Dynamics of R Coronae Borealis Stars

R Coronae Borealis (RCB) stars are rare, hydrogen-deficient supergiants that undergo abrupt and irregular declines in optical brightness, sometimes exceeding eight magnitudes. These declines arise from the sudden condensation of carbon dust in their extended atmospheres, followed by gradual dust dispersal and recovery of stellar flux. The underlying mechanism is widely attributed to the merger of a helium white dwarf with a carbon–oxygen white dwarf, leading to a hydrogen-poor, carbon-rich envelope supported by helium shell burning. Post-merger evolution drives complex convective motions, pulsational instabilities and episodic mass ejection, which shape the circumstellar dust geometry. Photometric variations on timescales from days to decades reflect a combination of stellar pulsations, shock-driven dust-formation sites and stochastic clump ejections distributed around the star. Infrared and spectroscopic monitoring reveal a multi-component circumstellar environment comprising hot freshly formed dust, cooler extended shells and asymmetric structures. Studies of isotopic abundances—particularly oxygen isotopes—provide crucial tests of merger models and nucleosynthesis pathways. The interplay between pulsation-induced shocks, convective mixing and radiative pressure on newly formed grains governs the rate and distribution of dust production, while long-term photometric archives allow assessment of secular evolution. These dynamic processes offer unique laboratories for studying late-stage stellar evolution, binary interactions and dust formation under hydrogen-poor conditions.

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Dynamics of R Coronae Borealis Stars publication trend

The graph below shows the total number of articles in dynamics of r coronae borealis stars across all publications each year (not limited to Nature Index journals).

Technical terms

R Coronae Borealis star: A hydrogen-deficient, carbon-rich supergiant exhibiting irregular deep declines in optical brightness due to circumstellar carbon dust formation.

White dwarf merger: The coalescence of two degenerate stellar cores—typically a helium and a carbon–oxygen white dwarf—leading to a single, hydrogen-poor remnant.

Double-degenerate scenario: A formation hypothesis in which two white dwarfs in a binary system merge to produce an RCB star.

Dust condensation: The process of carbon grain nucleation and growth in the cool outer layers of an RCB star’s atmosphere, triggering photometric declines.

Pulsation: Radial or non-radial oscillations of the stellar envelope that can drive shock waves and facilitate dust-formation episodes.

Dredge-up: The mixing of nuclear-processed material from the stellar interior into the observable atmosphere, affecting surface abundances of elements and isotopes.

References

1. An Infrared Census of R Coronae Borealis Stars II—Spectroscopic Classifications and Implications for the Rate of Low-mass White Dwarf Mergers. Publications of the Astronomical Society of the Pacific (2024).
2. IRAS 00450+7401 and the Mid-infrared Fade/Burst Cycle of R Coronae Borealis-type Stars. The Astronomical Journal (2023).
3. Hydrodynamic simulations of white dwarf–white dwarf mergers and the origin of R Coronae Borealis stars. Monthly Notices of the Royal Astronomical Society (2024).
4. Very Large Telescope Interferometer observations of the dust geometry around R Coronae Borealis stars*. Monthly Notices of the Royal Astronomical Society (2011).
5. R Coronae Borealis and dustless hydrogen-deficient carbon stars likely have different oxygen isotope ratios⋆. Astronomy & Astrophysics (2022).
6. Light curves for 10 R Coronae Borealis stars for longer than a century: secular evolution, dip statistics, and a general model for the shape of isolated light-curve dips. Monthly Notices of the Royal Astronomical Society (2023).
7. Improved Models of R Coronae Borealis Stars. The Astrophysical Journal (2022).