R Coronae Borealis variable

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Visual light curve for RY Sagittarii, 1988-2015, showing classic behaviour for this type of variable RYSgr lightcurve.png
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 [3] are currently known in our Galaxy while up to 1000 were expected, [4] making this class a very rare kind of star.

Contents

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 R Coronae Borealis light curve.png
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) [lower-alpha 1] ConstellationDiscovererDiscovery year Apparent magnitude (Maximum) [lower-alpha 2] Apparent magnitude (Minimum) [lower-alpha 2] Range of magnitudeSpectral classComment
UX Antliae Antlia Kilkenny & Westerhuys1990C 
S Apodis Apus Fleming 1896 [9] 5.6C(R3) 
U Aquarii Aquarius Peters 1881 [10] 7.6Cproposed Thorne–Żytkow object. [11]
UV Cassiopeiae Cassiopeia D'Esterre1913 [12] 4.7F0Ib-G5Ib 
DY Centauri Centaurus Dorrit Hoffleit 1930 [13] 4.4C-Hd/B5-6Ie [14] hot RCB and getting hotter. Binary?
UW Centauri Centaurus Henrietta Leavitt 1906 [15] 5.4Kin variable reflection nebula
V504 Centauri Centaurus McLeod1941 [16] 6.0 ?now considered to be an NL/VY Scl variable
V803 Centauri Centaurus Elvius19754.5pecnow listed as AM CVn variable
V854 Centauri Centaurus Dawes1964 [17] 8.1Ce [18]  
AE Circini Circinus Swope 1931 [19] 3.8 ? symbiotic variable, not RCB
V Coronae Australis Corona Australis Evelyn Leland 1896 [20] 7.5C (R0)"minority" RCB, iron-deficient
WX Coronae Australis Corona Australis Ida Woods 1928 [21] C (R5) 
R Coronae Borealis Corona Borealis Piggott 17959.09G0Iab:peprototype
V482 Cygni Cygnus Whitney 1936 [22] 3.7C-Hd [23]  
LT Draconis Draco Sergio Messina2000 [24] 8.2K5III [24] probably not an RCB star
W Mensae Mensa W. J. Luyten 1927 [25] F8:Iplocated in LMC
Y Muscae Musca Henrietta Leavitt 1906 [26] 1.6Fp 
RT Normae Norma Cannon 1910 [27] 5.8C(R) 
RZ Normae Norma Gaposchkin 1952 [28] 2.4C-Hd [29]  
V409 Normae Norma Elena V. Kazarovets 2011 [30] 7.2C(R) 
V2552 Ophiuchi Ophiuchus Erica Hesselbach2002 [31] 3.1C-Hd [32]  
SV Sagittae Sagitta Vladimir Albitsky 1929 [33] 4.7C0-3,2-3(R2) 
GU Sagittarii Sagittarius Luyten 1927 [34] 3.67C(R0) 
MV Sagittarii Sagittarius Ida Woods 1928 [34] 6.05B2p(HDCe)hot RCB with metal emission lines
RY Sagittarii Sagittarius Markwick 1893 [35] 8.2G0Iaep [36] weak emission lines
VZ Sagittarii Sagittarius Henrietta Leavitt 1904 [37] 4.2C 
V618 Sagittarii Sagittarius Swope 1935 [38] 5.5Me [38] symbiotic variable?
V3795 Sagittarii Sagittarius Dorrit Hoffleit 1972 [39] 4.0pec 
V5639 Sagittarii Sagittarius Greaves2007 [40] 2.7Ic 
FH Scuti Scutum Luyten 1937 [41] 3.4 ? 
SU Tauri Taurus Cannon 1908 [42] 7.76G0-1Iep 
RS Telescopii Telescopium Evelyn Leland 1910 [43] 6.9C (R4) 
Z Ursae Minoris Ursa Minor Benson, Priscilla 1994 [44] 8.2C 
  1. ↑ DY Persei is not included although it may be a related type of variable.
  2. 1 2 (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))

See also

Related Research Articles

A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.

<span class="mw-page-title-main">R Coronae Borealis</span> Variable star in the constellation Corona Borealis

R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Cor Bor class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades to as faint as 15th magnitude. Over successive months it then gradually returns to its normal brightness, giving it the nickname "reverse nova", after the more common type of star which rapidly increases in brightness before fading.

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

W Mensae is an unusual yellow supergiant star in the Large Magellanic Cloud in the southern constellation Mensa. It is an R Coronae Borealis variable and periodically decreases in brightness by several magnitudes.

<span class="mw-page-title-main">DY Persei</span> Variable carbon star in the constellation Perseus

DY Persei is a variable star and carbon star in the Perseus constellation. At maximum it is 11th magnitude and at its faintest it drops to 16th magnitude. DY Persei is the prototype of the very rare DY Persei class of variables that pulsate like red variables but also fade from sight like R Coronae Borealis variables.

