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

Abstract

Spectra of several R Coronae Borealis (RCB) stars at maximum light have been examined for the [CI] 9850- and 8727-Åabsorption lines. The 9850-Åline is variously blended with an FeII and CN lines, but positive identifications of the [CI] line are made for RCrB and SUTau. The 8727-Åline is detected in the spectrum of the five stars observed in this wavelength region. Carbon abundances are derived from the [CI] lines using the model atmospheres and atmospheric parameters used by Asplund et al. Although the observed strength of a CI 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 [CI] 9850-Åline seen clearly in RCrB and SUTau confirms the magnitude of the carbon problem revealed by the CI lines. The [CI] 8727-Åline measured in five stars shows an enhanced carbon problem. The gf-value required to fit the observed [CI] 8727-Åline 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.