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Title: The R Coronae Borealis stars: carbon abundances from forbidden carbon lines
Authors: Pandey, G
Lambert, D. L
Kameswara Rao, N
Gustafsson, B
Ryde, N
Yong, D
Keywords: Stars: Abundances;Stars: Atmospheres;Stars: Individual: RCRB;Stars: Individual: SUTau;Stars: Variables: Other
Issue Date: Sep-2004
Publisher: Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society, Vol. 353, No. 1, pp. 143 - 152
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.
ISSN: 0035-8711
Appears in Collections:IIAP Publications

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