Nature of shocks revealed by SOFIA OI observations in the Cepheus E
  protostellar outflow
  Protostellar jets and outflows are key features of the star-formation
process, and primary processes of the feedback of young stars on the
interstellar medium. Understanding the underlying shocks is necessary to
explain how jets and outflows are launched, and to quantify their chemical and
energetic impacts on the surrounding medium. We performed a high-spectral
resolution study of the [OI]$_{\rm 63 \mu m}$ emission in the outflow of the
intermediate-mass Class 0 protostar Cep E-mm. We present observations of the OI
$^3$P$_1 \rightarrow$ $^3$P$_2$, OH between $^2\Pi_{1/2}$ $J = 3/2$ and $J =
1/2$ at 1837.8 GHz, and CO (16-15) lines with SOFIA-GREAT at three positions in
the Cep E outflow: mm (the driving protostar), BI (in the southern lobe), and
BII (the terminal position in the southern lobe). The CO line is detected at
all three positions. The OI line is detected in BI and BII, whereas the OH line
is not detected. In BII, we identify three kinematical components in OI and CO,
already detected in CO: the jet, the HH377 terminal bow-shock, and the outflow
cavity. The OI column density is higher in the outflow cavity than in the jet,
which itself is higher than in the terminal shock. The terminal shock is where
the abundance ratio of OI to CO is the lowest (about 0.2), whereas the jet
component is atomic (ratio $\sim$2.7). In the jet, we compare the OI
observations with shock models that successfully fit the integrated intensity
of 10 CO lines: these models do not fit the OI data. The high intensity of OI
emission points towards the propagation of additional dissociative or
alternative FUV-irradiated shocks, where the illumination comes from the shock
itself. From the sample of low-to-high mass protostellar outflows where similar
observations have been performed, the effects of illumination seem to increase
with the mass of the protostar.
