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.