R Coronae Borealis
Epoch J2000.0 Equinox J2000.0
|Right ascension||15h 48m 34.4149s|
|Declination||+28° 09′ 24.296″|
|Apparent magnitude (V)||5.71 (var)|
|U−B color index||0.13|
|B−V color index||0.60|
|V−R color index||0.45|
|J−H color index||0.275|
|J−K color index||0.800|
|Variable type||R CrB|
|Radial velocity (Rv)||27.83 km/s|
|Proper motion (μ)||RA: -2.10 mas/yr
Dec.: −11.52 mas/yr
|Parallax (π)||0.04 ± 0.37 mas|
|Absolute magnitude (MV)||−5|
|Surface gravity (log g)||0.5 cgs|
R Coronae Borealis is a peculiar low-mass yellow supergiant star, and is the prototype of the rare RCB 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, in the constellation of Corona Borealis, but at intervals of several months to many years fades to as faint as magnitude 14. Over successive months it gradually returns to its normal brightness, giving it the nickname "Fade-Out star," or "Reverse Nova".
The variability of R CrB was discovered by the English astronomer Edward Pigott in 1795. It was known as Variabilis Coronae, "Variable of Corona". In 1935 it was the first star shown to have a different chemical composition than the sun via spectral analysis.
R Coronae Borealis is a variable star and the prototype of its class. It is one of only two R Coronae Borealis variables bright enough to be seen with the naked eye, along with RY Sagittarii. 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☉.
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. 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. This irregular fading is the defining characteristic of the R Coronae Borealis variables and is thought to be due to the condensation of carbon into "soot" surrounding the star.
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 the deep five year minimum of 1962-7. However, R Coronae Borealis then faded again to near 15th magnitude and as of August 2014 it had been below 10th magnitude for 7 years. In late 2014, R Coronae Borealis brightened quickly to 7th magnitude but then began to fade again. As of May 2015 it is around 8th magnitude.
R Coronae Borealis at maximum light shows the spectrum clearly of 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 as expected.
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.
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.
Modelling the pulsations suggests that the mass of R Coronae Borealis 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 1.6 kiloparsecs. The luminosity is estimated from helium star models to be 19,000 L☉ and the star has a radius around 100 R☉. The absolute magnitude of −5 is consistent with R CrB variables in the Large Magellanic Cloud whose distances are known quite accurately.
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. 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. Evolutionary models of post-AGB stars give a mass of 0.66 M☉ for R CrB, but with a considerable margin of error.
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 dust, 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.
Direct imaging with the Hubble Space Telescope shows extensive dust clouds out to a radius of around 2000 astronomical units from the star, corresponding with a stream of fine dust (composed of grains 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. The obscuration appears to happen closer to the star as clouds of carbon condense at shock regions in an expanding front. A shell about 4 pc wide containing 2 M☉ of dust at 25 K has been detected around R CrB.
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-  AAVSO Variable Star of the Season, January 2000