R Coronae Borealis variable

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Visual light curve for RY Sagittarii, 1988–2015, showing classic behaviour for this type of variable

An R Coronae Borealis variable (abbreviated RCB,[1] R CrB[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. Only about 150 RCB stars are currently known in our Galaxy while up to 1000 were expected [3], making this class a very rare kind of star.

It is increasingly suspected that R Coronae Borealis (RCB) stars – rare hydrogen-deficient and carbon-rich supergiant stars – are the product of mergers of white-dwarfs in the intermediary mass regime (0.6<M_Total<1.2M_Sun)[4]. 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.[5] The constant features are prominent carbon lines, strong atmospheric hydrogen deficiencies, and obviously the intermittent fadings.

The DY Persei variables have been considered a sub-class of R CrB variable, although they are less luminous carbon-rich AGB stars and may be unrelated.


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.[5]

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.[6] 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

This list contains all the R CrB stars listed in the GCVS,[7] as well as other notable examples.

Designation (name) Constellation Discoverer Discovery year Apparent magnitude (Maximum)[8] Apparent magnitude (Minimum)[8] Range of magnitude Spectral class Comment
UX Antliae Antlia Kilkenny & Westerhuys 1990 11m.85 <18m.0 >6.15 C  
S Apodis Apus Fleming 1896[9] 9m.6 15m.2 5.6 C(R3)  
U Aquarii Aquarius Peters 1881[10] 10m.8 18m.2 7.6 C proposed Thorne–Żytkow object.[11]
UV Cassiopeiae Cassiopeia D'Esterre 1913[12] 11m.8 16m.5 4.7 F0Ib-G5Ib  
DY Centauri Centaurus Dorrit Hoffleit 1930[13] 12m.0 16m.4 4.4 C-Hd/B5-6Ie[14] hot RCB and getting hotter. Binary?
UW Centauri Centaurus Henrietta Leavitt 1906[15] 9m.1 14m.5 5.4 K in variable reflection nebula
V504 Centauri Centaurus McLeod 1941[16] 12m.0 18m.0 6.0 ? now considered to be an NL/VY Scl variable
V803 Centauri Centaurus Elvius 1975 13m.2 17m.7 4.5 pec now listed as AM CVn variable
V854 Centauri Centaurus Dawes 1964[17] 7m.1 15m.2 8.1 Ce[18]  
AE Circini Circinus Swope 1931[19] 12m.2 16m.0 3.8 ? symbiotic variable, not RCB
V Coronae Australis Corona Australis Evelyn Leland 1896[20] 9m.4 17m.9 7.5 C (R0) "minority" RCB, iron-deficient
WX Coronae Australis Corona Australis Ida Woods 1928[21] 10m.25 <15m.2 >4.95 C (R5)  
R Coronae Borealis Corona Borealis Piggott 1795 5m.71 14m.8 9.09 G0Iab:pe prototype
V482 Cygni Cygnus Whitney 1936[22] 11m.8 15m.5 3.7 C-Hd[23]  
LT Draconis Draco Sergio Messina 2000[24] 10m.8 19m.0 8.2 K5III[24] probably not an RCB star
W Mensae Mensa W. J. Luyten 1927[25] 13m.4 <18m.3 >5.1 F8:Ip located in LMC
Y Muscae Musca Henrietta Leavitt 1906[26] 10m.5 12m.1 1.6 Fp  
RT Normae Norma Cannon 1910[27] 10m.6 16m.3 5.8 C(R)  
RZ Normae Norma Gaposchkin 1952[28] 10m.6 13m.0 2.4 C-Hd[29]  
V409 Normae Norma Elena V. Kazarovets 2011[30] 11m.8 19m.0 7.2 C(R)  
V2552 Ophiuchi Ophiuchus Erica Hesselbach 2002[31] 10m.5 13m.6 3.1 C-Hd[32]  
SV Sagittae Sagitta Vladimir Albitsky 1929[33] 11m.5 16m.2 4.7 C0-3,2-3(R2)  
GU Sagittarii Sagittarius Luyten 1927[34] 11m.33 15m.0 3.67 C(R0)  
MV Sagittarii Sagittarius Ida Woods 1928[34] 12m.0 16m.05 6.05 B2p(HDCe) hot RCB with metal emission lines
RY Sagittarii Sagittarius Markwick 1893[35] 5m.8 14m.0 8.2 G0Iaep[36] weak emission lines
VZ Sagittarii Sagittarius Henrietta Leavitt 1904[37] 10m.8 15m.0 4.2 C  
V618 Sagittarii Sagittarius Swope 1935[38] 11m.0 16m.5 5.5 Me[38] symbiotic variable?
V3795 Sagittarii Sagittarius Dorrit Hoffleit 1972[39] 11m.5 15m.5 4.0 pec  
V5639 Sagittarii Sagittarius Greaves 2007[40] 11m.2 13m.9 2.7 Ic  
FH Scuti Scutum Luyten 1937[41] 13m.4 16m.8 3.4 ?  
SU Tauri Taurus Cannon 1908[42] 9m.1 16m.86 7.76 G0-1Iep  
RS Telescopii Telescopium Evelyn Leland 1910[43] 9m.6 16m.5 6.9 C (R4)  
Z Ursae Minoris Ursa Minor Benson, Priscilla 1994[44] 10m.8 19m.0 8.2 C  

DY Persei is not included although it may be a related type of variable.

