Angular momentum transport and the formation of rotationally supported structures are major issues in our understanding of protostellar core formation. Whereas purely hydrodynamical simulations lead to large Keplerian disks, ideal magnetohydrodynamics (MHD) models yield the opposite result, with essentially no disk formation. This stems from the flux-freezing condition in ideal MHD, which leads to strong magnetic braking. In this paper, we provide a more accurate description of the evolution of the magnetic flux redistribution by including resistive terms in the MHD equations. We focus more particularly on the effect of ambipolar diffusion on the properties of the first Larson core and its surrounding structure, exploring various initial magnetisations and magnetic field versus rotation axis orientations of a 1 M collapsing prestellar dense core. We used the non-ideal magnetohydrodynamics version of the adaptive mesh refinement code RAMSES to carry out these calculations. The resistivities required to calculate the ambipolar diffusion terms were computed using a reduced chemical network of charged, neutral, and grain species. Including ambipolar diffusion leads to the formation of a magnetic diffusion barrier (also known as the decoupling stage) in the vicinity of the core, which prevents accumulation of magnetic flux in and around the core and amplification of the field above 0.1 G. The mass and radius of the first Larson core, however, remain similar between ideal and non-ideal MHD models. This diffusion plateau, preventing further amplification of the field and reorganising the field topology, has crucial consequences for magnetic braking processes, allowing the formation of disk structures. Magnetically supported outflows launched in ideal MHD models are weakened or even disappear when using non-ideal MHD. In contrast to ideal MHD calculations, misalignment between the initial rotation axis and the magnetic field direction does not significantly affect the results for a given magnetisation, showing that the physical dissipation processes truly dominate numerical diffusion. We demonstrate severe limits of the ideal MHD formalism; it yields unphysical behaviours in the long-term evolution of the system. This includes counter-rotation inside the outflow or magnetic tower, interchange instabilities, and flux redistribution triggered by numerical diffusion. These effects are not observed in non-ideal MHD. Disks with Keplerian velocity profiles are found to form around the protostar in all our non-ideal MHD simulations, with a final mass and size that strongly depend on the initial magnetisation. This ranges from a few 10 −2 M and ∼20−30 au for the most magnetised case (µ = 2) to ∼2 × 10 −1 M and ∼40−80 au for a lower magnetisation (µ = 5). In all cases, these disks remain significantly smaller than disks found in pure hydrodynamical simulations. Ambipolar diffusion thus bears a crucial impact on the regulation of magnetic flux and angular momentum transport during the collapse of a prestellar core and the...

We develop a detailed chemical network relevant to calculate the conditions that are characteristic of prestellar core collapse. We solve the system of time-dependent differential equations to calculate the equilibrium abundances of molecules and dust grains, with a size distribution given by size-bins for these latter. These abundances are used to compute the different non-ideal magneto-hydrodynamics resistivities (ambipolar, Ohmic and Hall), needed to carry out simulations of protostellar collapse. For the first time in this context, we take into account the evaporation of the grains, the thermal ionisation of Potassium, Sodium and Hydrogen at high temperature, and the thermionic emission of grains in the chemical network, and we explore the impact of various cosmic ray ionisation rates. All these processes significantly affect the non-ideal magneto-hydrodynamics resistivities, which will modify the dynamics of the collapse. Ambipolar diffusion and Hall effect dominate at low densities, up to n H = 10 12 cm −3 , after which Ohmic diffusion takes over. We find that the time-scale needed to reach chemical equilibrium is always shorter than the typical dynamical (free fall) one. This allows us to build a large, multi-dimensional multi-species equilibrium abundance table over a large temperature, density and ionisation rate ranges. This table, which we make accessible to the community, is used during first and second prestellar core collapse calculations to compute the non-ideal magneto-hydrodynamics resistivities, yielding a consistent dynamical-chemical description of this process.

