U Monocerotis

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U Monocerotis
UMonLightCurve.png
A visual band light curve for U Monocerotis, adapted from Pollard et al. (2006) [1]
Observation data
Epoch J2000.0       Equinox J2000.0
Constellation Monoceros
Right ascension 07h 30m 47.473s [2]
Declination −09° 46′ 36.79″ [2]
Apparent magnitude  (V)5.45 - 7.67 [3]
Characteristics
Spectral type F8eIb - K0pIb [4]
U−B color index +0.47 - +1.22 [5]
B−V color index +0.81 - +1.27 [5]
Variable type RVb [6]
Astrometry
Radial velocity (Rv)32 [7]  km/s
Proper motion (μ)RA: −10.124 [2]   mas/yr
Dec.: 3.268 [2]   mas/yr
Parallax (π)0.9161 ± 0.0915  mas [2]
Distance 3,600 ± 400  ly
(1,100 ± 100  pc)
Absolute magnitude  (MV)−4.516 [8]
Details
Mass 2.00+1.07
−0.72 [8]   M☉
Radius 100.3+18.9
−13.2 [8]   R☉
Luminosity 5,480+1,753
−882 [8]   L☉
Surface gravity (log g)0.0 [9]   cgs
Temperature 5,000 [8] (4,930-5,830 [5] )  K
Metallicity −0.8 [9]
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
SIMBAD data

U Monocerotis (U Mon) is a pulsating variable star and spectroscopic binary in the constellation Monoceros. The primary star is an RV Tauri variable, a cool luminous post-AGB star evolving into a white dwarf.

Contents

History

U Mon was reported to be variable in 1918 by the renown German astronomer Ernst Hartwig. [10] It was then included by Shapley in his list of Cepheid variables. [11] In the 1950s a series of papers established the fundamental observational parameters of the star, its period, brightness range, colour changes, and spectral variation. [4] [12] [13]

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]

Visibility

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]

System

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. [1]

Properties

The exact properties of U Mon are uncertain. It has a measured parallax, but with considerable uncertainty. 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. [16]

RV Tauri stars have low masses, although U Mon has been calculated to have one of the highest known masses for the class at about 2  M☉. Despite the low masses, they are highly extended cool stars of high luminosity. U Mon has a luminosity of 5,480 L☉, although this is both variable and highly uncertain. The spectral luminosity class is 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. [5] 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. [13]

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. [17]

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. [9]

Evolution

U Mon is likely a post-Asymptotic Giant Branch (AGB) star, a low to intermediate mass 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 Tauri variables give 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. [18]

Related Research Articles

<span class="mw-page-title-main">Variable star</span> Star whose brightness fluctuates, as seen from Earth

A variable star is a star whose brightness as seen from Earth changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

<span class="mw-page-title-main">Supergiant</span> Type of star that is massive and luminous

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.

<span class="mw-page-title-main">Mira variable</span> Type of variable star

Mira variables are a class of pulsating stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths. They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch (AGB), that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

<span class="mw-page-title-main">Long-period variable star</span> Cool luminous pulsating variable star

The descriptive term long-period variable star refers to various groups of cool luminous pulsating variable stars. It is frequently abbreviated to LPV.

<span class="mw-page-title-main">Red-giant branch</span> Portion of the giant branch before helium ignition

The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.

<span class="mw-page-title-main">RV Tauri</span> Star in the constellation Taurus

RV Tauri is a star in the constellation Taurus. It is a yellow supergiant and is the prototype of a class of pulsating variables known as RV Tauri variables. It is a post-AGB star and a spectroscopic binary about 4,700 light years away.

<span class="mw-page-title-main">RV Tauri variable</span> Class of luminous variable star

RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.

<span class="mw-page-title-main">119 Tauri</span> Star in the constellation Taurus

119 Tauri is a red supergiant star in the constellation Taurus. It is a semiregular variable and its angular diameter has been measured at about 10 mas. It is a similar star to Betelgeuse although redder and more distant.

<span class="mw-page-title-main">Yellow supergiant</span> Star that has a supergiant luminosity class, with a spectral type of F or G

A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.

<span class="mw-page-title-main">Stellar pulsation</span>

Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius cause corresponding changes in the luminosity of the star. Astronomers are able to deduce this mechanism by measuring the spectrum and observing the Doppler effect. Many intrinsic variable stars that pulsate with large amplitudes, such as the classical Cepheids, RR Lyrae stars and large-amplitude Delta Scuti stars show regular light curves.

<span class="mw-page-title-main">W Virginis</span> Variable star in the constellation Virgo

W Virginis is the prototype W Virginis variable, a subclass of the Cepheid variable stars. It is located in the constellation Virgo, and varies between magnitudes 9.46 and 10.75 over a period of approximately 17 days.

