Lithium in Wide Binaries: Effective Temperature Governs Depletion while Rotation Plays a Minor Role

Abstract

Using a sample of 116 wide binary systems as coeval and chemically homogeneous stellar pairs, we investigate the factors governing lithium depletion in main-sequence stars. We recover the well-established morphology of the lithium--effective temperature ($T_{\mathrm{eff}}$) relation, including the Li dip (6200--6600\,K), the Li plateau (6000--6200\,K), and a linear trend for cooler stars ($T_{\mathrm{eff}}$ $<$ 6000\,K), where lithium abundance increases by $\sim$0.15\,dex per 100\,K. We demonstrate that the apparent correlation between projected rotational velocity ($v\sin i$) and lithium abundance is secondary to the underlying $T_{\mathrm{eff}}$ dependence; $v\sin i$ is not an independent driver of lithium depletion in our sample. Notably, we identify an anomalous system within the Li dip where the primary star exhibits a $\sim$1.4\,dex lithium excess compared to its secondary companion at nearly identical $T_{\mathrm{eff}}$. We discuss two plausible origins for this anomaly: external enrichment via planetesimal accretion or planetary engulfment, and binary interactions with an unresolved tertiary companion. Our results confirm $T_{\mathrm{eff}}$ as the dominant parameter controlling lithium depletion, while highlighting that additional, non-standard processes can occasionally produce significant lithium enrichment.

Figures (11)

• Figure 1: Left panel: An example for A(Li) measurement. The black points are the observed spectra, and the red line is the best fit synthetic spectra with A(Li)=2.85 dex. The gray dashed and dotted lines are the uncertainties of the fitting with A(Li)=2.75 dex and 2.95 dex, respectively. Right panel: The abscissa represents the Li abundance determined by us, while the ordinate represents the Li abundance provided by GALAH DR3 (blue points) or GALAH DR4 (red points) .
• Figure 2: Upper: The 1D NLTE A(Li) – $T_{\rm eff}$ pattern for 116 pair WBs. Each binary pair is connected by a black dashed lines. Component stars are marked by filled circles which scaled in size according to their $v\sin i$ values, and color-coded by their [Fe/H], spanning the range of $\langle\rm[Fe/H]\rangle \pm 1.5\sigma$ (approximately $-0.18 \pm 0.32$ dex). The inset zooms in on binary pairs where both components have $T_{\mathrm{eff}}$ between 5700 K and 6200 K. Bottom: $\Delta$A(Li) as a function of $\Delta$$T_{\mathrm{eff}}$ between the primary and the secondary of each pair binary. The symbols are colored by the $T_{\mathrm{eff}}$ of the secondary stars. The symbols size are coded by the $|\Delta$$v\sin i|$ between the primary and secondary stars. The dashed black line represents a linear fit to the filled points (cool stars only), excluding symbols for Li dip binaries (inverted triangles), Li plateau binaries (filled squares), and systems where only the secondary is a cool star (crosses). The green dished box includes the binary pairs with negligible $T_{\mathrm{eff}}$ differences ($\Delta$$T_{\mathrm{eff}}$$<100$ K). The black asterisk marks the median value of A(Li) in the box.
• Figure 3: The Color-Magnitude Diagram (CMD) pattern of the anomalous systems (e.g., WB01, WB02, WB03 and WB04) that color-coded by their A(Li) and connected by the dashed gray lines for the primary and secondary stars. The black solid line represents the 1 Gyr MS isochrone, and the dashed and dashed-dot black lines are the isochrones for unresolved binaries with q = 0.7 and 1.0, respectively. The q represent the mass ratio of an unresolved binary.
• Figure 4: The Coordinates, Astrometry, Atmospheric Parameters and Li Abundances of the Binaries
• Figure 5: The Coordinates, Astrometry, Atmospheric Parameters and Li Abundances of the Binaries