2011A&A...533A.112H

Young circumstellar disks that are still embedded in dense molecular envelopes may differ from their older counterparts, but are historically difficult to study because emission from a disk can be confused with envelope or outflow emission. CO fundamental emission is a potentially powerful probe of the disk/wind structure within a few AU of young protostars. In this paper, we present high spectral (R=90000) and spatial (∼0.3") resolution VLT/CRIRES M-band spectra of 18 low-mass young stellar objects (YSOs) with dense envelopes in nearby star-froming regions to explore the utility of CO fundamental (Δv=1) 4.6µm emission as a probe of very young disks. CO fundamental emission is detected from 14 of the YSOs in our sample. The emission line profiles show a range of strengths and shapes, but can generally be classified into a broad, warm component and a narrow, cool component. The broad CO emission is detected more frequently from YSOs with bolometric luminosities of <15L_☉ than those with >15L_☉. The broad emission shares many of the same properties as CO fundamental emission seen from more mature disks around classical T Tauri stars (CTTSs) and is similarly attributed to the warm (∼1000K) inner AU of the disk. CO emission from the inner disk is not detected from most YSOs with a high bolometric luminosity. Instead, the CO emission from those objects is produced in cooler (∼320K), narrow lines in ¹²CO and in rarer isotopologues. From some objects, the narrow lines are blueshifted by up to ∼10 km/s, indicating a slow wind or outflow origin. For other sources the lines are located at the systemic velocity of the star and likely arise in the disk. For a few YSOs, spatially-extended CO and H₂ S(9) emission is detected up to ∼2'' from the central source and is attributed to interactions between the wind and surrounding molecular material. Warm CO absorption is detected in the wind of six objects with velocities up to 100km/s, often in discrete velocity components. That the wind is partially molecular where it is launched favors ejection in a disk wind rather than a coronal or chromospheric wind.