Definition
R Coronae Borealis (R CrB) is a prototype of a rare class of hydrogen‑deficient, carbon‑rich supergiant variable stars known as R Coronae Borealis variables. These stars exhibit irregular, dramatic declines in visual brightness caused by the rapid formation of carbon dust clouds that obscure their photospheres.
Overview
Located in the constellation Corona Borealis at a distance of approximately 1,400 parsecs (4,600 light‑years) from the Sun, R CrB has an apparent magnitude that typically varies between 5.8 and 6.0 but can plunge to fainter than magnitude 15 during dimming episodes. The star was first documented to undergo such declines in the late 18th century, and its unusual behavior led to the identification of a distinct variability class. R CrB is a luminous, evolved star thought to be in a short‑lived post‑asymptotic‑giant‑branch (post‑AGB) or post‑helium‑shell‑flash evolutionary stage.
Etymology/Origin
The designation “R Coronae Borealis” follows the traditional variable‑star naming convention, where the letter “R” indicates it was the first variable star discovered in the constellation Corona Borealis. The Latin “Corona Borealis” means “Northern Crown,” referring to the constellation’s shape.
Characteristics
| Property | Details |
|---|---|
| Spectral type | Typically listed as F‑G Ia e (hydrogen‑deficient, carbon‑rich) during quiescence. |
| Variability | Irregular declines of 6–9 magnitudes over weeks, followed by a gradual return to normal brightness over months to years. The declines are caused by condensation of amorphous carbon dust in the stellar wind. |
| Effective temperature | Approximately 6,500 K in the visual maximum, decreasing modestly during dimming due to dust obscuration. |
| Luminosity | Roughly 10,000 L☉, consistent with a supergiant status. |
| Mass | Estimated to be 0.8–1.0 M☉, typical for post‑AGB objects. |
| Chemical composition | Extremely hydrogen‑deficient (hydrogen abundance < 1 % of solar) and enriched in carbon, nitrogen, and sometimes s‑process elements. |
| Infrared excess | Strong infrared emission from circumstellar dust, detectable with IR telescopes (e.g., Spitzer, Herschel). |
| Pulsations | Small, semi‑regular radial pulsations with periods of ~40–50 days, detectable in radial‑velocity and photometric data. |
| Evolutionary scenario | Two principal models: (1) a final helium‑shell flash (late thermal pulse) in a post‑AGB star, or (2) the merger of a helium white dwarf with a carbon‑oxygen white dwarf. Both pathways can produce the observed hydrogen deficiency and carbon enrichment. |
Related Topics
- R Coronae Borealis variables – the class of hydrogen‑deficient, carbon‑rich variable stars named after R CrB.
- Hydrogen‑deficient carbon (HdC) stars – a broader group that includes R CrB variables and related objects lacking hydrogen.
- Final helium‑shell flash – a late thermal pulse event that can expel surface hydrogen and alter stellar surface composition.
- White dwarf merger – a proposed formation channel for some hydrogen‑deficient stars involving the coalescence of two white dwarfs.
- Circumstellar dust formation – the process by which carbon‑rich stellar winds condense into opaque dust clouds, leading to the characteristic dimming of R CrB stars.
- Variable star nomenclature – the systematic naming convention for variable stars based on constellation and discovery order.
References for further reading include peer‑reviewed articles on R Coronae Borealis and its class in journals such as Astronomy & Astrophysics, The Astrophysical Journal, and Monthly Notices of the Royal Astronomical Society.