Gaia Barium Dwarfs and Their Ostensibly Ordinary Counterparts

The recently identified Gaia population of main-sequence–white dwarf (MS+WD) binaries at separations of ∼1 au, often with moderate eccentricities, is not readily reproduced by binary population synthesis models. Barium (Ba) stars represent a closely related population whose enrichment in s-process elements both confirms the presence of a WD companion and attests to past binary interaction. It also indicates that mass transfer occurred at least during the late- and post–asymptotic giant branch (AGB) phases of the WD progenitor, when s-process elements are dredged up. In this work, we further explore the connection between the astrometrically identified Gaia MS+WD binaries and the classical Ba star population. To achieve this, we used high-resolution Fiber-fed Extended Range Optical Spectrograph spectroscopy to measure abundances for 30 Gaia Data Release 3 nonsingle-star binaries, identifying 14 as Ba-enriched. Together with our recent analysis of archival GALAH data, this yields a sample of 40 Ba dwarfs with dynamically measured WD masses, compared to only 6 previously known systems with known WD masses at these separations. We find that, in cases where metallicity is sufficiently low to facilitate efficient s-process production, Ba and yttrium enrichment is often detected. This enrichment is also identified in eccentric systems, suggesting that post-AGB mass transfer mechanisms are capable of pumping eccentricity into the orbit or occur without erasing it. Our results indicate that the Gaia MS+WD binaries trace the population from which Ba stars emerge. Treating the large Gaia-discovered population as an extension of known s-process enriched dwarfs opens an avenue to empirically constrain their formation and evolution.