Disc formation in turbulent cloud cores: Circumventing the magnetic braking catastrophe

We present collapse simulations of strongly magnetised, 100 M_sun, turbulent cloud cores. Around the protostars formed during the collapse Keplerian discs with typical sizes of up to 100 AU build up in contrast to previous simulations neglecting turbulence. Analysing the condensations in which the discs form, we show that the magnetic flux loss is not sufficient to explain the build-up of Keplerian discs. The average magnetic field is strongly inclined to the disc which might reduce the magnetic braking efficiency. However, the main reason for the reduced magnetic braking efficiency is the highly disordered magnetic field in the surroundings of the discs. Furthermore, due to the lack of a coherently rotating structure in the turbulent environment of the disc no toroidal magnetic field necessary for angular momentum extraction can build up. Simultaneously the angular momentum inflow remains high due to local shear flows created by the turbulent motions. We suggest that the "magnetic braking catastrophe" is an artefact of the idealised non-turbulent initial conditions and that turbulence provides a natural mechanism to circumvent this problem.Comment: 4 pages, 2 figures. To appear in the proceedings of 'The Labyrinth of Star Formation' (18-22 June 2012, Chania, Greece), published by Springe

The impact of turbulence and magnetic field orientation on star forming filaments

We present simulations of collapsing filaments studying the impact of turbulence and magnetic field morphologies on their evolution and star formation properties. We vary the mass per unit length of the filaments as well as the orientation of the magnetic field with respect to the major axis. We find that the filaments, which have no or a perpendicular magnetic field, typically reveal a smaller width than the universal width of 0.1 pc proposed by e.g. Arzoumanian et al. 2011. We show that this also holds in the presence of supersonic turbulence and that accretion driven turbulence is too weak to stabilize the filaments along their radial direction. On the other hand, we find that a magnetic field that is parallel to the major axis can stabilize the filament against radial collapse resulting in widths of 0.1 pc. Furthermore, depending on the filament mass and magnetic field configuration, gravitational collapse and fragmentation in filaments occurs either in an edge-on way, uniformly distributed across the entire length, or in a mixed way. In the presence of initially moderate density perturbations, a centralized collapse towards a common gravitational centre occurs. Our simulations can thus reproduce different modes of fragmentation observed recently in star forming filaments. Moreover, we find that turbulent motions influence the distance between individual fragments along the filament, which does not always match the results of a Jeans analysis.Comment: 14 pages, 8 figure, accepted for publication in MNRA

Tracing the ISM magnetic field morphology: The potential of multi-wavelength polarization measurements

$\textit{Aims.}$ We present a case study to demonstrate the potential of multi-wavelength polarization measurements. The aim is to investigate the effects that dichroic polarization and thermal re-emission have on tracing the magnetic field in the interstellar medium (ISM). Furthermore, we analyze the crucial influence of imperfectly aligned compact dust grains on the resulting synthetic continuum polarization maps.$\\ \textit{Methods.}$ We developed an extended version of the well-known 3D Monte-Carlo radiation transport code MC3D for multi-wavelength polarization simulations running on an adaptive grid.We investigated the interplay between radiation, magnetic fields and dust grains. Our results were produced by post-processing both ideal density distributions and sophisticated magnetohydrodynamic (MHD) collapse simulations with radiative transfer simulations. We derived spatially resolved maps of intensity, optical depth, and linear and circular polarization at various inclination angles and scales in a wavelength range from 7 $\mu m$ to 1 $mm$.$\\ \textit{Results.}$ We predict unique patterns in linear and circular polarization maps for different types of density distributions and magnetic field morphologies for test setups and sophisticated MHD collapse simulations. We show that alignment processes of interstellar dust grains can significantly influence the resulting synthetic polarization maps. Multi-wavelength polarization measurements allow one to predict the morphology of the magnetic field inside the ISM. The interpretation of polarization measurements of complex structures still remains ambiguous because of the large variety of the predominant parameters in the ISM.Comment: 14 pages, 12 figures, 1 table, Paper accepted 2014 by A&

