Program: GS-2020B-Q-218

Accretion disks around young stellar objects play an important role in the star formation process. While the accretion mechanism from disk to central star is still poorly understood, episodic accretion bursts are likely important in the growth of star and evolution of disk. FU Orionis-type objects (FUors) are observable evidence of episodic accretion. Observations show that FUor-type outbursts can happen at all evolutionary phases from the very embedded objects to the ones with only tenuous remnant envelopes. Moreover, there are claims that the outbursts help clear out the envelope and may drive the objects from the embedded Class I phase to the disk-only Class II phase. To understand the evolutionary phase of FUors, we have been conducting an IGRINS spectroscopic survey. In previous cycles, we already observed 20 objects and found spectral features that vary with evolutionary stage. Here, we propose to observe five more FUors to investigate the differences between the evolutionary stages. Our observations will provide a clue to constrain the powerful outburst mechanisms.