SOFIA FIFI-LS spectroscopy of DR21 Main: Energetics of the spatially resolved outflow from a high-mass protostar

Context. Massive star formation is associated with energetic processes that may influence the physics and chemistry of parental molecular clouds and impact galaxy evolution. The high-mass protostar DR21 Main in Cygnus X possesses one of the largest and most luminous outflows ever detected in the Galaxy, but the origin of its structure and driving mechanisms are still debated. Aims. Our aim is to spatially resolve the far-infrared line emission from DR21 Main and to investigate the gas physical conditions, energetics, and current mass loss rates along its outflow. Methods. Far-infrared SOFIA FIFI-LS spectra covering selected high-J CO lines, OH, [O <ns0:sc>I</ns0:sc>], [C <ns0:sc>II</ns0:sc>], and [O <ns0:sc>III</ns0:sc>] lines are analyzed across almost the full extent of the DR21 Main outflow using 2.00′ × 3.75′ mosaic. Results. The spatial extent of far-infrared emission closely follows the well-known outflow direction of DR21 Main in the case of high-J CO, [O <ns0:sc>I</ns0:sc>] 63.18 μm, and the OH line at 163.13 μm. On the contrary, the emission from the [C <ns0:sc>II</ns0:sc>] 157.74 μm and [O <ns0:sc>I</ns0:sc>] 145.53 μm lines arises mostly from the eastern part of the outflow, and is likely linked with a photodissociation region. Comparison of non-LTE radiative transfer models with the observed [O <ns0:sc>I</ns0:sc>] line ratios suggest H2 densities of ∼105 cm−3 in the western part of the outflow and ∼ 104 cm−3 in the east. Such densities are consistent with the predictions of UV-irradiated non-dissociative shock models for the observed ratios of CO and [O <ns0:sc>I</ns0:sc>] along the DR21 Main outflow. Assuming that the bulk of the emission arises in shocks, the outflow power of DR21 Main of 4.3–4.8 × 102 L⊙ and the mass loss rate of 3.3−3.7 × 10−3 M⊙ yr−1 are determined, consistent with estimates using HCO+ 1–0. Conclusions. Spatially resolved far-infrared emission of DR21 Main provides a strong support for its origin in outflow shocks, and the stratification of physical conditions along the outflow. The total line cooling provides additional evidence that DR21 Main drives one of the most energetic outflows in the Milky Way.