See also


  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. (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, P.; Clayton, G. C.; Bessell, M. S.; Welch, D. L.; Kamath, D.; Wood, P. R.; Wils, P.; Wyrzykowski, Ł et al. (2018-09-05). "A plethora of new R Coronae Borealis stars from a dedicated spectroscopic follow-up survey". arXiv:1809.01743 [astro-ph]. http://arxiv.org/abs/1809.01743.
  4. Clayton, Geoffrey C. (2012-06-15). "What are the R Coronae Borealis Stars?". arXiv:1206.3448 [astro-ph]. http://arxiv.org/abs/1206.3448.
  5. 5.0 5.1 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.
  6. 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.
  7. 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: 02025. Bibcode 2009yCat....102025S.
  8. 8.0 8.1 (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))
  9. Pickering, E. C.; Fleming, W. P. (1896). "Harvard College Observatory, circular no. 6. New variable stars". Astrophysical Journal 3: 296. Bibcode 1896ApJ.....3..296P. doi:10.1086/140219.
  10. Chandler, S. C. (1888). "Catalogue of variable stars". Astronomical Journal 8: 81. Bibcode 1888AJ......8...81C. doi:10.1086/101060.
  11. Andrew D. Vanture; Daniel Zucker; George Wallerstein (1999). "Is U Aquarii a Thorne-Żytkow Object?". The Astrophysical Journal 514 (2): 932–938. Bibcode 1999ApJ...514..932V. doi:10.1086/306956.
  12. d'Esterre, C. R.; Furuhjelm, R.; Deutschland, G. (1913). "Mitteilungen uber Veranderliche". Astronomische Nachrichten 196 (22): 301–306. Bibcode 1913AN....196..301D. doi:10.1002/asna.19131962205.
  13. Hoffleit, Dorrit (1930). "Variables in Milky Way Field 167". Harvard College Observatory Bulletin 874: 13. Bibcode 1930BHarO.874...13H.
  14. Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009–2014)". VizieR On-line Data Catalog: B/mk. Originally published in: Lowell Observatory (October 2014) 1: 2023. Bibcode 2014yCat....1.2023S.
  15. Pickering, Edward C. (1907). "Thirty-six new variable stars". Astronomische Nachrichten 173 (24): 379–384. Bibcode 1907AN....173..379P. doi:10.1002/asna.19061732408.
  16. McLeod, Noah W.; Swope, Henrietta H. (1941). "The Variable Stars of Milky Way Field 239". Harvard College Observatory Bulletin 915: 29. Bibcode 1941BHarO.915...29M.
  17. McNaught, R. H.; Dawes, G. (1986). "Nsv 6708". Information Bulletin on Variable Stars 2928: 1. Bibcode 1986IBVS.2928....1M.
  18. Whitney, Barbara A.; Clayton, Geoffrey C.; Schulte-Ladbeck, Regina E.; Meade, Marilyn R. (1992). "Spectropolarimetry of V854 Centauri at minimum light – Clues to the geometry of the dust and emission-line region". Astronomical Journal 103: 1652. Bibcode 1992AJ....103.1652W. doi:10.1086/116180.
  19. Swope, Henrietta H. (1931). "New Variable Stars in Centaurus and Circinus". Harvard College Observatory Bulletin 883: 23. Bibcode 1931BHarO.883...23S.
  20. Pickering, E. C.; Leland, E. F. (1896). "Harvard College Observatory, circular no. 10. Six new variable stars". Astrophysical Journal 4: 234. Bibcode 1896ApJ.....4..234P. doi:10.1086/140271.
  21. Woods, Ida E. (1928). "Forty New Variable Stars". Harvard College Observatory Bulletin 854: 4. Bibcode 1928BHarO.854....4W.
  22. Whitney, Balfour S. (1949). "A New Variable Star". Astrophysical Journal 109: 538. Bibcode 1949ApJ...109..538W. doi:10.1086/145160.
  23. Rao, N. K.; Lambert, David L. (1993). "On the absolute magnitude of V482 Cygni, an R Coronae Borealis star". Publications of the Astronomical Society of the Pacific 105: 574. Bibcode 1993PASP..105..574R. doi:10.1086/133196.
  24. 24.0 24.1 Messina, S.; Marino, G.; Rodonò, M.; Cutispoto, G. (2000). "Serendipitous discovery of an irregular and a semi-regular type variable in the field of BY Draconis". Astronomy and Astrophysics 364: 706. Bibcode 2000A&A...364..706M.