Context. The transport of angular momentum is fundamental during the formation of low-mass stars; too little removal and rotation ensures stellar densities are never reached, too much and the absence of rotation means no protoplanetary disks can form. Magnetic diffusion is seen as a pathway to resolving this long-standing problem. Aims. We aim to investigate the impact of including resistive magnetohydrodynamics (MHD) in simulations of the gravitational collapse of a 1 M gas sphere, from molecular cloud densities to the formation of the protostellar seed; the second Larson core. Methods. We used the adaptive mesh refinement code RAMSES to perform two 3D simulations of collapsing magnetised gas spheres, including self-gravity, radiative transfer in the form of flux-limited diffusion, and a non-ideal gas equation of state to describe H 2 dissociation which leads to the second collapse. The first run was carried out under the ideal MHD approximation, while ambipolar and ohmic diffusion was incorporated in the second calculation using resistivities computed from an equilibrium chemical network. Results. In the ideal MHD simulation, the magnetic field dominates the energy budget everywhere inside and around the first hydrostatic core, fueling interchange instabilities and driving a low-velocity outflow above and below the equatorial plane of the system. High magnetic braking removes essentially all angular momentum from the second core. On the other hand, ambipolar and ohmic diffusion create a barrier which prevents amplification of the magnetic field beyond 0.1 G in the first Larson core which is now fully thermally supported. A significant amount of rotation is preserved and a small Keplerian-like disk forms around the second core. The ambipolar and ohmic diffusions are effective at radii below 10 AU, indicating that a spatial resolution of at least ∼1 AU is necessary to investigate the angular momentum transfer and the formation of rotationally supported disks. Finally, when studying the radiative efficiency of the first and second core accretion shocks, we found that it can vary by several orders of magnitude over the 3D surface of the cores.Conclusions. This proves that magnetic diffusion is a pre-requisite to star formation. Not only does it enable the formation of protoplanetary disks in which planets will eventually form, it also plays a determinant role in the formation of the protostar itself.

Context. Star formation begins with the gravitational collapse of a dense core inside a molecular cloud. As the collapse progresses, the centre of the core begins to heat up as it becomes optically thick. The temperature and density in the centre eventually reach high enough values where fusion reactions can ignite, and the protostar is born. This sequence of events entails many physical processes, of which radiative transfer is of paramount importance. Simulated collapsing cores without radiative transfer rapidly become thermally supported before reaching high enough temperatures and densities, preventing the formation of stars. Aims. Many simulations of protostellar collapse make use of a grey treatment of radiative transfer coupled to the hydrodynamics. However, interstellar gas and dust opacities present large variations as a function of frequency, which can potentially be overlooked by grey models and lead to significantly different results. In this paper, we follow up on a previous paper on the collapse and formation of Larson's first core using multigroup radiation hydrodynamics (Paper I) by extending the calculations to the second phase of the collapse and the formation of Larson's second core. Methods. We have made the use of a non-ideal gas equation of state as well as an extensive set of spectral opacities in a spherically symmetric fully implicit Godunov code to model all the phases of the collapse of a 0.1, 1, and 10 M cloud cores. Results. We find that, for an identical central density, there are only small differences between the grey and multigroup simulations. The first core accretion shock remains supercritical while the shock at the second core border is found to be strongly subcritical with all the accreted energy being transfered to the core. The size of the first core was found to vary somewhat in the different simulations (more unstable clouds form smaller first cores) while the size, mass, and temperature of the second cores are independent of initial cloud mass, size, and temperature. Conclusions. Our simulations support the idea of a standard (universal) initial second core size of ∼3 × 10 −3 AU and mass ∼1.4 × 10 −3 M . The grey approximation for radiative transfer appears to perform well in one-dimensional simulations of protostellar collapse, most probably because of the high optical thickness of the majority of the protostar-envelope system. A simple estimate of the characteristic timescale of the second core suggests that the effects of using multigroup radiative transfer may be more important in the long-term evolution of the protostar.

The outflowing proper motions of 15 knots in the dominant northwestern lobe of the high‐excitation poly‐polar planetary nebula NGC 6302 have been determined by comparing their positions relative to those of faint stars in an image taken at the San Pedro Martir Observatory in 2007 to those in a South African Astronomical Observatory archival plate obtained by Evans in 1956. The Hubble‐type expansion of this lobe is now directly confirmed in a model‐independent way from these measurements. Furthermore, an unambigous distance to NGC 6302 of 1.17 ± 0.14 kpc is now determined. Also, all the velocity vectors of the 15 knots (and two others) point back to the central source. An eruptive event from within the central torus ≈2200 yr previously must have created the high‐speed lobes of NGC 6302.

