RCB R Coronae Borealis stars

- These are hydrogen-poor, carbon- and helium-rich, high-luminosity stars belonging to the spectral types Bpe-R, which are simultaneously eruptive and pulsating variables. They show slow non-periodic fadings by lm-9"' in V lasting from a month or more to several hundred days. These changes are superposed on cyclic pulsations with amplitudes up to several tenths of a magnitude and periods in the range of 30d-l00d. GCVS

Variables of the R Coronae Borealis type are hydrogen-poor, carbon- and helium-rich, high-luminosity stars belonging to the spectral types Bpe-R and are distinguished from other hydrogen-deficient objects by their spectacular dust-formation episodes. These stars are simultaneously eruptive and pulsating variables and exhibit some of the most spectacular behavior of any variable star.

They are apparently of low mass yet they possess high luminosity and at irregular intervals are known to manufacture thick dust clouds that can completely obscure the photosphere of the star. These changes are superposed on cyclic pulsations with amplitudes up to several tenths of a magnitude and with periods in the range 30-100 days (Figure 3.4).

The spectral class "R" is introduced here so a short explanation is needed. Within the Harvard spectral system, these stars are known as carbon stars. Carbon stars are late giant stars with strong bands of carbon

'"The Northern Crown, Corona Borealis, represents a crown give by Dionysus to Ariadne, the daughter of Minos of Crete.

of the prototype R~ type variable star, R Crb. Data provided by VSNET. Used with permission.

of the prototype R~ type variable star, R Crb. Data provided by VSNET. Used with permission.

compounds and no metallic oxide bands. There are actually two spectral types known as carbon stars: R-type, similar to G5-K0 stars and displaying strong carbon spectral bands; and N-type that displays even stronger carbon spectral bands. The major difference between R- and N-type stars is the presence of the elements carbon and oxygen. The R-type stars are divided into decimal subtypes, whereas the N stars are divided using subtypes a, b and c. Today, carbon stars are usually classified as C-type stars with R-type stars known as early (hotter) C-type and N-type stars known as late (cooler) C-type stars. The temperature sequence for carbon stars varies from approximately G4 through M4 and it is well known that many C stars are variable.

The RCB stars are thought to be the product of a final helium shell flash or the coalescence of a binary white-dwarf system. These stars are interesting and important, first because they represent a rare, or short-lived stage of stellar evolution, and second because these stars regularly produce large amounts of dust and so they serve as laboratories for the study of dust formation and evolution (Figure 3.5).

Hydrogen-deficient stars also include the extreme helium (EHe) stars and the hydrogen-deficient carbon (HdC) stars. These stars are all supergiants ranging from B- to G-type with very little hydrogen found within their atmospheres. The HdC stars are distinguished from RCB stars by the absence of large-scale variability and 1R excesses. IR excess is a phenomenon in which long-wavelength energy, that is infrared radiation, is detected at a level greater than can be simply explained as being produced by the star in question. Usually, the excess infrared radiation is accounted for by understanding that conception of an kcc lype variable star, illustrating the surrounding cloud of material responsible for obscuring the star's light. Copyright: Gerry A. Good

short-wave energy is absorbed by some material and then re-emitted as a long wave. The EHe stars are hotter and with the exception of three RCB-like stars, do not show large-scale variability.

Despite their eye-catching light curves and spectacular behavior, the number of RCB variables is small. The GCVS lists about 40 with 14 uncertain but several other sources indicate as many as 45 RCB variables may be known.

Observation Key

Bright stars /v^ Smoll amplitudes i i- Mixed periods <3> Visual, CCD/PEP

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