Young Stellar Objects

Classification of young stellar objects

The widely used class-based naming scheme for protostellar sources was introduced by Lada (1987) and is based on the slop of the spectral energy distribution in mid-infrared. Consider following expression for spectral index :

where is the flux density and being calculated in the wavelength range of . The initial three classes introduced by Lada (1987) were based on following range of values of :

Class	Spectral index ()	Evolutionary stage
I		Protostellar/circumstellar envelopes
II		T Tauri stars
III		Very late stage where the disk has been dissipated

Two more classes were retroactively added: Class 0 sources show a strong emission in the sub-mm, but very faint in and flat sources are self-explanatory where the slope is close to zero ().