Open AccessThe R Coronae Borealis stars: carbon abundances from forbidden carbon lines
Author(s) -
Pandey Gajendra,
Lambert David L.,
Rao N. Kameswara,
Gustafsson Bengt,
Ryde Nils,
Yong David
Publication year - 2004
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2004.08057.x
Subject(s) - physics , stars , astrophysics , carbon star , spectral line , line (geometry) , carbon fibers , absorption spectroscopy , astronomical spectroscopy , astronomy , absorption (acoustics) , wavelength , emission spectrum , optics , geometry , mathematics , materials science , composite number , composite material
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.