Ionisation impact of high-mass stars on interstellar filaments

Context. Ionising stars reshape their original molecular cloud and impact star formation, leading to spectacular morphologies such as bipolar nebulae around HII regions. Molecular clouds are structured in filaments where stars principally form, as revealed by the Herschel space observatory. The prominent southern hemisphere HII region, RCW 36, is one of these bipolar nebulae. Aims. We study the physical connection between the filamentary structures of the Vela C molecular cloud and the bipolar morphology of RCW 36, providing an in-depth view of the interplay occurring between ionisation and interstellar structures (bright-rims and pillars) around an HII region. Methods. We have compared Herschel observations in five far-infrared and submillimetre filters with the PACS and SPIRE imagers, to dedicated numerical simulations and molecular line mapping. Results. Our results suggest that the RCW 36 bipolar morphology is a natural evolution of its filamentary beginnings under the impact of ionisation. Conclusions. Such results demonstrate that, filamentary structures can be the location of very dynamical phenomena inducing the formation of dense clumps at the edge of HII regions. Moreover, these results could apply to better understanding the bipolar nebulae as a consequence of the expansion of an HII region within a molecular ridge or an interstellar filament

The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project

Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope, and high- precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD, and KORAL. Agreement among the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems

<i>Herschel</i> observations of the W3 GMC: clues to the formation of clusters of high-mass stars

The W3 GMC is a prime target for the study of the early stages of high-mass star formation. We have used Herschel data from the HOBYS key program to produce and analyze column density and temperature maps. Two preliminary catalogs were produced by extracting sources from the column density map and from Herschel maps convolved to 500 μm resolution. Herschel reveals that among the compact sources (FWHM < 0.45 pc), W3 East, W3 West, and W3 (OH) are the most massive and luminous and have the highest column density. Considering the unique properties of W3 East and W3 West, the only clumps with ongoing high-mass star formation, we suggest a "convergent constructive feedback" scenario to account for the formation of a cluster with decreasing age and increasing system/source mass toward the innermost regions. This process, which relies on feedback by high-mass stars to ensure the availability of material during cluster formation, could also lead to the creation of an environment suitable for the formation of Trapezium-like systems. In common with other scenarios proposed in other HOBYS studies, our results indicate that an active/dynamic process aiding in the accumulation, compression, and confinement of material is a critical feature of the high-mass star/cluster formation, distinguishing it from classical low-mass star formation. The environmental conditions and availability of triggers determine the form in which this process occurs, implying that high-mass star/cluster formation could arise from a range of scenarios: from large-scale convergence of turbulent flows to convergent constructive feedback or mergers of filaments

Herschel view of the Taurus B211/3 filament and striations: evidence of filamentary growth?

We present first results from the Herschel Gould Belt survey for the B211/L1495 region in the Taurus molecular cloud. Thanks to their high sensitivity and dynamic range, the Herschel images reveal the structure of the dense, star-forming filament B211 with unprecedented detail, along with the presence of striations perpendicular to the filament and generally oriented along the magnetic field direction as traced by optical polarization vectors. Based on the column density and dust temperature maps derived from the Herschel data, we find that the radial density profile of the B211 filament approaches power-law behavior, ρ ∝ r−2.0± 0.4, at large radii and that the temperature profile exhibits a marked drop at small radii. The observed density and temperature profiles of the B211 filament are in good agreement with a theoretical model of a cylindrical filament undergoing gravitational contraction with a polytropic equation of state: P ∝ ργ and T ∝ ργ−1, with γ = 0.97 ± 0.01 < 1 (i.e., not strictly isothermal). The morphology of the column density map, where some of the perpendicular striations are apparently connected to the B211 filament, further suggests that the material may be accreting along the striations onto the main filament. The typical velocities expected for the infalling material in this picture are ~0.5–1 km s-1, which are consistent with the existing kinematical constraints from previous CO observations