Vv. Smith et al., ABUNDANCES IN THE SYMBIOTIC STAR AG DRACONIS - THE BARIUM-SYMBIOTIC CONNECTION, Astronomy and astrophysics, 315(1), 1996, pp. 179-193
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.