Oxygen budget in low-mass protostars: the NGC 1333-IRAS4A R1 shock observed in [O I] at 63 μm with SOFIA-GREAT

In molecular outflows from forming low-mass protostars, most oxygen is expected to be locked up in water. However, Herschel observations have shown that typically an order of magnitude or more of the oxygen is still unaccounted for. To test if the oxygen is instead in atomic form, SOFIA-GREAT observed the R1 position of the bright molecular outflow from NGC 1333-IRAS4A. The [O I] 63 μm line is detected and spectrally resolved. From an intensity peak at +15 km s^-1, the intensity decreases until +50 km s^-1. The profile is similar to that of high-velocity (HV) H₂O and CO 16-15, the latter observed simultaneously with [O I]. A radiative transfer analysis suggests that 15% of the oxygen is in atomic form toward this shock position. The CO abundance is inferred to be 10^-4 by a similar analysis, suggesting that this is the dominant oxygen carrier in the HV component. These results demonstrate that a large portion of the observed [O I] emission is part of the outflow. Further observations are required to verify whether this is a general trend.