R Coronae Borealis

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R Coronae Borealis
Corona Borealis constellation map.svg
Red circle.svg
Location of R Coronae Borealis (circled)
Observation data
Epoch J2000.0       Equinox J2000.0
Constellation Corona Borealis
Right ascension 15h 48m 34.4147s [1]
Declination +28° 09′ 24.295″ [1]
Apparent magnitude  (V)5.71 - 14.8 [2]
Characteristics
Spectral type G0Iep [3]
U−B color index 0.13 [4]
B−V color index 0.60 [4]
V−R color index 0.45 [4]
J−H color index 0.275 [5]
J−K color index 0.800 [5]
Variable type R CrB [6] [2]
Astrometry
Radial velocity (Rv)27.83 [7]  km/s
Proper motion (μ)RA: -2.10 [8]   mas/yr
Dec.: −11.52 [8]   mas/yr
Parallax (π)0.73 ± 0.27  mas [1]
Distance 1,400 [9]   pc
Absolute magnitude  (MV)−5 [9]
Details
Mass 0.8-0.9 [10]   M☉
Radius 85 [9]   R☉
Luminosity ~10,000 [11]   L☉
Surface gravity (log g)0.5 [12]   cgs
Temperature 6,750 [11]   K
Other designations
R  Coronae Borealis, R  CrB, GSC2  N1330022410, 2MASS  J15483440+2809242, AG+28° 1513, GSC  02039-01605, BD+28° 2477, HD  141527, PLX  3581, TYC  2039-1605-1, CDS 886, PPM  104338, GC  21257, HIP  77442, RAFGL  4219, GCRV  9116, HR  5880, AAVSO  1544+28A, IRAS  15465+2818, SAO  84015.
Database references
SIMBAD data

R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Coronae Borealis variable 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.

Contents

Nomenclature

R Coronae Borealis is a faint naked eye star, but does not have any traditional names. Johann Bayer did not give it a Greek letter designation although it is marked on his map. John Flamsteed numbered all the Bayer stars but did not add any additional designations for fainter stars, so R Coronae Borealis does not appear in either of these two catalogues. [13]

At its discovery it was described simply as "the variable in the Northern crown". [13] It was later referred to as Variabilis Coronae, "Variable (star) of Corona (Borealis)". [14] It has also been called a "reverse nova" because of its habit of fading from sight. [15] The variable star designation R Coronae Borealis was introduced, as "Coronae R" by Friedrich Wilhelm Argelander in 1850. [16]

Variability

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

The variability of R Coronae Borealis was discovered by English astronomer Edward Pigott in 1795. [13] In 1935 it was the first star shown to have a different chemical composition to the Sun via spectral analysis. [17]

R Coronae Borealis is the prototype of the R Coronae Borealis class of variable stars. It is one of only two R Coronae Borealis variables bright enough to be seen with the naked eye, along with RY Sagittarii. [18] Much of the time it shows variations of around a tenth of a magnitude with poorly defined periods that have been reported as 40 and 51 days. These correspond to the first overtone and fundamental radial pulsation modes for an extreme helium star slightly under one M☉. [10]

At irregular intervals a few years or decades apart R Coronae Borealis fades from its normal brightness near 6th magnitude for a period of months or sometimes years. There is no fixed minimum, but the star can become fainter than 15th magnitude in the visual range. [19] The fading is less pronounced at longer wavelengths. Typically the star starts to return to maximum brightness almost immediately from its minimum, although occasionally this is interrupted by another fade. The cause of this behaviour is believed to be a regular build-up of carbon dust in the star's atmosphere. The sudden drop in brightness may be caused by a rapid condensation of carbon-rich dust similar to soot, resulting in much of the star's light being blocked. The gradual restoration to normal brightness results from the dust being dispersed by radiation pressure. [20]

In August 2007, R Coronae Borealis began a fade to an unprecedented minimum. It fell to 14th magnitude in 33 days, then continued to fade slowly, dropping below 15th magnitude in June 2009. It then began an equally slow rise, not reaching 12th magnitude until late 2011. This was an unusually deep and exceptional long minimum, longer even than a deep five year minimum which had occurred in 1962–7. It then faded again to near 15th magnitude, and by August 2014 it had been below 10th magnitude for 7 years. In late 2014, it brightened quickly to 7th magnitude but then began to fade again. [20] By mid-2017, it had been below its "normal" brightness for ten years. It also reached a new record faintest at magnitude 15.2. [19]

