Origin of distinct low-abundance populations in the subgiant sample

Ascertain the origin of the distinct stellar populations with exceptionally low [X/Fe] values for Zr, La, Ce, Pr, Nd, Sm, and Eu identified in the LAMOST subgiant sample analyzed with PhDLspec, and determine whether these populations are artifacts of the measurement process.

Background

Applying PhDLspec to 116,611 LAMOST subgiants, the authors recover expected chemical trends but also report distinct populations with unusually low abundances for some neutron-capture elements (e.g., Zr, La, Ce, Pr, Nd, Sm, Eu).

Such features could reflect real astrophysical substructures or systematic measurement effects in low-resolution spectra. The authors explicitly state that the origin of these populations is not yet clear and suggest they may be artifacts, leaving their true nature as an unresolved question.

References

For a subset of elements (Zr, La, Ce, Pr, Nd, Sm, and Eu), we identify distinct stellar populations exhibiting exceptionally low abundances compared to the bulk sample. The origin of these populations is not yet clear, but we anticipate that they may be artifacts of the measurement process.

— 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 5 (The LAMOST subgiant abundance catalog)

Above them, two elements stand out as exceptions. N exhibits unusually large dispersions compared to other light elements, and its scatter declines mildly with increasing age. Nd, however, shows exceptionally small intrinsic dispersions among heavy elements. The reason for these exceptional results remains unclear.

— Testing Chemical Tagging with LAMOST: Intrinsic Abundance Dispersion of Subgiant Stars in the Galactic Disk  (2609.11555 - Yaqian et al., 10 Sep 2026) in Section 4.1, discussion of Figure 6