Turbulence and its effect on protostellar disk formation

We analyse simulations of turbulent, magnetised molecular cloud cores focussing on the formation of Class 0 stage protostellar discs and the physical conditions in their surroundings. We show that for a wide range of initial conditions Keplerian discs are formed in the Class 0 stage already. Furthermore, we show that the accretion of mass and angular momentum in the surroundings of protostellar discs occurs in a highly anisotropic manner, by means of a few narrow accretion channels. The magnetic field structure in the vicinity of the discs is highly disordered, revealing field reversals up to distances of 1000 AU. These findings demonstrate that as soon as even mild turbulent motions are included, the classical disc formation scenario of a coherently rotating environment and a well-ordered magnetic field breaks down.Comment: Invited contribution to the NIC proceedings 2016 for the John von Neumann-Institut f\"ur Computing (NIC) Symposium 201

Revealing the dynamics of Class 0 protostellar discs with ALMA

We present synthetic ALMA observations of Keplerian, protostellar discs in the Class 0 stage studying the emission of molecular tracers like $^{13}$CO, C$^{18}$O, HCO$^+$, H$^{13}$CO$^+$, N$_2$H$^+$, and H$_2$CO. We model the emission of discs around low- and intermediate-mass protostars. We show that under optimal observing conditions ALMA is able to detect the discs already in the earliest stage of protostellar evolution, although the emission is often concentrated to the innermost 50 AU. Therefore, a resolution of a few 0.1" might be too low to detect Keplerian discs around Class 0 objects. We also demonstrate that under optimal conditions for edge-on discs Keplerian rotation signatures are recognisable, from which protostellar masses can be inferred. For this we here introduce a new approach, which allows us to determine protostellar masses with higher fidelity than before. Furthermore, we show that it is possible to reveal Keplerian rotation even for strongly inclined discs and that ALMA should be able to detect possible signs of fragmentation in face-on discs. In order to give some guidance for future ALMA observations, we investigate the influence of varying observing conditions and source distances. We show that it is possible to probe Keplerian rotation in inclined discs with an observing time of 2 h and a resolution of 0.1", even in the case of moderate weather conditions. Furthermore, we demonstrate that under optimal conditions, Keplerian discs around intermediate-mass protostars should be detectable up to kpc-distances.Comment: 17 pages, 17 figures, accepted for publication by MNRA

Accretion and magnetic field morphology around Class 0 stage protostellar discs

We analyse simulations of turbulent, magnetised molecular cloud cores focussing on the formation of Class 0 stage protostellar discs and the physical conditions in their surroundings. We show that for a wide range of initial conditions Keplerian discs are formed in the Class 0 stage already. In particular, we show that even subsonic turbulent motions reduce the magnetic braking efficiency sufficiently in order to allow rotationally supported discs to form. We therefore suggest that already during the Class 0 stage the fraction of Keplerian discs is significantly higher than 50%, consistent with recent observational trends but significantly higher than predictions based on simulations with misaligned magnetic fields, demonstrating the importance of turbulent motions for the formation of Keplerian discs. We show that the accretion of mass and angular momentum in the surroundings of protostellar discs occurs in a highly anisotropic manner, by means of a few narrow accretion channels. The magnetic field structure in the vicinity of the discs is highly disordered, revealing field reversals up to distances of 1000 AU. These findings demonstrate that as soon as even mild turbulent motions are included, the classical disc formation scenario of a coherently rotating environment and a well-ordered magnetic field breaks down. Hence, it is highly questionable to assess the magnetic braking efficiency based on non-turbulent collapse simulation. We strongly suggest that, in addition to the global magnetic field properties, the small-scale accretion flow and detailed magnetic field structure have to be considered in order to assess the likelihood of Keplerian discs to be present.Comment: 14 pages, 6 figures, accepted for publication in MNRAS, updated to final versio

Morphologies of protostellar outflows: An ALMA view

The formation of stars is usually accompanied by the launching of protostellar outflows. Observations with the Atacama Large Millimetre/sub-millimetre Array (ALMA) will soon revolutionalise our understanding of the morphologies and kinematics of these objects. In this paper, we present synthetic ALMA observations of protostellar outflows based on numerical magnetohydrodynamic collapse simulations. We find significant velocity gradients in our outflow models and a very prominent helical structure within the outflows. We speculate that the disk wind found in the ALMA Science Verification Data of HD 163296 presents a first instance of such an observation