<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">UX Antliae</span> Star in the constellation Antlia


UX Antliae is a post-AGB and R Coronae Borealis variable star that has a base apparent magnitude of around 11.85, with irregular dimmings down to below magnitude 18.0.

<span class="mw-page-title-main">RW Cephei</span> Star in the constellation Cepheus

RW Cephei is a K-type hypergiant and a semirregular variable star in the constellation Cepheus, at the edge of the Sharpless 132 H II region and close to the small open cluster Berkeley 94. It is among the largest stars known with a radius of almost 1,000 times that of the Sun (R☉), nearly as large as the orbit of Jupiter.

FG Sagittae is a supergiant star in the constellation Sagitta at a distance of 4000 light-years. When first noted in 1943, it was identified to be a variable star, and it was found to be a hot, blue star of stellar spectral type B in 1955. Since then it has expanded and cooled, becoming a yellow G-type star by 1991, and then further cooling to become an orange K-type star. It started to pulsate when becoming an A-type star with a period of 15 days. This period later increased to over 100 days.

An extreme helium star is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe. Since there are no known conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.

<span class="mw-page-title-main">V605 Aquilae</span> Star in the constellation Aquila

V605 Aquilae, in the constellation Aquila, is the variable central star of the planetary nebula Abell 58. It is a highly unusual hydrogen-deficient carbon-rich star.

<span class="mw-page-title-main">SU Andromedae</span> Star in the constellation Andromeda

SU Andromedae is a carbon star in the constellation of Andromeda. It is a variable star classified as a slow irregular pulsating supergiant, and varies from an apparent visual magnitude of 8.5 at minimum brightness to a magnitude of 8.0 at maximum brightness with no clear period.

<span class="mw-page-title-main">RY Sagittarii</span> Yellow supergiant star in the constellation Sagittarius

RY Sagittarii is a yellow supergiant and an R Coronae Borealis type variable star in the constellation Sagittarius. Although it ostensibly has the spectrum of a G-type star, it differs markedly from most in that it has almost no hydrogen and much carbon.

<span class="mw-page-title-main">Z Ursae Minoris</span> Star in the constellation Ursa Minor

Z Ursae Minoris is a carbon star and R Coronae Borealis variable in the constellation Ursa Minor.

<span class="mw-page-title-main">RS Telescopii</span> Star in the constellation Telescopium

RS Telescopii, abbreviated RS Tel, is a variable star in the southern constellation of Telescopium. It is a dim star with an apparent visual magnitude of 10.67, which is much too faint to be visible without a telescope. The variability of this star was discovered by Evelyn F. Leland and announced by Edward C. Pickering in 1910. It was first studied by Cecilia H. Payne in 1928 at the Harvard College Observatory.

<span class="mw-page-title-main">V Coronae Australis</span> Variable star in the constellation Corona Australis

V Coronae Australis is a R Coronae Borealis variable (RCB) star in the constellation Corona Australis. These are extremely hydrogen-deficient supergiants thought to have arisen as the result of the merger of two white dwarfs; fewer than 100 have been discovered as of 2012. V Coronae Australis dimmed in brightness from 1994 to 1998.

<span class="mw-page-title-main">WX Coronae Australis</span> Variable star in the constellation Corona Australis

WX Coronae Australis is an R Coronae Borealis star in the constellation Corona Australis, one of the brightest examples of this extremely rare class of variable star. Despite the rarity, Corona Australis hosts another R CrB star, V Coronae Australis.

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

S Apodis, also known as HD 133444 is a variable star located in the southern circumpolar constellation Apus. It has an apparent magnitude ranging from 9.6 to 17, which is below the limit for naked eye visibility. The object is located relatively far at a distance of approximately 15,000 light years based on Gaia DR3 parallax measurements, but it is drifting closer with a heliocentric radial velocity of −75 km/s.

<span class="mw-page-title-main">Hydrogen-deficient star</span> Star that has little or no hydrogen in its atmosphere

A hydrogen-deficient star is a type of star that has little or no hydrogen in its atmosphere. Hydrogen deficiency is unusual in a star, as hydrogen is typically the most common element in a stellar atmosphere. Despite being rare, there are a variety of star types that display a hydrogen deficiency.

<span class="mw-page-title-main">DY Centauri</span> Star in the constellation Centaurus

DY Centauri is a variable star in the constellation Centaurus. From its brightness, it is estimated to be 7000 parsecs (23000 light-years) away from Earth.

<span class="mw-page-title-main">AF Andromedae</span> Luminous blue variable star in the constellation Andromeda

AF Andromedae is a luminous blue variable (LBV), a type of variable star. The star is one of the most luminous variables in M31, the Andromeda Galaxy.

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. 1 2 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.
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  13. ↑ Hoffleit, Dorrit (1930). "Variables in Milky Way Field 167". Harvard College Observatory Bulletin. 874: 13. Bibcode:1930BHarO.874...13H.
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  24. 1 2 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.
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  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 .
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