R Coronae Borealis 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.

This article is about the type of variable star. For the specific star, see R Coronae Borealis.
Visual light curve for RY Sagittarii, 1988–2015, showing classic behaviour for this type of variable

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 (total mass between 0.6 and 1.2 M☉).[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.

Diversity

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.

Physics

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

References
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  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 (published January 1996). 456: 750. Bibcode:1996ApJ...456..750I. doi:10.1086/176694.CS1 maint: multiple names: authors list (link)
  3. Tisserand, P.; Clayton, G. C.; Bessell, M. S.; Welch, D. L.; Kamath, D.; Wood, P. R.; Wils, P.; Wyrzykowski, Ł; Mróz, P. (2018-09-05). "A plethora of new R Coronae Borealis stars from a dedicated spectroscopic follow-up survey". arXiv:1809.01743 [astro-ph.SR].
  4. Clayton, Geoffrey C. (2012-06-15). "What are the R Coronae Borealis Stars?". arXiv:1206.3448 [astro-ph.SR].
  5. 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.
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  8. (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))
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  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.
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  30. Kazarovets, E. V.; Samus, N. N. (2011). "NSV 07212 and NSV 07329: Two Probable RCB Stars". Peremennye Zvezdy. 31 (4): 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.
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  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.
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