Emergence of a lithium dip in ~35 Myr "Snake" Open Clusters

TL;DR

This study shows that a lithium dip can emerge as early as $35\pm5$ Myr in the young, coeval Snake open cluster, contradicting the longstanding view that Li-dips form only in older populations. Using high-resolution spectra from GALAH DR4 and a dedicated spectral-synthesis approach, the authors derive Li abundances for $211$ Snake members with NLTE and LTE treatments and examine their dependence on $T_{ m eff}$ and $v\sin i$, revealing a dip in the $6200$–$6800$ K range with a depth of $ΔA({\rm Li}) \approx 0.40$ dex centered near $T_{ m eff} \sim 6500$ K. They also find a significant anti-correlation between rotation and Li within the dip, indicating faster rotators experience stronger Li depletion due to rotational shear enhancing turbulent mixing at the convective–radiative boundary. Additionally, the data show a lower-temperature edge of the Li plateau reaching $\sim 5500$ K in this young age, suggesting an age-dependent broadening of the plateau. Collectively, these results constrain angular momentum transport and mixing processes in early stellar evolution and motivate broader surveys to map Li–$T_{ m eff}$ relations across environments and ages.

Abstract

We report the discovery of a lithium dip (Li-dip) in the stellar "Snake" (age = $35 \pm 5$ Myr), challenging the classical view that Li-dips emerge only at ages $\gtrsim 150$ Myr. Using high-resolution spectra from GALAH DR4 ($R \sim 28,000$) for 211 member stars, we identify a clear depletion feature in a $T_{\mathrm{eff}}$ range of 6200--6800 K with a depth of $ΔA(\mathrm{Li}) \approx 0.40$ dex. Our analysis reveals two key advances: the Li-dip appears $\gtrsim 100$ Myr earlier than the previous observations, and within the dip temperature range, a significant correlation is found between rotational velocity and lithium depletion. Specifically, fast rotators ($v \sin i > 25$ km s$^{-1}$) exhibit stronger lithium depletion than slow rotators ($v \sin i < 25$ km s$^{-1}$). This trend suggests that faster rotators develop stronger rotational shear at the convective-radiative boundary, which enhances turbulent mixing and accelerates lithium destruction. It is also found that the lower temperature edge of the lithium plateau can reach as low as 5500 K for the young open clusters.

Figures (4)

• Figure 1: The best-fit 35 Myr isochrone for our stellar sample is displayed in the figure, with the 11,634 candidate member stars of the Snake highlighted in green. Among these, the 211 sources cross-matched with galah DR4 are marked by squares, which are colored according to their $T_{\mathrm{eff}}$. The mean values in three consecutive bins of $G_{\rm bp}$ - $G_{\rm rp}$, spanning 0.0 to 1.5 mag in steps of 0.5 mag, are 7069 K, 5914 K, and 5100 K, respectively.
• Figure 2: The comparison of our measured lithium abundances with those from galah DR4. Our values are systematically higher than those from galah pipeline, especially for the rapidly rotating stars ($v \sin i$). The color bar is coded by the values of $v \sin i$.
• Figure 3: $A\mathrm{(Li)}$ as a function of $T_{\mathrm{eff}}$. Top: a Li-dip is clearly present among the 211 Snake member stars in the $T_{\mathrm{eff}}$ range 6200--6800 K, with a depletion depth $\Delta A\mathrm{(Li)} \simeq 0.40$ dex. Points are colour-coded by the values of $v \sin i$; hotter stars generally rotate faster. Error bars represent the typical measurement uncertainty of $A\mathrm{(Li)} = 0.1$ dex. Bottom: comparison of the $A\mathrm{(Li)}$--$T_{\mathrm{eff}}$ distributions with those of other open clusters: the Pleiades Bouvier2018, M 48 Sun_2023, Praesepe and Hyades Cummings_2017, and NGC 188 Sun_2025.
• Figure A.1: An example of Li abundances determination. Purple points are the observed spectra, and the red line is the best NLTE fitting synthetic spectra which A(Li) is 3.24 dex, and the LTE fitting of same A(Li) is also provided with the green line. The gray dashed line is under-estimated Li abundance with A(Li)=3.14 dex, while the gray dotted line is over-estimated Li abundance with A(Li) = 3.34 dex.