U Monocerotis

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U Monocerotis
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Monoceros
Right ascension 07h 30m 47.46738s[1]
Declination −09° 46′ 36.8036″[1]
Apparent magnitude (V) 5.45 - 7.67[2]
Spectral type F8eIb - K0pIb[3]
U−B color index +0.47 - +1.22[4]
B−V color index +0.81 - +1.27[4]
Variable type RVb[5]
Radial velocity (Rv)32[6] km/s
Proper motion (μ) RA: −2.59[1] mas/yr
Dec.: 3.49[1] mas/yr
Parallax (π)1.47 ± 0.99[1] mas
Distance770[7] pc
Absolute magnitude (MV)−3.36[8]
Mass0.645[9] M
Radius94[10] R
Diameter94[10] D
Luminosity3,800 ± 2,700 L
Surface gravity (log g)0.0 cgs
Temperature5,000 (4,930-5,830[4]K
Other designations
U Mon, HIP 36521, 2MASS J07304746-0946366, BD−09°2085, HD 59693, TYC 5400-4699-1, GCRV 5005, AAVSO 0726-09, IRAS 07284-0940
Database references

U Monocerotis (U Mon) is a variable star and spectroscopic binary in the constellation Monoceros.


U Mon was reported to be variable in 1918 by the renown German astronomer Ernst Hartwig.[11] It was then included by Shapley in his list of Cepheid variables.[12] In the 1950s a series of papers established the fundamental observational parameters of the star, its period, brightness range, colour changes, and spectral variation.[3][13][10]

In 1970, U Mon was discovered to have a large infrared excess. The double-peaked spectral energy distribution and polarization are strongly indicative of a dust shell around the star.[14]


U Mon can often be seen with the naked eye between Sirius and Procyon, but drops below naked eye visibility at deep minima. It lies about two degrees west of α Mon, at fourth magnitude the brightest star in Monoceros. At its brightest U Mon can reach magnitude 5.45. At a shallow minimum it drops to about magnitude 6.0, but at its deepest minima it is below magnitude 7.5. The period is given as 92.23 days, although this varies slightly from cycle to cycle. The brightness of the main pulsations varies over a long secondary period. This takes about 2,500 days, more than twice the length of the secondary period in any other RV Tauri variable.[15]


U Mon is a binary system with a dusty ring surrounding both stars. The companion cannot be observed directly or in the spectrum. Its existence is inferred by radial velocity changes as it orbits every 2,597 days. This is approximately the same time as the long secondary period that modulates the brightness variations. One model for these longterm variations is a periodic eclipse by a circumbinary dust disc.[16]


The exact properties of U Mon are uncertain. It has a measured parallax, but it is too small and uncertain to give an accurate distance. The properties can be inferred by other methods such as spectral line profiles and atmospheric modelling, but these methods are also uncertain for unusual stars like U Mon. RV Tauri stars have been shown to follow a period-luminosity relationship, and this can be used to confirm the luminosity and distance.[17]

RV Tauri stars have low masses, but are highly extended cool stars of high luminosity. U Mon has a luminosity of 3,800 L, although this is both variable and highly uncertain. The spectral luminosity class is of a bright supergiant, indicating the rarefied nature of its atmosphere and low surface gravity. The surface gravity varies during the pulsations, dropping to extremely low values as the star passes through its largest size. The temperature varies by about 1,000 K, being hottest when the star is rising towards a maximum.[4] Integrated radial velocities indicate that during the largest pulsations, the location of the reversing layer in the atmosphere moves by nearly 90% of the average stellar radius.[10]

U Mon is a metal-poor star, as expected for a low-mass post-AGB object. It shows some enhancement of Carbon, but only to about 80% of the oxygen abundance. There is no suggestion of s-process elements being over-abundant. This is consistent with first dredge-up abundances, suggesting that most RV Tauri stars were not massive enough to experience a third dredge-up.[18]

U Mon is surrounded by a dusty circumstellar disc, a common feature of RV Tauri variables. It is likely to be created by interaction with a binary companion.[7]


U Mon is likely a post-Asymptotic Giant Branch (AGB) star, an originally sun-like star which is in the end stages of its life just prior to the expulsion of a planetary nebula and contraction to a white dwarf. RV Tau gives an insight into the lives and deaths of stars like the Sun. Evolution models show it takes about 10 billion years for a 1 solar mass (1 M) star to reach the Asymptotic Giant Branch.[19]


