Abstract
Abstract We present velocity-resolved Stratospheric Observatory for Infrared Astronomy (SOFIA)/upgrade German REceiver for Astronomy at Terahertz Frequencies observations of [O i] and [C ii] lines toward a Class I protostar, L1551 IRS 5, and its outflows. The SOFIA observations detect [O i] emission toward only the protostar and [C ii] emission toward the protostar and the redshifted outflow. The [O i] emission has a width of ∼100 km s−1 only in the blueshifted velocity, suggesting an origin in shocked gas. The [C ii] lines are narrow, consistent with an origin in a photodissociation region. Differential dust extinction from the envelope due to the inclination of the outflows is the most likely cause of the missing redshifted [O i] emission. Fitting the [O i] line profile with two Gaussian components, we find one component at the source velocity with a width of ∼20 km s−1 and another extremely broad component at −30 km s−1 with a width of 87.5 km s−1, the latter of which has not been seen in L1551 IRS 5. The kinematics of these two components resemble cavity shocks in molecular outflows and spot shocks in jets. Radiative transfer calculations of the [O i], high-J CO, and H2O lines in the cavity shocks indicate that [O i] dominates the oxygen budget, making up more than 70% of the total gaseous oxygen abundance and suggesting [O]/[H] of ∼1.5 × 10−4. Attributing the extremely broad [O i] component to atomic winds, we estimate an intrinsic mass-loss rate of (1.3 ± 0.8) × 10−6 M ⊙ yr−1. The intrinsic mass-loss rates derived from low-J CO, [O i], and H i are similar, supporting the model of momentum-conserving outflows, where the atomic wind carries most momentum and drives the molecular outflows.
Highlights
Fitting the [O i] line profile with two Gaussian components, we find one component at the source velocity with a width of ∼20 km s−1 and another extremely broad component at −30 km s−1 with a width of 87.5 km s−1, the latter of which has not been seen in L1551 IRS 5
Fridlund et al (2002) derived a source velocity of 6.5 km s−1 for L1551 IRS 5, while Yıldız et al (2013) found a slightly lower source velocity of 6.2 km s−1, which has since been adopted by several studies of high-J CO and water emission (e.g., Kristensen et al 2017b)
We present velocity-resolved spectra of the [O i] 63 μm and [C ii] 158 μm lines observed towards L1551 IRS 5 with Stratospheric Observatory for Infrared Astronomy (SOFIA)
Summary
Studies using observations of the Herschel Space Observatory surveyed the [O i] 63 μm line from low-mass to high-mass protostars with many results suggesting that the line originates in UV-irradiated shocks (Karska et al 2014b; Nisini et al 2015; Karska et al 2018). Toward NGC 1333 IRAS 4A, a low-mass protostar, Kristensen et al (2017a) used SOFIA to observe the [O i] 63 μm line in the R1 shock knot along the outflow. The line profile of the [O i] emission is similar to that of CO J = 16 → 15 and shock-excited H2O lines, implying that they trace the same shock component They suggest that CO is the major carrier of volatile oxygen in the shock and only ∼ 15% of the oxygen is atomic. We present velocity-resolved spectra of the [O i] 63 μm and [C ii] 158 μm lines observed towards L1551 IRS 5 with SOFIA.
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