Spectrum

R Coronae Borealis at maximum light shows the spectrum a late F or early G yellow supergiant, but with marked peculiarities. Hydrogen lines are weak or absent, while carbon lines and molecular bands of cyanogen (CN) and C2 are exceptionally strong. Helium lines and metals such as calcium are also present. [17] The spectrum is variable, most obviously during the brightness fades. The normal absorption spectrum is replaced by emission lines, especially HeI, CaII, NaI, and other metals. The lines are typically very narrow at this stage. Helium emission lines sometimes show P Cygni profiles. In deep minima, many of the metal lines disappear although the Ca doublet remains strong. Forbidden "nebular" lines of [OI], [OII], and [NII] can be detected at times. [20]

The spectrum at maximum indicates that hydrogen in R Coronae Borealis is strongly depleted, helium is the dominant element, and carbon is strongly enhanced. At minimum, the spectrum shows the development of carbon clouds that obscure the photosphere, leaving chromospheric lines visible at times.

Properties

R Coronae Borealis in optical light R Coronae Borealis.jpg
R Coronae Borealis in optical light

R Coronae Borealis is about 90% helium and less than 1% hydrogen. The majority of the remainder is carbon. [21] This classifies it as a carbon-enhanced extreme helium star. Modelling the pulsations suggests that the star's mass is 0.8-0.9 M☉. The temperature at maximum is reasonably well known at 6,900K and appears to decrease during the fades as the photosphere is obscured by condensing dust.

The distance of R Coronae Borealis is not known exactly, but is estimated at 1.4 kiloparsecs from assumptions about its intrinsic brightness. The absolute magnitude of −5 is calculated by comparison with R CrB variables in the Large Magellanic Cloud whose distances are known quite accurately. The luminosity is estimated from helium star models to be 19,000 L☉ and the star has a radius around 100 R☉. [22] The Gaia data release 1 parallax also gives a distance of 1.4 kpc although with a considerable margin of error. [1]

There is a fainter star 3" away from R Coronae Borealis, but it is believed to be a distant class K dwarf. Its colour and apparent magnitude are not consistent with being at the same distance as R Coronae Borealis. [20]

Formation

There are two main models for the formation of R CrB stars: the merger of two white dwarfs; or a very late helium flash in a post-AGB star. Models of post-AGB stars calculate that a star with the appearance of R CrB would have a mass around 0.6 M☉ so it is thought to have formed by the merger of a carbon-oxygen white dwarf and a helium white dwarf. [23] The detection of significant lithium in the atmosphere is not easily explained by the merger model, but is a natural consequence of a late helium flash. [22] Evolutionary models of post-AGB stars give a mass of 0.66 M☉ for R CrB, but with a considerable margin of error. [24]

Circumstellar material

Direct imaging with the Hubble Space Telescope shows extensive dust clouds out to a radius of around 2000 astronomical units from R Coronae Borealis, corresponding to a stream of fine dust (composed of grains about 5 nm in diameter) associated with the star's stellar wind, and coarser dust (composed of grains with a diameter of around 0.14 µm) ejected periodically. [25] The obscuration appears to happen closer to the star as clouds of carbon condense at shock regions in an expanding front. "Puffs" of dust emitted from the star condense at about 85 R☉ from the surface, and are visible as cometary knots when they lie to side of the star. [20] There is also a 2 M☉ shell about 4 pc wide containing dust at 25 K, which may be a fossil planetary nebula. [22]

Related Research Articles

<span class="mw-page-title-main">Corona Borealis</span> Constellation in the northern celestial hemisphere

Corona Borealis is a small constellation in the Northern Celestial Hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Its brightest stars form a semicircular arc. Its Latin name, inspired by its shape, means "northern crown". In classical mythology Corona Borealis generally represented the crown given by the god Dionysus to the Cretan princess Ariadne and set by her in the heavens. Other cultures likened the pattern to a circle of elders, an eagle's nest, a bear's den or a smokehole. Ptolemy also listed a southern counterpart, Corona Australis, with a similar pattern.

<span class="mw-page-title-main">Supergiant</span> Type of star that is massive and luminous

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.

<span class="mw-page-title-main">Red supergiant</span> Stars with a supergiant luminosity class with a spectral type of K or M

Red supergiants (RSGs) are stars with a supergiant luminosity class and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

<span class="mw-page-title-main">Mira variable</span> Type of variable star

Mira variables are a class of pulsating stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths. They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch (AGB), that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

<span class="mw-page-title-main">Chi Cygni</span> Star in the constellation Cygnus

Chi Cygni is a Mira variable star in the constellation Cygnus, and also an S-type star. It is around 500 light years away.

<span class="mw-page-title-main">La Superba</span> Variable star in the constellation Canes Venatici

La Superba is a strikingly red giant star in the constellation Canes Venatici. It is a carbon star and semiregular variable.

