The R Coronae Borealis stars - carbon abundances from forbidden carbon lines

Abstract

Spectra of several R Coronae Borealis (RCB) stars at maximum light were examined for the [C I] 9850 A and 8727 A absorption lines. The 9850 A line is variously blended with a Fe II and CN lines but positive identifications of the [C I] line are made for R CrB and SU Tau. The 8727 A line is detected in the spectrum of the five stars observed in this wavelength region. Carbon abundances are derived from the [C I] lines using the model atmospheres and atmospheric parameters used by Asplund et al. (2000). Although the observed strength of a C I line is constant from cool to hot RCB stars, the strength is weaker than predicted by an amount equivalent to a factor of four reduction of a line's gf-value. Asplund et al. dubbed this 'the carbon problem' and discussed possible solutions. The [C I] 9850 A line seen clearly in R CrB and SU Tau confirms the magnitude of the carbon problem revealed by the C I lines. The [C I] 8727 A line measured in five stars shows an enhanced carbon problem. The gf-value required to fit the observed [C I] 8727 A line is a factor of 15 less than the well-determined theoretical gf-value. We suggest that the carbon problem for all lines may be alleviated to some extent by a chromospheric-like temperature rise in these stars. The rise far exceeds that predicted by our non-LTE calculations, and requires a substantial deposition of mechanical energy.

arXiv comment11 pages (embedded 5 figures and 3 tables), accepted for publication in MNRAS

References in corpus (3)

• A Reappraisal of the Solar Photospheric C/O Ratio
• The 1995-1996 Decline of R Coronae Borealis - High Resolution Optical Spectroscopy
• The circumstellar shell of the post-AGB star HD 56126: the ${}^{12}$CN/${}^{13}$CN isotope ratio and fractionation

Cited by in corpus (12)

• Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars
• A Widely-Separated, Highly-Occluded Companion to the Nearby Low-Mass T Tauri Star TWA 30
• Fluorine in R Coronae Borealis Stars
• On the CO Near-IR Band and the Line Splitting Phenomenon in the Yellow Hypergiant Rho Cassiopeiae
• Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies
• The Galactic R Coronae Borealis stars : the C2 Swan bands, the carbon problem, and the 12C/13C ratio
• Gas Phase diagnostics of Protoplanetary disk extension
• Carbon Abundances of Three Carbon-Enhanced Metal-Poor Stars from High-Resolution Gemini-S/bHROS Spectra of the 8727A [C I] Line
• Abundance analyses of the new R Coronae Borealis stars: ASAS-RCB-8 and ASAS-RCB-10
• Do Hydrogen-deficient Carbon Stars have Winds?
• Revised surface abundances of R Coronae Borealis stars
• High resolution spectroscopy of the relatively hydrogen-poor metal rich giants in the globular cluster Omega Centauri