U Geminorum

U Geminorum (U Gem), in the constellation Gemini, is an archetypal example of a dwarf nova. The binary star system consists of a white dwarf closely orbiting a red dwarf. Every few months it undergoes an outburst that greatly increases its brightness. The dwarf nova class of variable stars are often referred to as U Geminorum variables after this star.

U Geminorum

AAVSO light curve of U Geminorum's outburst behavior. Different colors reflect different bandpasses. Day numbers are Julian day.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Gemini
Right ascension 07h 55m 05.24s[1]
Declination +22° 00′ 05.1″[1]
Apparent magnitude (V) 8.2 - 14.9[2]
Characteristics
Spectral type DA[3] + M4.5Ve[4]
Variable type U Gem + eclipsing[2]
Astrometry
Radial velocity (Rv)+42.0[5] km/s
Proper motion (μ) RA: −27.363±0.049[6] mas/yr
Dec.: −40.398±0.027[6] mas/yr
Parallax (π)10.7121 ± 0.0299[6] mas
Distance304.5 ± 0.8 ly
(93.4 ± 0.3 pc)
Orbit[7]
PrimaryWhite dwarf
CompanionRed dwarf
Period (P)0.1769062 days
Semi-major axis (a)1.55±0.02 R☉
Eccentricity (e)0.027
Inclination (i)69.7±0.7°
Semi-amplitude (K1)
(primary)		107±2 km/s
Semi-amplitude (K2)
(secondary)		310±5 km/s
Details
White dwarf
Mass1.2±0.05[7] M☉
Radius0.008[8] R☉
Surface gravity (log g)7.90[3] cgs
Temperature29,200[3] K
Red dwarf
Mass0.42±0.04[7] M☉
Radius0.43±0.06[8] R☉
Other designations
U Gem, BD+22 1807, HD 64511.[9]
Database references
SIMBADdata

Discovery

U Geminorum was discovered by J.R. Hind in 1855 who initially thought it was a nova; it quickly faded below his telescope's limiting magnitude. Its true nature was revealed three months later when it was re-observed in outburst by Pogson.[10] The star has been monitored by amateur and professional astronomers ever since, though its location near the zodiac means that some outbursts are undoubtedly missed due to the seasonal gap.

Orbit

The U Geminorum binary has a very short orbital period of 4 hours and 11 minutes; this orbit alone makes the system variable, as the components transit and eclipse each other with each revolution. Normally, the combined apparent magnitude varies between 14.0 and 15.1; during an outburst however, the star can brighten a hundredfold, to above 9th magnitude. Though the average interval between outbursts is 102 days,[2] the period is in fact highly irregular, varying from as little as 62 days to as long as 257. As is the case with dwarf novae, the outbursts are theorized results of a periodic surge of influx from the white dwarf's accretion disk, caused by instability in the disk itself.

The orbital motion of the two stars causes their spectral lines to shift due to the doppler effect. However, the wavelengths of the white dwarf spectral lines are also changed due to its gravitational redshift. This complicates the derivation of an accurate orbit. The properties of the stars implied by their orbit are somewhat different from those directly observed or typical for stars of their type.[8]

Distance

Distance estimates for U Geminorum have varied from 52 parsecs (170 light-years) to 112 parsecs (370 light-years).[11] The GAIA DR2 star catalog gives a distance of 93.4 parsecs (305 ly), with a likely margin of error around 0.3 parsecs (0.98 ly).[6]

See also

References
  1. Cutri, R. M.; et al. (June 2003). "2MASS All Sky Catalog of point sources". The IRSA 2MASS All-Sky Point Source Catalog. NASA/IPAC. Bibcode:2003tmc..book.....C.
  2. 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.
  3. Kleinman, S. J.; Kepler, S. O.; Koester, D.; Pelisoli, Ingrid; Peçanha, Viviane; Nitta, A.; Costa, J. E. S.; Krzesinski, J.; Dufour, P.; Lachapelle, F.-R.; Bergeron, P.; Yip, Ching-Wa; Harris, Hugh C.; Eisenstein, Daniel J.; Althaus, L.; Córsico, A. (2013). "SDSS DR7 White Dwarf Catalog". The Astrophysical Journal Supplement Series. 204 (1): 5. arXiv:1212.1222. Bibcode:2013ApJS..204....5K. doi:10.1088/0067-0049/204/1/5. S2CID 53866516.
  4. Stauffer, J.; Spinrad, H.; Thorstensen, J. (1979). "Spectra of the M dwarf companion in U Geminorum". Publications of the Astronomical Society of the Pacific. 91: 59. Bibcode:1979PASP...91...59S. doi:10.1086/130440.
  5. Kraft, Robert P. (March 1962). "Binary Stars among Cataclysmic Variables. I. U Geminorum Stars (dwarf Novae)". Astrophysical Journal. 135: 408. Bibcode:1962ApJ...135..408K. doi:10.1086/147280.
  6. 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.
  7. Echevarría, Juan (2007). "U Geminorum: A Test Case for Orbital Parameter Determination". The Astronomical Journal. 134 (1): 262–273. arXiv:0704.1641. Bibcode:2007AJ....134..262E. doi:10.1086/518562. S2CID 14955616.
  8. Naylor, T. (2005). "The masses, radii and luminosities of the components of U Geminorum". Monthly Notices of the Royal Astronomical Society. 361 (3): 1091–1101. arXiv:astro-ph/0506351. Bibcode:2005MNRAS.361.1091N. doi:10.1111/j.1365-2966.2005.09262.x. S2CID 16271370.
  9. "U Geminorum". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-10-03.
  10. Turner, H. H. (1906). "U Geminorum, Pogson's observations of, edited". Monthly Notices of the Royal Astronomical Society. 67: 119. Bibcode:1906MNRAS..67..119T. doi:10.1093/mnras/67.2.119.
  11. Long, Knox S.; Gilliland, Ronald L. (1999). "GHRS observations of the white dwarf in U Geminorum". Astrophysical Journal. 511 (2): 916–924. Bibcode:1999ApJ...511..916L. doi:10.1086/306721.

Further reading
  • Burnham, Robert. Burnham's Celestial Handbook. New York: Dover Publications, Inc., 1978. ISBN 0-486-23568-8 pp. 925–34.
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