Fast Atomic Shocks in the Outflow of L1551 IRS 5 Identified with SOFIA-upGREAT Observations of [OI]
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Yao-Lun Yang
University of Virginia
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Protostellar outflows are the primary process of angular momentum removal in protostellar accretion disks, indirectly probing the ongoing accretion through the disk around the central protostar. The [OI] fine-structure emission is thought to trace the more energetic outflows compared to molecular lines. We present velocity-resolved SOFIA/upGREAT observations of [OI] and [CII] lines toward a Class I protostar, L1551 IRS 5, and its outflows. The [OI] emission, only detected toward the protostar, appears to have a total width of ~100 km/s only in the blue-shifted velocity, tracing the shocked gas due to the outflow, while narrow [CII] lines appear toward the protostar and the red-shifted outflow, tracing the photodissociation region. I will argue that the dust extinction in the envelope may lead to the missing red-shifted [OI] emission. The comparison with Herschel observations suggests that shocks dominate the emission of both [OI] and [CII]. We decompose the [OI] line profile into two components, a broad component at the source velocity tracing the cavity shocks or disk wind and an extremely broad component with a width of ~90 km/s from the jet. This extremely broad component suggests an origin of dissociative shocks in the jet, which has never been directly detected in L1551 IRS 5. For the shocked gas in the outflow cavity and/or disk wind, atomic oxygen dominates the volatile oxygen budget, making up 70-90% of the total budget. Knowing the origin of each component allows us to constrain the intrinsic outflow rate derived from [OI], which is consistent with that estimated from CO and HI after correction of inclination. There is no variability of the intrinsic outflow rate over a factor of 3 even though L1551 IRS 5 is thought to be a FU Orionis-like object.

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