  1. 1.0 1.1 1.2 1.3 1.4 Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653. arXiv:0708.1752. Bibcode 2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. Watson, C.; Henden, A. A.; Price, A. (2015). "AAVSO International Variable Star Index VSX". CDS/ADC Collection of Electronic Catalogues 1. Bibcode 2015yCat....1.2027W.
  3. 3.0 3.1 Rosino, L. (1951). "The Spectra of Variables of the RV Tauri and Yellow Semiregular Types". Astrophysical Journal 113: 60. Bibcode 1951ApJ...113...60R. doi:10.1086/145377.
  4. 4.0 4.1 4.2 4.3 Dawson, D. W. (1979). "A photometric investigation of RV Tauri and yellow semiregular variables". Astrophysical Journal Supplement Series 41: 97. Bibcode 1979ApJS...41...97D. doi:10.1086/190610.
  5. Samus, N. N.Expression error: Unrecognized word "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.
  6. Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington: 0. Bibcode 1953GCRV..C......0W.
  7. 7.0 7.1 7.2 Ruyter, S; Winckel; Dominik; Waters; Dejonghe (2005). "Strong dust processing in circumstellar discs around 6 RV Tauri stars. Are dusty RV Tauri stars all binaries?". Astronomy and Astrophysics 435 (1): 161–166. arXiv:astro-ph/0503290v1. Bibcode 2005A&A...435..161D. doi:10.1051/0004-6361:20041989.
  8. Wahlgren, Glenn M. (1992). "The metallicity and luminosity of RV Tauri variables from medium-resolution spectra". Astronomical Journal 104: 1174. Bibcode 1992AJ....104.1174W. doi:10.1086/116306.
  9. Stasińska, G.; Szczerba, R.; Schmidt, M.; Siódmiak, N. (2006). "Post-AGB stars as testbeds of nucleosynthesis in AGB stars". Astronomy and Astrophysics 450 (2): 701. arXiv:astro-ph/0601504. Bibcode 2006A&A...450..701S. doi:10.1051/0004-6361:20053553.
  10. 10.0 10.1 10.2 Abt, Helmut A.; Monocerotis, Helmut A. (1955). "Studies of RV Tauri Stars. I. U Monocerotis". Astrophysical Journal 122: 72. Bibcode 1955ApJ...122...72A. doi:10.1086/146056.
  11. Lua error in Module:Citation/CS1 at line 886: bad argument #1 to 'sub' (string expected, got table).
  12. Shapley, H. (1918). "Studies based on the colors and magnitudes in stellar clusters. VIII. The luminosities and distances of 139 Cepheid variables". Astrophysical Journal 48: 279. Bibcode 1918ApJ....48..279S. doi:10.1086/142435.
  13. Joy, Alfred H. (1952). "The Semiregular Variable Stars of the RV Tauri and Related Classes". Astrophysical Journal 115: 25. Bibcode 1952ApJ...115...25J. doi:10.1086/145506.
  14. Gehrz, R. D.; Woolf, N. J. (1970). "R V Tauri Stars: A New Class of Infrared Object". Astrophysical Journal 161: L213. Bibcode 1970ApJ...161L.213G. doi:10.1086/180605.
  15. Percy, John R.; Bakos, Akos (1998). "AC Her and U Mon: RV Tauri Stars in the AASVO Photoelectric Photometry Program". The Journal of the American Association of Variale Star Observers 26: 112. Bibcode 1998JAVSO..26..112P.
  16. Pollard, K. R.; McSaveney, J. A.; Cottrelll, P. L. (2006). "The long-term phenomenon in U Mon". Memorie della Società Astronomica Italiana 77: 527. Bibcode 2006MmSAI..77..527P.
  17. Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A.; Bennett, D. P.; Cook, K. H.; Freeman, K. C. et al. (1998). "The MACHO Project LMC Variable Star Inventory. VII. The Discovery of RV Tauri Stars and New Type II Cepheids in the Large Magellanic Cloud". The Astronomical Journal 115 (5): 1921. arXiv:astro-ph/9708039. Bibcode 1998AJ....115.1921A. doi:10.1086/300317.
  18. Giridhar, Sunetra; Lambert, David L.; Gonzalez, Guillermo (2000). "Abundance Analyses of Field RV Tauri Stars. V. DS Aquarii, UY Arae, TW Camelopardalis, BT Librae, U Monocerotis, TT Ophiuchi, R Scuti, and RV Tauri". The Astrophysical Journal 531: 521. arXiv:astro-ph/9909081. Bibcode 2000ApJ...531..521G. doi:10.1086/308451.
  19. Bloecker, T. (1995). "Stellar evolution of low- and intermediate-mass stars. II. Post-AGB evolution". Astronomy and Astrophysics 299: 755. Bibcode 1995A&A...299..755B.

Template:Stars of Monoceros