Inner Warm Disk of ESO Hα 279a Revealed by NA i and CO Overtone Emission Lines

We present an analysis of near-infrared, high-resolution spectroscopy toward the flat-spectrum young stellar object (YSO) ESO Hα 279a (∼1.5M _o) in the Serpens star-forming region at a distance of 429 pc. Using the Immersion GRating INfrared Spectrometer (IGRINS; R ≈ 45,000), we detect emission lines originating from the accretion channel flow, jet, and inner disk. Specifically, we identify hydrogen Brackett series recombination, [Fe ii], [Fe iii], [Fe iv], Ca i, Na i, H₂, H₂O, and CO overtone emission lines. By modeling five bands of CO overtone emission lines and the symmetric double-peaked line profile for Na i emission lines, we find that ESO Hα 279a has an actively accreting Keplerian disk. From our Keplerian disk model, we find that Na i emission lines originate between 0.04 and 1.00 au, while the CO overtone emission lines are from the outer part of the disk, in the range between 0.22 and 3.00 au. The model reveals that the neutral atomic Na gas is a good tracer of the innermost region of the actively accreting disk. We derive a mass accretion rate of M _o yr^-1 from the measured Brγ emission luminosity of 1.78(±0.31) × 10³¹ erg s^-1.