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

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

2004-05-29

arxiv.org/abs/astro-ph/0405600v1

Mon.Not.Roy.Astron.Soc. 353 (2004) 143

Astronomy and Astrophysics

Astrophysics

11 pages (embedded 5 figures and 3 tables), accepted for publication in MNRAS

Scientific paper

10.1111/j.1365-2966.2004.08057.x

Spectra of several R Coronae Borealis (RCB) stars at maximum light were examined for the [C I] 9850 A and 8727 A absorption lines. The 9850 A line is variously blended with a Fe II and CN lines but positive identifications of the [C I] line are made for R CrB and SU Tau. The 8727 A 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] 9850 A 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 A line measured in five stars shows an enhanced carbon problem. The gf-value required to fit the observed [C I] 8727 A 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.

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