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Astron. Astrophys. 318, 521-534 (1997)


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Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars

Martin Asplund 1, Bengt Gustafsson 1, Dan Kiselman 2, 3 and Kjell Eriksson 1

1 Astronomiska observatoriet, Box 515, S-751 20  Uppsala, Sweden
2 NORDITA, Blegdamsvej 17, DK-2100  Copenhagen O, Denmark
3 present address: Stockholms observatorium, S-133 36  Saltsjöbaden, Sweden
e-mail: martin@astro.uu.se; bg@astro.uu.se; dan@astro.su.se; ke@astro.uu.se

Received 1 May 1996 / Accepted 2 July 1996

Abstract

We have constructed line-blanketed model atmospheres for the hydrogen-deficient and carbon-rich R Coronae Borealis (R CrB) stars, as well as for the similar hydrogen-deficient carbon (HdC) stars and the cool extreme helium (EHe) stars. Improved continuum opacities have been used together with realistic line absorption data for atomic and molecular transitions. The observed dereddened fluxes of R CrB are compared with the calculated model fluxes and found to agree best with a model effective temperature of 6 900 K, while the infrared flux method gives between 6 600 and 6 900 K, depending on the nature of the flux excess in the J and H bands compared to the model fluxes. The excess may correspond to a recently formed dust cloud close to the star, with a typical temperature around 2 000 K and a dust mass of [FORMULA] [FORMULA]. The agreement for the ultraviolet flux distribution is also very satisfactory as seen from IUE spectra of R CrB. Theoretical broad band photometry is presented and effective temperatures of R CrB and HdC stars estimated.

The constructed models show a significantly steeper temperature gradient compared to previously existing models as a result of the line opacity. Due to the cool surface and high abundance of carbon, molecular bands of e.g. C2 and CO are visible in the spectra even at as high effective temperatures as 7 000 K. Furthermore, the high temperatures encountered at depth explain the observed He I and C II lines for [FORMULA] down to [FORMULA] K. In the inner layers ([FORMULA]) the models show density inversions related to the ionization zone of helium. For certain low gravity models the luminosity exceeds the local Eddington limit and hence gas pressure inversions occur as well, which could be related to the decline events of R CrB stars.

Key words: stars: model atmospheres – stars: AGB and post-AGB – stars: variables: R Coronae Borealis – stars: variables: luminous blue variables – radiative transfer – instabilities

Send offprint requests to: Martin Asplund

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998

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