Classical barium stars are binary systems which consist of a late-type giant enriched in carbon and slow neutron capture ( s-process ) elements and an evolved white dwarf ( WD ) that is invisible at optical wavelengths .
The youngest observed barium stars are surrounded by planetary nebulae ( PNe ) , ejected soon after the wind accretion of polluted material when the WD was in its preceeding asymptotic giant branch ( AGB ) phase .
Such systems are rare but powerful laboratories for studying AGB nucleosynthesis as we can measure the chemical abundances of both the polluted star and the nebula ejected by the polluter .
Here we present evidence for a barium star in the PN Hen 2-39 ( PN G283.8 - 04.2 ) as one of only a few known systems .
The polluted giant is very similar to that found in WeBo 1 ( PN G135.6 + 01.0 ) .
It is a cool ( T _ { \mathrm { eff } } = 4250 \pm 150 K ) giant enhanced in carbon ( [ C/H ] = 0.42 \pm 0.02 dex ) and barium ( [ Ba/Fe ] = 1.50 \pm 0.25 dex ) .
A spectral type of C-R3 C _ { 2 } 4 nominally places Hen 2-39 amongst the peculiar early R-type carbon stars , however the barium enhancement and likely binary status mean that it is more likely to be a barium star with similar properties , rather than a true member of this class .
An AGB star model of initial mass 1.8 M _ { \odot } and a relatively large carbon pocket size can reproduce the observed abundances well , provided mass is transferred in a highly conservative way from the AGB star to the polluted star ( e.g .
wind Roche-lobe overflow ) .
It also shows signs of chromospheric activity and photometric variability with a possible rotation period of \sim 5.5 days likely induced by wind accretion .
The nebula exhibits an apparent ring morphology in keeping with the other PNe around barium stars ( WeBo 1 and A 70 ) and shows a high degree of ionization implying the presence of an invisible hot pre-WD companion that will require confirmation with UV observations .
In contrast to A 70 , the nebular chemical abundance pattern is consistent with non-Type I PNe , in keeping with the trend found from nebular s-process studies that non-Type I PNe are more likely to be s-process enhanced .