Companions to Seven Evolved Intermediate-Mass Stars". Publications of the Astronomical Society of Japan. 64 (6). 135. arXiv:. Bibcode:2012PASJ...64..135S. doi:10.1093/pasj/64.6.135.
12. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
13. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
14. ^
15. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
16. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16
17. ^ Hoffleit, D. (October 1992), "Do all Three Vary: omicron UMa, 23 UMa and HR 3245?", Information Bulletin on Variable Stars, 3789: 1, Bibcode:1992IBVS.3789....1H
18. ^ Eggleton, P. P.; Tokovinin, A. A. (September 2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869–879. arXiv:. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x.
19. ^ Haakonsen, Christian Bernt; Rutledge, Robert E. (September 2009), "XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources", The Astrophysical Journal Supplement, 184 (1): 138–151, arXiv:, Bibcode:2009ApJS..184..138H, doi:10.1088/0067-0049/184/1/138
20. ^ Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x