Ba enrichment in Gaia MS+WD binaries: Tracing s-process element production

Barium abundance
Barium abundance as a function of iron abundance of the primary and mass of the secondary (which we assume is a WD). Median error bars are shown in gray. We note that enhanced Ba abundances are found almost exclusively in the region of M2 < 0.75 M⊙ and [Fe/H] < −0.08 (bounded in green). (Rekhi et al. 2024).

A large population of intermediate-separation binaries, consisting of a main-sequence (MS) star and a white dwarf (WD), recently emerged from Gaia's third data release (DR3), posing challenges to current models of binary evolution. Here we examine the s-process element abundances in these systems using data from GALAH DR3. Following refined sample analysis with parameter estimates based on GALAH spectra, we find a distinct domain where enhanced s-process elemental abundances depend on both the WD mass and metallicity, consistent with parameter spaces identified in previous asymptotic giant branch (AGB) nucleosynthesis studies having higher s-process yields. Notably, these enhanced abundances show no correlation with the systems' orbital parameters, supporting a history of accretion in intermediate-separation MS+WD systems. Consequently, our results form direct observational evidence of a connection between AGB masses and s-process yields. We conclude by showing that the GALAH DR3 survey includes numerous Ba dwarf stars, within and beyond the mass range covered in our current sample, which can further elucidate s-process element distributions in MS+WD binaries.

In a follow-up study, we used high-resolution Fiber-fed Extended Range Optical Spectrograph spectroscopy to measure abundances for 30 additional systems, identifying 14 as Ba-enriched. Together with the 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.

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