Clarify RSG–AGB classification below log(L/Lsun) = 4

Determine whether cool luminous stars with bolometric luminosity log(L/Lsun) < 4 should be classified as red supergiants or as asymptotic giant branch stars, thereby establishing a robust lower-luminosity boundary for red supergiant classification and reducing contamination in red supergiant samples.

Background

In constructing their Hertzsprung–Russell diagram, the authors restrict attention to the region typically associated with red supergiants (RSGs). They note that although catalogs may include candidates with luminosities as low as log(L/Lsun) ≈ 3.2, the commonly adopted lower limit for bona fide RSGs is around log(L/Lsun) = 4 to avoid contamination by asymptotic giant branch (AGB) stars.

The paper explicitly highlights that the boundary at log(L/Lsun) = 4 remains ambiguous, with uncertainty over whether stars below this threshold are RSGs or AGB stars. Resolving this ambiguity is important for building clean RSG samples, interpreting their evolutionary status, and comparing observations to stellar evolutionary models.