C3PO. V. Comoving Pairs Indicate Rotational Spin-down Drives the Main-sequence Li-Dip

Abstract The lithium-dip observed in mid-F dwarfs remains a long-standing challenge for stellar evolution models. We present high-precision stellar parameters and A (Li) for 22 new comoving pairs, primarily located on the hotter side of the Li-Dip. Combined with pairs from the C3PO catalog, our sample includes 124 stars with T eff between 6000 and 7300 K, encompassing and extending slightly beyond the Li-Dip. Among them, 49 comoving pairs (98 stars) have both components within the temperature range of interest. Using this expanded set of comoving pairs observed with high-resolution spectroscopy, we show that rotational spin down is the dominant process responsible for Li depletion in the Li-Dip. First, within comoving pairs, the star with v sin i > 12 km s −1 shows higher A (Li) than its more slowly rotating companion within the Li-Dip, indicating that rotation-dependent mixing drives lithium depletion. Second, we observe a correlation between A (Li) and v sin i : fast rotators retain higher A (Li) with less scatter, while slow rotators show lower A (Li) and greater dispersion. Third, among slow rotators, A (Li) varies widely, suggesting that differences in initial rotation rates and spin-down histories influence how much Li is depleted. Some stars may have formed as fast rotators and spun down rapidly, leading to more Li depletion, while others may have started as slow rotators and retained more of their initial Li. These results demonstrate that rotational induced mixing plays an important role in shaping the Li-Dip beyond the effects of stellar age and mass.