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

Spectra of several R Coronae Borealis (RCB) stars at maximum light were examined for the [C i] 9850Å and 8727Å absorption lines. The 9850Å line is variously blended with a Fe ii and CN lines but positive identifications of the [C i] line are made for RCrB and SUTau. The 8727Å 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] 9850Å line seen clearly in RCrB and SUTau 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-LTE calculations, and requires a substantial deposition of mechanical energy.