The magnetic field in the Flame nebula

Context. Star formation drives the evolution of galaxies and the cycling of matter between different phases of the interstellar medium and stars. The support of interstellar clouds against gravitational collapse by magnetic fields has been proposed as a possible explanation for the low observed star formation efficiency in galaxies and the Milky Way. The Planck satellite provided the first all-sky map of the magnetic field geometry in the diffuse interstellar medium on angular scales of 5-15′. However, higher spatial resolution observations are required to understand the transition from diffuse, subcritical gas to dense, gravitationally unstable filaments. Aims. NGC 2024, also known as the Flame Nebula, is located in the nearby Orion B molecular cloud. It contains a young, expanding HII region and a dense supercritical filament. This filament harbors embedded protostellar objects and is likely not supported by the magnetic field against gravitational collapse. Therefore, NGC 2024 provides an excellent opportunity to study the role of magnetic fields in the formation, evolution, and collapse of dense filaments, the dynamics of young HII regions, and the effects of mechanical and radiative feedback from massive stars on the surrounding molecular gas. Methods. We combine new 154 and 216 μm dust polarization measurements carried out using the HAWC+ instrument aboard SOFIA with molecular line observations of 12CN(1–0) and HCO+(1–0) from the IRAM 30-meter telescope to determine the magnetic field geometry, and to estimate the plane of the sky magnetic field strength across the NGC 2024 HII region and the surrounding molecular cloud. Results. The HAWC+ observations show an ordered magnetic field geometry in NGC 2024 that follows the morphology of the expanding HII region and the direction of the main dense filament. The derived plane of the sky magnetic field strength is moderate, ranging from 30 to 80 μG. The strongest magnetic field is found at the northern west edge of the HII region, characterized by the highest gas densities and molecular line widths. In contrast, the weakest field is found toward the main, dense filament in NGC 2024. Conclusions. We find that the magnetic field has a non negligible influence on the gas stability at the edges of the expanding HII shell (gas impacted by the stellar feedback) and the filament (site of the current star formation).