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BD - 21° 3873: another yellow-symbiotic barium star *
V.V. Smith 1, 2,
K. Cunha 3,
** 4 and
H.M.J. Boffin 5
Received 9 December 1996 / Accepted 20 January 1997
An abundance analysis of the yellow symbiotic system reveals it to be a metal-poor K-giant ([Fe/H] = -1.3) which is enriched in the heavy s-process elements. In that respect, this star appears to be a twin of AG Dra, another yellow symbiotic system analyzed in a previous paper (Smith et al., 1996). The heavy-element abundance distributions of AG Dra and are almost identical, and are best reproduced by a s-process with a neutron exposure parameter of 1.2 - 1.3 mb-1 and a neutron density (as derived from the Rb/Zr ratio). These two systems thus link the symbiotic stars to the binary barium and CH stars which are also s-process enriched. These binary systems, which exhibit overabundances of the heavy elements, owe their abundance peculiarities to mass transfer from thermally-pulsing asymptotic giant branch stars, which have since evolved to become white-dwarf companions of the cool stars we now view as the chemically-peculiar primaries.
The spectroscopic orbits of (derived from CORAVEL measurements) and AG Dra are similar to those of barium and CH stars. With an orbital period of 281.6 d, is one of the closest systems in these families, and its light curve indeed suggests that variations due to reflection and ellipticity effects are present. The amplitude of the ellipsoidal variations indicates that the giant must be close to filling its Roche lobe. However, no acceptable solution simultaneously satisfies the constraints from the light curve, the orbital elements and the evolutionary tracks in the framework of the standard Roche lobe geometry. We suggest that this discrepancy may be resolved by taking into account the deformation of the Roche lobe caused by the force driving the large mass loss of the giant.
Key words: binaries: symbiotic stars: individual: BD - 21O 3873 stars: abundances stars: peculiar
* Based in part on observations carried out at the European Southern Observatory (La Silla, Chile), and the Swiss telescope at the Haute-Provence Observatory (France)
Present address: Dept. of Physics and Astronomy, University of Wales, Cardiff, CF2 3YB, United Kingdom
Send offprint requests to: A. Jorissen
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998