Spectral types of 1576 optical stars in ONC

Spectral types were initially estimated for F, G, and early K stars from atomic line indices while for late K and M stars, which constitute the majority of our sample, indices involving TiO and VO bands were used.

To ensure proper classification, particularly for reddened, veiled, or nebula-contaminated stars, all spectra were then visually examined for type verification or refinement. We provide an updated spectral type table that supersedes previous work, increasing the percentage of optically visible ONC stars with spectral type information from 68% to 90%. However, for many objects, repeated observations have failed to yield spectral types primarily due to the challenges of adequate sky subtraction against a bright and spatially variable nebular background.