W Ursae Majoris

W Ursae Majoris (W UMa) is the variable star designation for a binary star system in the northern constellation of Ursa Major. It has an apparent visual magnitude of about 7.9,[2] which is too faint to be seen with the naked eye. However, it can be viewed with a small telescope.[8] Parallax measurements place it at a distance of roughly 169 light years (52 parsecs) from Earth.[1]

W Ursae Majoris
The red dot shows the location of W Ursae Majoris in Ursa Major.
Observation data
Epoch J2000      Equinox J2000
Constellation Ursa Major
Right ascension  09h 43m 45.4705s[1]
Declination  +55h 57m 09.0667s[1]
Apparent magnitude (V) 7.90[2] (7.75–8.48)
Characteristics
Spectral type F8Vp + F8Vp[3]
U−B color index +0.08[2]
B−V color index +0.66[2]
Variable type W UMa
Astrometry
Radial velocity (Rv)−46[4] km/s
Proper motion (μ) RA: 17.150±0.049[1] mas/yr
Dec.: −29.226±0.050[1] mas/yr
Parallax (π)19.2775 ± 0.0334[1] mas
Distance169.2 ± 0.3 ly
(51.87 ± 0.09 pc)
Orbit[5]
Period (P)0.3336 days
Semi-major axis (a)2.443 R☉[6]
Inclination (i)86.0°
Details
Mass1.190 / 0.570[5] M☉
Radius1.084 / 0.775[6] R☉
Rotational velocity (v sin i)144.40 ± 6.52[7] km/s
Other designations
BD+56 1400, HD 83950, SAO 27364, ADS 7494, CCDM 09438+5557, HIP 47727.[3]
Database references
SIMBADThe system
A
B

In 1903, the luminosity of this system was found to vary by the German astronomers Gustav Müller and Paul Kempf. It has since become the prototype and eponym for a class of variable stars called W Ursae Majoris variables.[9] This system consists of a pair of stars in a tight, circular orbit with a period of 0.3336 days, or eight hours and 23 seconds.[5] During every orbital cycle, each star eclipses the other, resulting in a decrease in magnitude. The maximum magnitude of the pair is 7.75 mag. During the eclipse of the primary, the net magnitude drops by 0.73 mag, while the eclipse of the secondary causes a magnitude decrease of 0.68 mag.[10]

The two stars in W Ursae Majoris are so close together that their outer envelopes are in direct contact, making them a contact binary system. As a result, they have the same stellar classification of F8Vp, which matches the spectrum of a main-sequence star that is generating energy through the nuclear fusion of hydrogen. However, the primary component has a larger mass and radius than the secondary, with 1.19 times the Sun's mass and 1.08 times the Sun's radius. The secondary has 0.57 solar masses and 0.78 solar radii.[5][6]

The orbital period of the system has changed since 1903, which may be the result of mass transfer or the braking effects of magnetic fields. Star spots have been observed on the surface of the stars and strong X-ray emissions have been detected, indicating a high level of magnetic activity that is common to W UMa variables. This magnetic activity may play a role in regulating the timing and magnitude of mass transfer occurs.[9]

W Ursae Majoris has a 12th magnitude companion star with the designation ADS 7494B. They may be moving together through space.[11]

References
  1. 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.
  2. Eggen, O. J. (September 1963), "Three-color photometry of the components in 228 wide double and multiple systems", Astronomical Journal, 68: 483–514, Bibcode:1963AJ.....68..483E, doi:10.1086/109000
  3. "W UMa -- Spectroscopic binary", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-12
  4. Wilson, Ralph Elmer (1953), General Catalogue of Stellar Radial Velocities, Washington: Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W
  5. Bilir, S.; et al. (February 2005), "Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation", Monthly Notices of the Royal Astronomical Society, 357 (2): 497–517, arXiv:astro-ph/0411291, Bibcode:2005MNRAS.357..497B, doi:10.1111/j.1365-2966.2005.08609.x
  6. Gazeas, K.; Stȩpień, K. (November 2008), "Angular momentum and mass evolution of contact binaries", Monthly Notices of the Royal Astronomical Society, 390 (4): 1577–1586, arXiv:0803.0212, Bibcode:2008MNRAS.390.1577G, doi:10.1111/j.1365-2966.2008.13844.x
  7. White, Russel J.; Gabor, Jared M.; Hillenbrand, Lynne A. (June 2007), "High-Dispersion Optical Spectra of Nearby Stars Younger Than the Sun", The Astronomical Journal, 133 (6): 2524–2536, arXiv:0706.0542, Bibcode:2007AJ....133.2524W, doi:10.1086/514336
  8. Sherrod, P. Clay; Koed, Thomas L. (2003), A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations, Astronomy Series, Courier Dover Publications, p. 9, ISBN 0-486-42820-6
  9. Morgan, N.; Sauer, M.; Guinan, E. (1997), "New Light Curves and Period Study of the Contact Binary W Ursae Majoris", Information Bulletin on Variable Stars, 4517: 1, Bibcode:1997IBVS.4517....1M
  10. Malkov, O. Yu.; et al. (February 2006), "A catalogue of eclipsing variables" (PDF), Astronomy and Astrophysics, 446 (2): 785–789, Bibcode:2006A&A...446..785M, doi:10.1051/0004-6361:20053137
  11. Rucinski, S. M.; Lu, W.-X.; Shi, J. (September 1993), "Spectral-line broadening functions of W UMa-type binaries. III - W UMa", Astronomical Journal, 106 (3): 1174–1180, Bibcode:1993AJ....106.1174R, doi:10.1086/116716
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