We propose a high-resolution near-infrared spectroscopic observation to determine the ages of pre-main sequence (PMS) stars in ρ Ophiuchus (ρ Oph) molecular cloud. The age of PMS stars can be estimated precisely by the surface gravity determinations using equivalent width ratios (e.g., Takagi et al. 2011). The equivalent width ratio of nearby absorption lines allows us to determine the surface gravity with no contamination by the uncertainties of distance, extinction, and veiling. The disk dissipation timescale of PMS stars in the ρ Oph cloud has been estimated based on this method, which is 1.2 Myr (Takagi et al. 2015). The targets were limited to stars located in the surface area of the molecular cloud. To reveal the detailed star-forming process in the ρ Oph cloud, we have observed several PMS stars that are embedded in the cloud and found that the disk lifetime may be longer in the inner part of the cloud, but the number of the observed target was few. For further discussion, we will determine the ages of 28 low-mass pre-main sequence stars which are highly embedded, by using NIR WFS which is a powerful tool to observe targets with large extinctions. The result of this observation will reveal the star-formation process in the ρ Oph cloud.
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