Barium Stars Across the Milky Way: Probing Their Origins via the GALAH Survey

Abstract

Barium stars are unusually enriched in barium ([Ba/Fe] >= 1.0 dex) and not predicted by current Galactic chemical evolution models. Previous observations of barium stars have found evidence that they form through mass transfer from a companion asymptotic giant branch (AGB) star or through radiative levitation. The chemical abundance and kinematic information of barium stars may help constrain AGB stellar nucleosynthesis, binary star evolution, and internal evolutionary processes that affect surface abundances. Using ~450,000 stars from the GALactic Archaeology with Hermes (GALAH) survey, we identify nearly 3000 new barium-rich stars and separate them into hot (Teff > 6000 K) and cool (Teff < 6000 K) populations. Cross-matching with Gaia DR3, we find that 47.7% of our barium stars within 1 kpc have elevated re-normalized unit weight error (RUWE >= 1.4), compared to 16.3% of a comparable sample of the GALAH field, suggesting multiplicity plays an important role in the formation of both populations of barium stars. A subset of hot barium stars exhibit low RUWE (RUWE < 1.2) and [alpha/Fe] < -0.2, supporting radiative levitation as an origin as well. We determine Galactic memberships using both kinematics and chemistry and find that barium stars exist in the thin disk, thick disk, and halo though they are slightly more prevalent at lower metallicities. Overall, we show evidence for barium stars produced by mass transfer and for those produced by radiative levitation, with both formation mechanisms occurring ubiquitously across the Galaxy.

Figures (12)

• Figure 1: Left: [Ba/Fe] versus [Fe/H] for our sample (see Section \ref{['subsec:using data']}) of the GALAH field. Stars in the shaded region are deemed barium stars and have $\text{[Ba/Fe]} \ge 1$. Right: Spectral comparisons for barium-enhanced (solid line) and barium-normal (dashed line) stars for a K giant, G dwarf, G giant, and F dwarf from top to bottom respectively. The barium stars and the comparison GALAH stars are chosen such that they have similar temperatures, surface gravity, and metallicities. This figure confirms the existence of a population of stars in the GALAH survey that are barium-rich and qualitatively confirms their Ba enhancement spectroscopically.
• Figure 2: $T_{\text{eff}}$ distributions within 1 kpc for barium stars (purple) and the non-barium-enhanced GALAH field (dashed black). The histograms are normalized to integrate to unity. Barium stars tend to be hotter on average relative to their non-barium-enhanced counterparts. The underrepresentation of K and G type barium stars could be explained by mass transfer (See Section \ref{['discuss:masstrans']}).
• Figure 3: Normalized RUWE distributions for nearby ($\le$ 1 kpc) "hot" (top panel, $T_{\text{eff}}$ > 6000 K) and "cool" (bottom panel, $T_{\text{eff}}$$<$ 6000 K) barium stars with a dashed black line marking $\text{RUWE} = 1.4$, the value above which stars tend to be in binary systems (see Section \ref{['subsec:gaiaandruwe']}). The GALAH field subsamples in both panels do not include barium-rich stars and are selected to have the same $T_{\text{eff}}$ ranges as the barium stars. Both panels show an excess of barium stars with RUWE values greater than 1.4 relative to the field. This supports the mass transfer formation theory for both hot and cool barium star populations.
• Figure 4: [X/Fe] distributions of $s$-process elements for barium-rich stars (purple) and the GALAH field with barium-rich stars removed (black). Barium stars tend to be richer in $s$-process elements than the background field. This implies that processes which foster barium enhancement also lead to enrichment in the other $s$-process elements. Y shows the strongest and most distinct correlation to barium abundance among the $s$-process elements. This motivates our exploration of Ba and Y abundances in Fig. \ref{['fig:ba_y_ruwe']} and Fig. \ref{['fig:meshgrid']}.
• Figure 5: Equivalent distributions as in Figure \ref{['fig:sprocessspectra']} but for [C/Fe] in dwarfs (left), all stars (middle), and giants (right). We note that barium dwarfs show significant enhancements in [C/Fe] ($+0.29\pm0.01$ dex on average) relative to the field sample while barium giants do not ($+0.02\pm0.01$ dex).