DY Persei variables are a subclass of R Coronae Borealis variables. They are carbon-rich asymptotic giant branch (AGB) stars that exhibit pulsational variability of AGB stars and irregular fades similar to R CrB stars.

<span class="mw-page-title-main">R Coronae Borealis variable</span> Type of eruptive variable star

An R Coronae Borealis variable is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation, 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, making this class a very rare kind of star.

<span class="mw-page-title-main">S Coronae Borealis</span> Star in the constellation Corona Borealis

S Coronae Borealis is a Mira variable star in the constellation Corona Borealis. Its apparent magnitude varies between 5.3 and 13.6, with a period of 360 days—just under a year. Within the constellation, it lies to the west of Theta Coronae Borealis, and around 1 degree southeast of the eclipsing binary star U Coronae Borealis.

<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.

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.

<span class="mw-page-title-main">TX Piscium</span> Carbon star visible to the naked eye in the constellation Pisces

TX Piscium is a variable red giant star in the constellation Pisces. It is amongst the reddest naked eye stars, with a significant reddish hue when seen in binoculars. It is approximately 800 light years from Earth.

<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">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">IK Tauri</span> Mira variable star in the constellation Taurus

IK Tauri or NML Tauri is a Mira variable star located about 280 parsecs (910 ly) from the Sun in the zodiac constellation of Taurus.

<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.