  25. Luyten, W. J. (1927). "A New Irregular Variable of the R Coronae Type". Harvard College Observatory Bulletin 846 (846): 31–33. Bibcode 1927BHarO.846...33L. 1927BHarO.846...33L.
  26. Pickering, Edward C. (1906). "Thirty-one new variable stars". Astronomische Nachrichten 173 (16–17): 263–268. Bibcode 1906AN....173R.263P. doi:10.1002/asna.19061731604.
  27. Cannon, Annie J.; Pickering, Edward C. (1910). "22 New Variable Stars in Harvard Map, No. 52". Harvard College Observatory Circular 162: 1. Bibcode 1910HarCi.162....1C.
  28. Milone, L. A. (1990). "The Position of RZ Normae". Information Bulletin on Variable Stars 3505 (3505): 1–3. Bibcode 1990IBVS.3505....1M.
  29. Tisserand, P.; Clayton, G. C.; Welch, D. L.; Pilecki, B.; Wyrzykowski, L.; Kilkenny, D. (2013). "The ongoing pursuit of R Coronae Borealis stars: The ASAS-3 survey strikes again". Astronomy & Astrophysics 551: A77. arXiv:1211.2475. Bibcode 2013A&A...551A..77T. doi:10.1051/0004-6361/201220713.
  30. Kazarovets, E. V.; Samus, N. N. (2011). "NSV 07212 and NSV 07329: Two Probable RCB Stars". Peremennye Zvezdy 31: 4. Bibcode 2011PZ.....31....4K.
  31. Hesselbach, E.; Clayton, G. C.; Smith, P. S. (2002). "Study of Suggested New R Coronae Borealis Stars HAD V98 and HD 172468". American Astronomical Society 201: 1128. Bibcode 2002AAS...201.1711H.
  32. Hesselbach, E.; Clayton, Geoffrey C.; Smith, Paul S. (2003). "The Newly Active R Coronae Borealis Star, V2552 Ophiuchi". The Publications of the Astronomical Society of the Pacific 115 (813): 1301–1303. arXiv:astro-ph/0309331. Bibcode 2003PASP..115.1301H. doi:10.1086/379746.
  33. Albitzky, V. (1929). "Nine new variable stars". Astronomische Nachrichten 235 (17): 317–318. Bibcode 1929AN....235..317A. doi:10.1002/asna.19292351707.
  34. 34.0 34.1 Hoffleit, D. (1959). "Six variable stars of unusual type in Sagittarius". Astronomical Journal 64: 241. Bibcode 1959AJ.....64..241H. doi:10.1086/107927.
  35. Shears, Jeremy (2012). "Ernest Elliott Markwick: Variable stars and military campaigns". The Journal of the British Astronomical Association 122 (6): 335–48. arXiv:1109.4234. Bibcode 2011arXiv1109.4234S. https://arxiv.org/ftp/arxiv/papers/1109/1109.4234.pdf.
  36. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  37. Leavitt, Henrietta; Pickering, Edward C. (1904). "Sixteen New Variable Stars in Sagittarius". Harvard College Observatory Circular 91: 1. Bibcode 1904HarCi..91....1L.
  38. 38.0 38.1 Kilkenny, D. (1989). "Spectroscopy of 'RCB' stars – III. V618 SGR and MT PUP". The Observatory 109: 229. Bibcode 1989Obs...109..229K.
  39. Hoffleit, D. (1972). "Six Variable Stars in Sagittarius". Information Bulletin on Variable Stars 617: 1. Bibcode 1972IBVS..617....1H.
  40. Greaves, J. (2007). "An R Coronae Borealis Variable in Sagittarius". Peremennye Zvezdy 27: 7. Bibcode 2007PZ.....27....7G.
  41. Luyten, W. J. (1937). "Neue Veränderliche am Südhimmel". Astronomische Nachrichten 261 (23): 451–458. Bibcode 1937AN....261..451L. doi:10.1002/asna.19362612304.
  42. Pickering, E. C. (1909). "The variable star SU Tauri". Astronomische Nachrichten 182 (24): 401–404. Bibcode 1909AN....182..401P. doi:10.1002/asna.19091822404.
  43. Pickering, Edward C. (1910). "20 new variable stars in Harvard Map, Nos. 2, 5, 32, 44 and 53". Astronomische Nachrichten 184: 5–8. Bibcode 1910AN....184....5P. doi:10.1002/asna.19101840104.
  44. Benson, Priscilla J.; Clayton, Geoffrey C.; Garnavich, Peter; Szkody, Paula (1994). "Z Ursa Minoris – a new R Coronae Borealis variable". The Astronomical Journal 108 (1): 247–50. Bibcode 1994AJ....108..247B. doi:10.1086/117063. http://articles.adsabs.harvard.edu//full/1994AJ....108..247B/0000247.000.html.

External links

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