ABUNDANCES IN THE SYMBIOTIC STAR AG DRACONIS - THE BARIUM-SYMBIOTIC CONNECTION

Citation
Vv. Smith et al., ABUNDANCES IN THE SYMBIOTIC STAR AG DRACONIS - THE BARIUM-SYMBIOTIC CONNECTION, Astronomy and astrophysics, 315(1), 1996, pp. 179-193
Citations number
108
Categorie Soggetti
Astronomy & Astrophysics
Journal title
ISSN journal
00046361
Volume
315
Issue
1
Year of publication
1996
Pages
179 - 193
Database
ISI
SICI code
0004-6361(1996)315:1<179:AITSSA>2.0.ZU;2-C
Abstract
An abundance analysis of the yellow symbiotic system AG Draconis revea ls it to be a metal-poor K-giant ([Fe/H] = -1.3) which is enriched in the heavy s-process elements. This star thus provides a link between t he symbiotic stars and 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 trans fer from thermally-pulsing asymptotic giant branch stars, which have s ince evolved to become white-dwarf companions of the cool stars we now view as the chemically peculiar primaries. A comparison of the heavy- element abundance distribution in AG Dra with theoretical nucleosynthe sis calculations shows that the s-process is defined by a relatively l arge neutron exposure (tau=1.3 mb(-1)), while an analysis of the rubid ium abundance suggests that the s-process occurred at a neutron densit y of about 2 10(8) cm(-3). The derived spectroscopic orbit of AG Dra i s similar to the orbits of barium and CH stars. Because the luminosity function of low-metallicity K giants is skewed towards higher luminos ities by about 2 magnitudes relative to solar-metallicity giants, it i s argued that the lower metallicity K giants have larger mass-loss rat es. It is this larger mass-loss rate that drives the symbiotic phenome na in AG Dra and we suggest that the other yellow symbiotic stars are probably low-metallicity objects as well.