Nature of heavy-element abundance outliers in M67

Determine the nature and cause of the outlier heavy-element abundance measurements for [Zr/Fe], [Ce/Fe], [Pr/Fe], [Sm/Fe], and [Eu/Fe] that appear significantly lower than the cluster mean among M67 member stars in the PhDLspec analysis of LAMOST low-resolution spectra, and explain why these stars exhibit abnormally low values with large measurement uncertainties.

Background

After two-step external calibration, the authors validate elemental abundances for M67 open cluster member stars. While most elements show flat [X/Fe] trends versus log g with small scatter, they note a substantial subset of stars with anomalously low heavy-element abundances (e.g., Zr, Ce, Pr, Sm, Eu).

These outliers also show larger reported measurement errors. Because open clusters are typically chemically homogeneous within small dispersions, the presence of such low-abundance outliers raises the unresolved issue of whether these measurements reflect astrophysical variations, data reduction issues, or modeling limitations in low-resolution spectra analyzed by PhDLspec.

References

Nevertheless, Figure~\ref{fig:m67_cali} shows some substantial portion of outliers whose abundance ratios are significantly lower than the average value for some heavy elements, such as Zr, Ce, Pr, Sm, and Eu. The nature of these outlier measurements is unclear, but we mention that they have abnormally low values and much larger measurement errors than typical values.

— PhDLspec: physical-prior embedded deep learning method for spectroscopic determination of stellar labels in high-dimensional parameter space  (2604.02730 - Wu et al., 3 Apr 2026) in Section 4.3 (Validation with M67 member stars)