Let Sleeping Stars Li: Mapping Turnoff Lithium, the Lithium Dip, & Testing Standard Model Predictions in GALAH DR4 Subgiants

Lithium is an excellent tracer of stellar mixing. We reconstruct the end of the main sequence lithium depletion pattern using the GALAH DR4 3D-NLTE lithium dataset for 22,442 stars, complemented by masses and subgiant lithium dilution factors inferred from YREC stellar models. We recover the two canonical features seen in clusters: a cool plateau (${\mathrm{M}}_{[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}{]}_{\odot }}\sim 1.2{\mathrm{M}}_{\odot }$) with a median $A(Li)=2.42$, and a steep plunge into the lithium dip (${\mathrm{M}}_{[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}{]}_{\odot }}\sim 1.35{\mathrm{M}}_{\odot }$) dominated by upper limits at $A(Li)<1.5$. Above the dip, our data reveal a striking new pattern with a wide range of abundances. We argue this reflects severe rotationally induced mixing in most stars, with lithium preserved in a minority where gravitational settling suppresses mixing. Lithium depletion with intrinsic dispersion is universal across the sample. Predicted and observed dilution trends agree well below the dip. For higher-mass stars, dilution corrections agree with standard models, but our upper envelope lies below open cluster values. We attribute this to a shallower lithium preservation zone and further evidence for gravitational settling and radiative levitation. We detect lithium in $\approx 25\mathrm{\%}$ of dip stars, whereas clusters show none; neither blurring of a narrow mass feature nor binary contamination fully explains this. The cool plateau is 0.8 dex below the solar meteoritic value across a wide range of $[Fe\mathrm{/}H]$. After accounting for pre-main sequence depletion, the plateau is elevated at $\ge [Fe\mathrm{/}H{]}_{\odot }$, possibly evidence for lithium production.