<span class="mw-page-title-main">Kappa Pavonis</span> Variable star in the constellation Pavo

Kappa Pavonis is a variable star in the constellation Pavo. It is the brightest W Virginis variable in the sky.

<span class="mw-page-title-main">S Vulpeculae</span> Variable star in the constellation Vulpecula

S Vulpeculae is a variable star located in the constellation Vulpecula. A supergiant star, it is around 382 times the diameter of the Sun.

<span class="mw-page-title-main">BL Boötis</span> Star in the constellation Boötes

BL Boötis is a pulsating star in the constellation Boötes. It is the prototype of a class of anomalous Cepheids which is intermediate in the H-R diagram between the type I classical Cepheids and the type II Cepheids.

<span class="mw-page-title-main">R Sagittae</span> Star in the constellation Sagitta

R Sagittae is an RV Tauri variable star in the constellation Sagitta that varies from magnitude 8.0 to 10.5 in 70.77 days. It is a post-AGB low mass yellow supergiant that varies between spectral types G0Ib and G8Ib as it pulsates. Its variable star designation of "R" indicates that it was the first star discovered to be variable in the constellation. It was discovered in 1859 by Joseph Baxendell, though classified as a semi regular variable until RV Tauri variables were identified as a distinct class in 1905.

<span class="mw-page-title-main">AC Herculis</span> Spectroscopic binary star in the constellation Hercules

AC Herculis, is an RV Tauri variable and spectroscopic binary star in the constellation of Hercules. It varies in brightness between apparent magnitudes 6.85 and 9.0.

<span class="mw-page-title-main">HP Lyrae</span> Variable star in the constellation Lyra

HP Lyrae is a variable star in the constellation Lyra, with a visual magnitude varying between 10.2 and 10.8. It will likely be an RV Tauri variable, an unstable post-AGB star losing mass before becoming a white dwarf.

<span class="mw-page-title-main">SZ Tauri</span> Variable star in Taurus

SZ Tauri is a variable star in the equatorial constellation of Taurus. The brightness of this star varies from an apparent visual magnitude of 6.39 down to 6.69 with a period of 3.149 days, which is near the lower limit of visibility to the naked eye. The distance to this star is approximately 2,070 light years based on parallax measurements. There is some indication this may be a binary system, but the evidence is inconclusive.

<span class="mw-page-title-main">Y Tauri</span> Variable star in the constellation Taurus

Y Tauri is a carbon star located in the constellation Taurus. Parallax measurements by Gaia put it at a distance of approximately 2,170 light-years.

<span class="mw-page-title-main">X Cygni</span> Variable star in the constellation Cygnus

X Cygni is a variable star in the northern constellation of Cygnus, abbreviated X Cyg. This is a Delta Cephei variable that ranges in brightness from an apparent visual magnitude of 5.85 down to 6.91 with a period of 16.386332 days. At it brightest, this star is dimly visible to the naked eye. The distance to this star is approximately 628 light years based on parallax measurements. It is drifting further away with a radial velocity of 8.1 km/s. This star is a likely member of the open cluster Ruprecht 173.

References

  1. 1 2 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.
  2. 1 2 3 4 5 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.
  3. ↑ Watson, C.; Henden, A. A.; Price, A. (2015). "AAVSO International Variable Star Index VSX". CDS/ADC Collection of Electronic Catalogues. 1.
  4. 1 2 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 .
  5. 1 2 3 4 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 .
  6. ↑ 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.
  7. ↑ Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Washington: 0. Bibcode:1953GCRV..C......0W.
  8. 1 2 3 4 5 Bódi, A.; Kiss, L. L. (2019). "Physical properties of galactic RV Tauri stars from Gaia DR2 data". The Astrophysical Journal. 872 (1): 60. arXiv: 1901.01409 . Bibcode:2019ApJ...872...60B. doi: 10.3847/1538-4357/aafc24 . S2CID   119099605.
  9. 1 2 3 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. S2CID   54547984.
  10. ↑ Muller, Gustav; Hartwig, Ernst (1918). Geschichte und Literatur des Lichtwechsels der bis Ende 1915 ALS sicher veranderlich anerkannten Sterne : nebst einem Katalog der Elemente ihres Lichtwechsels. Leipzig : In Kommission bei Poeschel & Trepte.
  11. ↑ 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.
  12. ↑ 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.
  13. 1 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 .
  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 Variable Star Observers. 26 (2): 112. Bibcode:1998JAVSO..26..112P.
  16. ↑ Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A.; Bennett, D. P.; Cook, K. H.; Freeman, K. C.; Griest, K.; Lawson, W. A.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Peterson, B. A.; Pollard, Karen R.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Sutherland, W.; Tomaney, A.; Welch, D. L. (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. S2CID   3614156.
  17. ↑ 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 (1): 521–536. arXiv: astro-ph/9909081 . Bibcode:2000ApJ...531..521G. doi:10.1086/308451. S2CID   119408774.
  18. ↑ 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.
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