The magnetic field in the Flame Nebula
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Star formation drives galaxy evolution and the cycling of baryonic matter between different phases of the interstellar medium and stars. The magnetic field support of interstellar clouds against gravitational collapse has been invoked as a possible explanation for the observed low star formation efficiency. 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 needed to characterize the transition from diffuse, subcritical gas to dense, gravitationally unstable filaments.
The Flame Nebula, NGC2024, located in the nearby Orion B molecular cloud, harbors a young, expanding HII region and a dense supercritical filament, which contains embedded protostellar objects and is thus not supported by the magnetic field against gravitational collapse. This makes NGC2024 an excellent laboratory for studying the role of magnetic fields in the formation, evolution, and possible collapse of dense filaments, the dynamics of young HII regions, and the role of mechanical and radiative feedback from massive stars on the surrounding molecular gas.
We combine new 154 and 216 μm dust polarization measurements carried out using the HAWC+ instrument aboard SOFIA with molecular line observations of CN and HCO+ 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. The HAWC+ observation shows an ordered magnetic field geometry in NGC2024 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 relatively high, ranging from 30 to 90 μG. The strongest magnetic field is found at the NE edge of the HII region, characterized by the highest gas densities and molecular line widths. We find that the UV-illuminated gas at the edge of the HII region, which is impacted by the stellar feedback, is supported by the magnetic field against gravitational collapse.
Figure: Flame Nebula (NGC~2024) observed at different wavelengths. Panel 1: ESO VISTA composite image. Panel 2: The integrated intensity brightness temperature of the 12CN(1-0) emission obtained by the IRAM 30-meter telescope. Panel 3: The background image of panel 3 represents the Stokes I intensity of dust polarization measurements obtained by SOFIA HAWC+ at 14“. Black lines indicate the direction of the magnetic field. The bottom panels show the spectrum of 12CN(1-0) taken at different positions indicated on panel 2. The colored line represents the best model describing observed 12CN(1-0).
