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

Spectra of several R Coronae Borealis (RCB) stars at maximum light have been examined for the [C  i ] 9850‐ and 8727‐Å absorption lines. The 9850‐Å line is variously blended with an Fe  ii and CN lines, but positive identifications of the [C  i ] line are made for R CrB and SU Tau. The 8727‐Å 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. Although the observed strength of a C  i line is constant from cool to hot RCB stars, the strength is lower than predicted by an amount equivalent to a factor of 4 reduction of the gf ‐value of a line. Asplund et al. dubbed this ‘the carbon problem’ and discussed possible solutions. The [C  i ] 9850‐Å 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‐Å line measured in five stars shows an enhanced carbon problem. The gf ‐value required to fit the observed [C  i ] 8727‐Å 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‐local thermodynamic equilibrium calculations, and requires a substantial deposition of mechanical energy.