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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
⁴ National Centre for Nuclear Research, Pasteura 7, 02-093 Warszawa, Poland
⁵ Astronomical Observatory of the Jagiellonian University, Orla 171, 30-244 Kraków, Poland
⁶ Deutsches SOFIA Institut, University of Stuttgart, Pfaffenwaldring 29, 70569 Stuttgart, Germany
⁷ Institute For Interdisciplinary Research in Science and Education (IFIRSE), ICISE, 07 Science Avenue, Ghenh Rang Ward, 55121 Quy Nhon City, Binh Dinh Province, Vietnam
⁸ Star and Planet Formation Laboratory, RIKEN Pioneering Research Institute, Wako-shi, Saitama, 351-0106, Japan
⁹ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903, USA
¹⁰ Department of Astronomy, University of Illinois, 1002 West Green St, Urbana, IL 61801, USA

^★★ Corresponding author.

Received: 21 November 2024
Accepted: 17 March 2025

Abstract

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 I], [C II], and [O III] 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 I] 63.18 μm, and the OH line at 163.13 μm. On the contrary, the emission from the [C II] 157.74 μm and [O I] 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 I] 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 I] 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.