R Coronae Borealis variable

This article is about the type of variable star. For the specific star, see R Coronae Borealis.

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^[3] are currently known in our Galaxy while up to 1000 were expected,^[4] making this class a very rare kind of star.

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_☉).^[5] 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

Light curve of R Coronae Borealis from 1990 to 2017, showing the unprecedented deep minimum

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.^[6] 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.^[6]

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.^[7] 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,^[8] as well as other notable examples.

Designation (name)[a]	Constellation	Discoverer	Discovery year	Apparent magnitude (Maximum)[b]	Apparent magnitude (Minimum)[b]	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

1. DY Persei is not included although it may be a related type of variable.
2. (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))

See also

• Thorne–Żytkow object

References

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. 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 (published January 1996): 750. Bibcode:1996ApJ...456..750I. doi:10.1086/176694.
3. "Tracking down R Coronae Borealis". Retrieved 2022-05-17.
4. Tisserand, P.; Clayton, G. C.; Bessell, M. S.; Welch, D. L.; Kamath, D.; Wood, P. R.; Wils, P.; Wyrzykowski, Ł; Mróz, P. (2020). "A plethora of new R Coronae Borealis stars discovered from a dedicated spectroscopic follow-up survey". Astronomy & Astrophysics. 635: A14. arXiv:1809.01743. Bibcode:2020A&A...635A..14T. doi:10.1051/0004-6361/201834410. S2CID 119547939.
5. Clayton, Geoffrey C. (2012-06-15). "What are the R Coronae Borealis Stars?". Journal of the American Association of Variable Star Observers (Jaavso). 40 (1): 539. arXiv:1206.3448. Bibcode:2012JAVSO..40..539C.
6. 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.
7. 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. S2CID 118653032.
8. 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: 02025. Bibcode:2009yCat....102025S.
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. hdl:2027/coo1.ark:/13960/t2n593755.
11. Andrew D. Vanture; Daniel Zucker; George Wallerstein (1999). "Is U Aquarii a Thorne-Żytkow Object?". The Astrophysical Journal. 514 (2) (published April 1, 1999): 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. 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. S2CID 59060842.
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. S2CID 16561076.
33. Albitzky, V. (1929). "Nine new variable stars". Astronomische Nachrichten. 235 (17): 317–318. Bibcode:1929AN....235..317A. doi:10.1002/asna.19292351707.
34. 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:2012JBAA..122..335S.
36. "V RY Sgr". The International Variable Star Index. Retrieved 8 August 2013.
37. Leavitt, Henrietta; Pickering, Edward C. (1904). "Sixteen New Variable Stars in Sagittarius". Harvard College Observatory Circular. 91: 1. Bibcode:1904HarCi..91....1L.
38. 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): 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.

External links

• R Coronae Borealis stars, by C. Simon Jeffrey, Armagh Observatory Northern Ireland
• Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
• The R Coronae Borealis Stars, by Geoffrey C. Clayton, from SAO/NASA Astrophysics Data System (ADS)
• Variable Star of the Month, January, 2000: R Coronae Borealis, at the AAVSO website