The magnetic field in the Flame nebula

International audienceContext. 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 H II 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 H II regions, and the effects of mechanical and radiative feedback from massive stars on the surrounding molecular gas.Methods. We combined 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-m telescope to determine the magnetic field geometry, and to estimate the plane of the sky magnetic field strength across the NGC 2024 H II 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 H II 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 eastern edge of the H II 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 H II shell (gas impacted by stellar feedback) and the filament (site of current star formation)

Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H2O and its isotopologues towards Orion KL

We report the detection of more than 48 velocity-resolved ground rotational state transitions of H 16 2 O, H 18 2 O, and H 17 2 O – most for the first time – in both emission and absorption toward Orion KL using Herschel/HIFI. We show that a simple fit, constrained to match the known emission and absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measured H 18 2 O line fluxes, which are less affected by line opacity than their H 16 2 O counterparts, and an escape probability method, the column densities of H 18 2 O associated with each emission component are derived. We infer total water abundances of 7.4 × 10−5, 1.0× 10−5, and 1.6 × 10−5 for the plateau, hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL water abundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher than water abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation from (and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources

Herschel observations of deuterated water towards Sgr B2(M)

Observations of HDO are an important complement for studies of water, because they give strong constraints on the formation processes – grain surfaces versus energetic process in the gas phase, e.g. in shocks. The HIFI observations of multiple transitions of HDO in Sgr B2(M) presented here allow the determination of the HDO abundance throughout the envelope, which has not been possible before with ground-based observations only. The abundance structure has been modeled with the spherical Monte Carlo radiative transfer code RATRAN, which also takes radiative pumping by continuum emission from dust into account. The modeling reveals that the abundance of HDO rises steeply with temperature from a low abundance (2.5 × 10−11) in the outer envelope at temperatures below 100 K through a medium abundance (1.5 × 10−9) in the inner envelope/outer core at temperatures between 100 and 200 K, and finally a high abundance ( 3.5 × 10−9) at temperatures above 200 K in the hot core

Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL

We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules (“weeds”), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 × 1013 cm−2 for the HF column density responsible for the broad absorption component. Using our best estimate of the H2 column density within the low-velocity molecular outflow, we obtain a lower limit of ∼1.6 × 10−10 for the HF abundance relative to hydrogen nuclei, corresponding to ∼0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J = 2−1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 × 10−9 derived by Neufeld et al. for cold, foreground clouds on the line of sight towards G10.6-0.4

Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies

Aims. To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular cores, whose asymmetries trace infall and expansion motions. Methods. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN. Results. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity (HCN to H13CN). This is most evident in the HCN 12–11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field changes from infall in the outer part to expansion in the inner part. Conclusions. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important