Anomalous Spectral Types and Intrinsic Colors of Young Stars

TL;DR

Young stars exhibit wavelength-dependent spectral types and intrinsic colors that differ from main-sequence stars, necessitating specific observational approaches for accurate characterization and evolutionary analysis.

Contribution

This paper emphasizes the importance of using optical spectral types and young-star-specific intrinsic colors for accurate pre-main sequence star analysis.

Findings

Near-infrared spectral types are 3-5 subclasses later than optical types.

Intrinsic colors of young stars differ from main-sequence stars at the same spectral type.

Using appropriate intrinsic colors prevents systematic errors in extinction and age estimates.

Abstract

We highlight differences in spectral types and intrinsic colors observed in pre-main sequence (pre-MS) stars. Spectral types of pre-MS stars are wavelength-dependent, with near-infrared spectra being 3-5 spectral sub-classes later than the spectral types determined from optical spectra. In addition, the intrinsic colors of young stars differ from that of main-sequence stars at a given spectral type. We caution observers to adopt optical spectral types over near-infrared types, since Hertzsprung-Russell (H-R) diagram positions derived from optical spectral types provide consistency between dynamical masses and theoretical evolutionary tracks. We also urge observers to deredden pre-MS stars with tabulations of intrinsic colors specifically constructed for young stars, since their unreddened colors differ from that of main sequence dwarfs. Otherwise, V-band extinctions as much as ~0.6 mag erroneously higher than the true extinction may result, which would introduce systematic errors in the H-R diagram positions and thus bias the inferred ages.