Gaia Barium Dwarfs and Their Ostensibly Ordinary Counterparts

TL;DR

Gaia DR3 reveals a population of MS+WD binaries at separations around ${\sim}1~{\rm AU}$ with notable eccentricities that challenge standard binary evolution. By combining high-resolution FEROS spectroscopy of 30 Gaia DR3 MS+WD systems with GALAH data, the study identifies ${\sim}10$ Ba-enriched stars and builds a Ba-dwarf sample of ${\sim}38$ with dynamically constrained WD masses, showing Ba and Y enrichment linked to WD mass and metallicity. Ba enrichment persists in systems with eccentricities up to ${e}\approx0.4$, implying mass transfer in the late- and post-AGB phases can occur without fully erasing eccentricity, though enrichment is not universal among high-$e$ systems. The Gaia AMRF MS+WD binaries appear to be the progenitors of Ba dwarfs, extending the Ba-star population in parameter space and offering an empirical path to constrain the formation and evolution of mass-transfer binaries, with a concrete follow-up plan using the upcoming MAST/HighSpec survey for ~1000 targets.

Abstract

The recently identified Gaia population of main-sequence--white dwarf (MS+WD) binaries at separations of ${\sim}\,1~{\rm AU}$, often with moderate eccentricities, is not readily reproduced by binary population synthesis models. Barium 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-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 barium star population. To achieve this, we used high-resolution FEROS spectroscopy to measure abundances for 30 Gaia DR3 non-single-star binaries, identifying 10 as Ba-enriched. Together with our recent analysis of archival GALAH data, this yields a sample of 38 barium 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, barium 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 barium 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.

Figures (10)

• Figure 1: Eccentricity-M$_\text{WD}$ diagram of the FEROS sample (blue). The sample includes 21 systems with $e>0.2$ and M$_\text{WD}$$<0.65$$M_\odot$, complemented with a comparison sample of 12 systems with $e<0.1$ and M$_\text{WD}$$>0.75$$M_\odot$. These systems probe regions of parameter space largely unexplored by s-process-enhanced binaries in the GALAH sample of Rekhi_2024_BaEnrichment (orange). The Shahaf_2024_TriageGaia sample is shown in grey for reference.
• Figure 2: [Ba/Fe] as a function of (a) eccentricity and (b) M$_\text{WD}$, colored by [Fe/H]. The GALAH sample from Rekhi_2024_BaEnrichment is also plotted for reference. The green dashed line denotes the boundary beyond which Ba is enhanced. We detect 8 Ba-enhanced systems at $e>0.2$, doubling the Rekhi_2024_BaEnrichment eccentric s-process-enriched sample. However, an additional 6-8 eccentric systems having commensurate metallicities are not s-process enriched, indicating unknown factors influencing mass-transfer in eccentric systems. We also identify 6 Ba-enhanced systems hosting massive WDs (M$_\text{WD}$$\in [0.75,0.1]$$M_\odot$) at low eccentricities, demonstrating that enrichment can occur even where s-process yields are expected to be suppressed due to their relatively high progenitor masses. [Y/Fe] plots are shown in Appendix \ref{['app:addn plots']}, demonstrating that Y and Ba enhancements occur in tandem.
• Figure 3: Period-Eccentricity diagram of the Shahaf_2024_TriageGaia AMRF sample (gray), its Ba-enriched subset from this work and Rekhi_2024_BaEnrichment (red) and Ba/CH/CEMP-s stars from literature Jorissen_2016_BinaryPropertiesEscorza_2019_BariumRelatedEscorza_2023_BariumRelated. Our samples uncover a significantly larger population of Ba dwarfs at $e$ > 0.1 at P < 1000 d, which is underrepresented in literature. The AMRF sample thus provides a systematic route to identifying large samples of Ba dwarfs with orbital parameters and companion WD masses from the Gaia NSS catalog.
• Figure 4: Primary and WD masses of the Ba star samples from Figure \ref{['fig:P e general']}. Systems with P > 1000 d are plotted as open circles. The WD companions of Ba dwarfs have masses around 0.6 $M_\odot$ in both samples, indicating progenitor core development similar to field WDs. The generally higher WD masses in the Ba giants sample likely reflect a detection bias, as these systems tend towards higher primary masses.
• Figure 5: Stacked distance histogram of the Ba star samples from Figure \ref{['fig:P e general']} having P < 1000 d. The AMRF method increases the volume probed for Ba Dwarfs by an order of magnitude.