Context. Numerical simulations of star formation are becoming ever more sophisticated, incorporating new physical processes in increasingly realistic set-ups. These models are being compared to the latest observations through state-of-the-art synthetic renderings that trace the different chemical species present in the protostellar systems. The chemical evolution of the interstellar and protostellar matter is very topical, with more and more chemical databases and reaction solvers available online to the community. Aims. The current study was developed to provide a database of relatively simple numerical simulations of protostellar collapse as a template library for observations of cores and very young protostars, and for researchers who wish to test their chemical modelling under dynamic astrophysical conditions. It was also designed to identify statistical trends that may appear when running many models of the formation of low-mass stars by varying the initial conditions. Methods. A large set of 143 calculations of the gravitational collapse of an isolated sphere of gas with uniform temperature and a Bonnor-Ebert-like density profile was undertaken using a 1D fully implicit Lagrangian radiation hydrodynamics code. The parameter space covered initial masses from 0.2 to 8 M , temperatures of 5-30 K, and radii 3000 ≤ R 0 ≤ 30, 000 AU. Results. A spread due to differing initial conditions and optical depths, was found in the thermal evolutionary tracks of the runs. Within less than an order of magnitude, all first and second Larson cores had masses and radii essentially independent of the initial conditions. Radial profiles of the gas density, velocity, and temperature were found to vary much more outside of the first core than inside. The time elapsed between the formation of the first and second cores was found to strongly depend on the first core mass accretion rate, and no first core in our grid of models lived for longer than 2000 years before the onset of the second collapse. Conclusions. The end product of a protostellar cloud collapse, the second Larson core, is at birth a canonical object with a mass and radius of about 3 M J and 8 R J , independent of its initial conditions. The evolution sequence which brings the gas to stellar densities can, however, proceed in a variety of scenarios, on different timescales or along different isentropes, but each story line can largely be predicted by the initial conditions. All the data from the simulations are publicly available.

We present the first detailed kinematical analysis of the planetary nebula Abell 63, which is known to contain the eclipsing close‐binary nucleus UU Sge. Abell 63 provides an important test case in investigating the role of close‐binary central stars on the evolution of planetary nebulae. Longslit observations were obtained using the Manchester echelle spectrometer combined with the 2.1‐m San Pedro Martir Telescope. The spectra reveal that the central bright rim of Abell 63 has a tube‐like structure. A deep image shows collimated lobes extending from the nebula, which are shown to be high‐velocity outflows. The kinematic ages of the nebular rim and the extended lobes are calculated to be 8400 ± 500 and 12 900 ± 2800 yr, respectively, which suggests that the lobes were formed at an earlier stage than the nebular rim. This is consistent with expectations that disc‐generated jets form immediately after the common envelope phase. A morphological–kinematical model of the central nebula is presented and the best‐fitting model is found to have the same inclination as the orbital plane of the central binary system; this is the first proof that a close‐binary system directly affects the shaping of its nebula. A Hubble‐type flow is well‐established in the morphological–kinematical modelling of the observed line profiles and imagery. Two possible formation models for the elongated lobes of Abell 63 are considered, (i) a low‐density, pressure‐driven jet excavates a cavity in the remnant asymptotic giant branch (AGB) envelope; (ii) high‐density bullets form the lobes in a single ballistic ejection event.

We present in this paper a multigroup model for radiation hydrodynamics to account for variations of the gas opacity as a function of frequency. The entropy closure model (M1) is applied to multigroup radiation transfer in a radiation hydrodynamics code. In difference from the previous grey model, we are able to reproduce the crucial effects of frequency-variable gas opacities, a situation omnipresent in physics and astrophysics. We also account for the energy exchange between neighbouring groups which is important in flows with strong velocity divergence. These terms were computed using a finite volume method in the frequency domain. The radiative transfer aspect of the method was first tested separately for global consistency (reversion to grey model) and against a well established kinetic model through Marshak wave tests with frequency dependent opacities. Very good agreement between the multigroup M1 and kinetic models was observed in all tests. The successful coupling of the multigroup radiative transfer to the hydrodynamics was then confirmed through a second series of tests. Finally, the model was linked to a database of opacities for a Xe gas in order to simulate realistic multigroup radiative shocks in Xe. The differences with the previous grey models are discussed.Comment: 27 pages, 11 figures, Accepted for publication in JQSR