R Coronae Borealis variable

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This article is about the type of variable star. For the variable star, see R Coronae Borealis.
An AAVSO light curve of fadings by R Coronae Borealis, the prototype star

An R Coronae Borealis variable (abbreviated RCB,[1] RCrB[2]) is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation (a few tenths of a magnitude), and one irregular unpredictably sudden fading by 1 to 9 magnitudes. The prototype star R Coronae Borealis was discovered by the English amateur astronomer Edward Pigott in 1795, who first observed the enigmatic fadings of the star. Since then, only about 100 RCB variables have been identified,[3] making this class a very rare kind of star.

The fading is caused by condensation of carbon to soot, making the star fade in visible light while measurements in infrared light exhibit no real luminosity decrease. R Coronae Borealis variables are typically supergiant stars in the spectral classes F and G (by convention called "yellow"), with typical C2 and CN molecular bands, characteristic of yellow supergiants. RCB star atmospheres do however lack hydrogen by an abundance of 1 part per 1,000 down to 1 part per 1,000,000 relative to helium and other chemical elements, while the universal abundance of hydrogen is about 3 to 1 relative to helium.


There is a considerable variation in spectrum between various RCB specimens. Most of the stars with known spectrum are either F to G class ("yellow") supergiants, or a comparatively cooler C-R type carbon star supergiant. Three of the stars are however of the "blue" B type, for example VZ Sagittarii. Four stars are unusually and inexplicably poor in iron absorption lines in the spectrum.[4] The constant features are prominent Carbon lines, strong atmospheric Hydrogen deficiencies, and obviously the intermittent fadings.


Two main models for carbon dust formation near the R Coronae Borealis stars have been proposed, one model that presumes the dust forms at a distance of 20 star radii from the center of the star, and one model that presumes that the dust forms in the photosphere of the star. The rationale for the 20 radii formation is that the carbon condensation temperature is 1,500 K, while the photospheric dust model was formulated by the 20 radii model's failure to explain the fast decline of the RCBs' light curves just before reaching minimum. The 20 radii model requires a large and thereby long-time buildup of the obstructing dust cloud, making the fast light decline hard to comprehend.

The alternate theory of photospheric buildup of carbon dust in a 4,500-6,500 K temperature environment could be explained by condensations in the low pressure parts of shock fronts – being detected in the atmosphere of RY Sagittarii – a condensation that causes local runaway cooling, allowing carbon dust to form.[4]

The formation of the stars themselves is also unclear. Standard stellar evolution models do not produce large luminous stars with essentially zero hydrogen. The two main theories to explain these stars are both somewhat exotic, perhaps befitting such rare stars. In one, a merger occurs between two white dwarf stars, one a Helium white dwarf and the other a carbon-oxygen white dwarf. White dwarfs are naturally lacking in hydrogen and the resultant star would also lack that element. The second model postulates a massive convective event at the onset of burning of an outer helium shell, causing the little remaining atmospheric Hydrogen to be turned over into the interior of the star.[5] It is possible that the diversity of R CrB stars is caused by a diversity of formation mechanisms, relating them to extreme helium stars and hydrogen-deficient carbon stars.

List of stars[edit]

Designation (name) Constellation Discovery Apparent magnitude (Maximum)[6] Apparent magnitude (Minimum)[6] Range of magnitude Period Spectral class Comment
UX Antliae Antlia Kilkenny & Westerhuys, 1990 11m.85 <18m.0 >6.15 C  
U Aquarii Aquarius 10m.8 18m.2 7.6 C proposed Thorne–Żytkow object.[7]
V Coronae Australis Corona Australis 9m.4 17m.9 7.5 C (R0)  
WX Coronae Australis Corona Australis 10m.25 <15m.2 >4.95 C (R5)  
R Coronae Borealis Corona Borealis Piggott, 1795 5m.71 14m.8 9.09 G0Iab:pe prototype
W Mensae Mensa W. J. Luyten, 1927 13m.4 <18m.3 >5.1 F8:Ip located in Large Magellanic Cloud
RY Sagittarii Sagittarius Markwick, 1893 5m.8 14m.0 8.2 G0Iaep  
SU Tauri Taurus   9m.1 16m.86 7.76 G0-1Iep  
RS Telescopii Telescopium   9m.6 16m.5 6.9 C (R4)  
Z Ursae Minoris Ursa Minor Benson, Priscilla, 1994 10m.8 19m.0 8.2 C  


  1. ^ Rosenbush, A. E. (1996). "What causes the R Corona Borealis type minimum: dust cloud or dust shell?". Hydrogen deficient stars - Astronomical Society of the Pacific Conference Series (Astronomical Society of the Pacific) 96: 91. Bibcode:1996ASPC...96...91R. 
  2. ^ Iben, Icko, Jr.; Tutukov, Alexander V.; Yungelson, Lev R. (January 1996). "On the Origin of Hydrogen-deficient Supergiants and Their Relation to R Coronae Borealis Stars and Non-DA White Dwarfs". Astrophysical Journal 456: 750. Bibcode:1996ApJ...456..750I. doi:10.1086/176694. 
  3. ^ Tisserand; Clayton; Welch; Pilecki; Wyrzykowski; Kilkenny (2012). "The ongoing pursuit of R Coronae Borealis stars: ASAS-3 survey strikes again". arXiv:1211.2475v2 [astro-ph.SR]. 
  4. ^ a b Clayton, G. C. (1996). "The R Coronae Borealis Stars". Publications of the Astronomical Society of the Pacific 108: 225. Bibcode:1996PASP..108..225C. doi:10.1086/133715.  edit
  5. ^ Hema, B. P.; Pandey, G.; Lambert, D. L. (2012). "The Galactic R Coronae Borealis Stars: The C2 Swan Bands, the Carbon Problem, and the 12C/13C Ratio". The Astrophysical Journal 747 (2): 102. arXiv:1201.1357. Bibcode:2012ApJ...747..102H. doi:10.1088/0004-637X/747/2/102.  edit
  6. ^ a b (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))
  7. ^ Andrew D. Vanture; Daniel Zucker; George Wallerstein (April 1, 1999). "Is U Aquarii a Thorne-Żytkow Object?". The Astrophysical Journal 514 (2). Bibcode:1999ApJ...514..932V. doi:10.1086/306956. 

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