References

  1. 1 2 3 4 Gaia Collaboration; et al. (November 2016). "Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties". Astronomy & Astrophysics. 595: 23. arXiv: 1609.04172 . Bibcode:2016A&A...595A...2G. doi:10.1051/0004-6361/201629512. S2CID   1828208. A2.
  2. 1 2 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. Bibcode:2009yCat....102025S.
  3. ↑ Klochkova, V. G. (1997). "Supergiants with large IR excesses". Bulletin of the Special Astrophysical Observatory. 44: 5. Bibcode:1997BSAO...44....5K.
  4. 1 2 3 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  5. 1 2 Cutri, Roc M.; Skrutskie, Michael F.; Van Dyk, Schuyler D.; Beichman, Charles A.; Carpenter, John M.; Chester, Thomas; Cambresy, Laurent; Evans, Tracey E.; Fowler, John W.; Gizis, John E.; Howard, Elizabeth V.; Huchra, John P.; Jarrett, Thomas H.; Kopan, Eugene L.; Kirkpatrick, J. Davy; Light, Robert M.; Marsh, Kenneth A.; McCallon, Howard L.; Schneider, Stephen E.; Stiening, Rae; Sykes, Matthew J.; Weinberg, Martin D.; Wheaton, William A.; Wheelock, Sherry L.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.
  6. ↑ Mattei, Janet A.; Waagen, Elizabeth O.; Foster, E. Grant (1991). "R Coronae Borealis light curves 1843-1990". AAVSO Monograph. Bibcode:1991rcbl.book.....M.
  7. ↑ White, Russel J.; Gabor, Jared M.; Hillenbrand, Lynne A. (2007). "High-Dispersion Optical Spectra of Nearby Stars Younger Than the Sun". The Astronomical Journal. 133 (6): 2524–2536. arXiv: 0706.0542 . Bibcode:2007AJ....133.2524W. doi:10.1086/514336. S2CID   122854.
  8. 1 2 Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv: 0708.1752 . Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID   18759600.
  9. 1 2 3 Montiel, Edward J.; Clayton, Geoffrey C.; Marcello, Dominic C.; Lockman, Felix J. (2015). "What is the Shell Around R Coronae Borealis?". The Astronomical Journal. 150 (1): 14. arXiv: 1505.04173 . Bibcode:2015AJ....150...14M. doi:10.1088/0004-6256/150/1/14. S2CID   119271048.
  10. 1 2 Saio, H. (2008). "Radial and Nonradial Pulsations in RCB and EHe-B Stars". Hydrogen-Deficient Stars ASP Conference Series. 391: 69. Bibcode:2008ASPC..391...69S.
  11. 1 2 Clayton, Geoffrey C.; Geballe, T. R.; Zhang, Wanshu (2013). "Variable Winds and Dust Formation in R Coronae Borealis Stars". The Astronomical Journal. 146 (2): 23. arXiv: 1305.5047 . Bibcode:2013AJ....146...23C. doi:10.1088/0004-6256/146/2/23. S2CID   118385818.
  12. ↑ Asplund, M.; Gustafsson, B.; Lambert, D. L.; Rao, N. K. (2000). "The R Coronae Borealis stars - atmospheres and abundances". Astronomy and Astrophysics. 353: 287. Bibcode:2000A&A...353..287A.
  13. 1 2 3 Pigott, Edward; Englefield, Henry C. (1797). "On the Periodical Changes of Brightness of Two Fixed Stars. By Edward Pigott, Esq. Communicated by Sir Henry C. Englefield, Bart. F. R. S". Philosophical Transactions of the Royal Society of London. 87. The Royal Society: 133–141. Bibcode:1797RSPT...87..133P. doi: 10.1098/rstl.1797.0007 . JSTOR   106921. S2CID   186214528.
  14. ↑ Allen, R. H. (1963). Star Names: Their Lore and Meaning . Dover Publications, Inc. p.  178. ISBN   978-0486210797.
  15. ↑ Bone, Neil (2006). "Skky notes for April and May". Journal of the British Astronomical Association. 116 (2): 102. Bibcode:2006JBAA..116..102B.
  16. ↑ Alexander von Humboldt (1850). Humboldts Kosmos: In verkürzter Gestalt. Greiner und Pfeiffer.
  17. 1 2 Berman, Louis (1935). "The Spectrum Analysis of the Hot Carbon Star, R Coronae Borealis". The Astrophysical Journal. 81: 369. Bibcode:1935ApJ....81..369B. doi:10.1086/143644.
  18. ↑ Skuljan, L.; Cottrell, P. L. (2002). "Recent declines of RS Telescopii, UW Centauri, and V Coronae Australis". The Observatory. 122: 322. Bibcode:2002Obs...122..322S.
  19. 1 2 Watson, C. L. (2006). "The International Variable Star Index (VSX)". The Society for Astronomical Sciences 25th Annual Symposium on Telescope Science. Held May 23–25. 25: 47. Bibcode:2006SASS...25...47W.
  20. 1 2 3 4 5 Howell, Steve B.; Rector, Travis A.; Walter, Donald (2013). "Optical Spectroscopy at Deep Light Minimum of R Coronae Borealis". Publications of the Astronomical Society of the Pacific. 125 (930): 879–888. Bibcode:2013PASP..125..879H. doi: 10.1086/672163 . S2CID   120129277.
  21. ↑ Searle, Leonard (1961). "An Abundance Analysis of R Coronae Borealis". Astrophysical Journal. 133: 531. Bibcode:1961ApJ...133..531S. doi:10.1086/147056.
  22. 1 2 3 Clayton, Geoffrey C.; Sugerman, Ben E. K.; Adam Stanford, S.; Whitney, B. A.; Honor, J.; Babler, B.; Barlow, M. J.; Gordon, K. D.; Andrews, J. E.; Geballe, T. R.; Bond, Howard E.; De Marco, O.; Lawson, W. A.; Sibthorpe, B.; Olofsson, G.; Polehampton, E.; Gomez, H. L.; Matsuura, M.; Hargrave, P. C.; Ivison, R. J.; Wesson, R.; Leeks, S. J.; Swinyard, B. M.; Lim, T. L. (2011). "The Circumstellar Environment of R Coronae Borealis: White Dwarf Merger or Final-Helium-Shell Flash?". The Astrophysical Journal. 743 (1): 44. arXiv: 1110.3235 . Bibcode:2011ApJ...743...44C. doi:10.1088/0004-637X/743/1/44. S2CID   119240518.
  23. ↑ Zhang, X.; Jeffery, C. S.; Chen, X.; Han, Z. (2014). "Post-merger evolution of carbon-oxygen + helium white dwarf binaries and the origin of R Coronae Borealis and extreme helium stars". Monthly Notices of the Royal Astronomical Society. 445 (1): 660–673. arXiv: 1408.5500 . Bibcode:2014MNRAS.445..660Z. doi:10.1093/mnras/stu1741. S2CID   118506564.
  24. ↑ Stasińska, G.; Szczerba, R.; Schmidt, M.; Siódmiak, N. (2006). "Post-AGB stars as testbeds of nucleosynthesis in AGB stars". Astronomy and Astrophysics. 450 (2): 701. arXiv: astro-ph/0601504 . Bibcode:2006A&A...450..701S. doi:10.1051/0004-6361:20053553. S2CID   12040452.
  25. ↑ Jeffers, S. V.; Min, M.; Waters, L. B. F. M.; Canovas, H.; Rodenhuis, M.; De Juan Ovelar, M.; Chies-Santos, A. L.; Keller, C. U. (2012). "Direct imaging of a massive dust cloud around R Coronae Borealis". Astronomy & Astrophysics. 539 (A56): A56. arXiv: 1203.1265 . Bibcode:2012A&A...539A..56J. doi:10.1051/0004-6361/201117138. S2CID   55589182.
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