Collapse of massive magnetized dense cores using radiation-magneto-hydrodynamics: early fragmentation inhibition

We report the results of radiation-magneto-hydrodynamics calculations in the context of high mass star formation, using for the first time a self-consistent model for photon emission (i.e. via thermal emission and in radiative shocks) and with the high resolution necessary to resolve properly magnetic braking effects and radiative shocks on scales <100 AU. We investigate the combined effects of magnetic field, turbulence, and radiative transfer on the early phases of the collapse and the fragmentation of massive dense cores. We identify a new mechanism that inhibits initial fragmentation of massive dense cores, where magnetic field and radiative transfer interplay. We show that this interplay becomes stronger as the magnetic field strength increases. Magnetic braking is transporting angular momentum outwards and is lowering the rotational support and is thus increasing the infall velocity. This enhances the radiative feedback owing to the accretion shock on the first core. We speculate that highly magnetized massive dense cores are good candidates for isolated massive star formation, while moderately magnetized massive dense cores are more appropriate to form OB associations or small star clusters.Comment: Accepted for publication in ApJL, 19 pages, 4 figure

Cosmic-ray propagation at small scale: a support for protostellar disc formation

As long as magnetic fields remain frozen into the gas, the magnetic braking prevents the formation of protostellar discs. This condition is subordinate to the ionisation fraction characterising the inmost parts of a collapsing cloud. The ionisation level is established by the number and the energy of the cosmic rays able to reach these regions. Adopting the method developed in our previous studies, we computed how cosmic rays are attenuated as a function of column density and magnetic field strength. We applied our formalism to low- and high-mass star formation models obtained by numerical simulations of gravitational collapse that include rotation and turbulence. In general, we found that the decoupling between gas and magnetic fields, condition allowing the collapse to go ahead, occurs only when the cosmic-ray attenuation is taken into account with respect to a calculation in which the cosmic-ray ionisation rate is kept constant. We also found that the extent of the decoupling zone also depends on the dust grain size distribution and is larger if large grains (of radius about 0.1 microns) are formed by compression and coagulation during cloud collapse. The decoupling region disappears for the high-mass case due to magnetic field diffusion that is caused by turbulence and that is not included in the low-mass models. We infer that a simultaneous study of the cosmic-ray propagation during the cloud's collapse may lead to values of the gas resistivity in the innermost few hundred AU around a forming protostar that is higher than generally assumed.Comment: 8 pages, CRISM 2014 conference proceeding

Public employment and urban dynamics: some elements for analysis.

The public service presents important territorial implications, as its own objective is to allow the State to fulfil its missions of general interest in matter of security, justice, keeping public order, economic guidance, responding to the social demand of improving living conditions, cementing social cohesion ... So its presence is imperative on the whole territory(1) but raises true questions when it concerns a territory situated at the border of main decision centres : there lies the very public function and there rises the spectre of demographic decline, of economic crisis in small centres of Ford-type production, the drop of the financial capacities of local territorial communities. On these border territories dominated by small and medium-sized towns, when faced with the announced obligations of adaptability of the public service and the rising of market economy, what becomes of the expression of the public function performance, that is to say public employment ? Taking into account the need to analyse the various territorial components at stake, we have chosen a plural conception of the public service which combines the criteria of the employer in its large version with that of the public service understood according to a dynamic perspective. Given the extent of the range of situations offered by public employment, we have selected three sectors : the Post Office, National Education, Health Service ; altogether they offer a representative variety of such public employment and are present in the studied territories of the small and medium-sized towns ; sometimes they are situated in metropolitan areas around Lyons (such as Roanne, Macon and even the small towns of the southern part of Saone-et-Loire department), sometimes they are to be found in sub-metropolitan areas, such as those of Ardeche and the towns of Annonay, Aubenas, Privas. Three sets of questions orientate our research ; -What does public employment represent in these configurations, which place does it occupy in the whole sphere of employment ? Does it maintain an important part of the activity of the small centres where local economy is declining ? Does it allow to organize employment basins around towns and of it does according which modes ? -In terms of society, does public employment generate a renewal of populations revealed by the "turn over", in specific sectors and categories of employment ? Is it possible to find dynamics in public employment, as regards function, qualification, "status"... Are there specific trajectories(2) according to the types of employment and those of urban units ? -In the public employment sector, concerned with a necessary adaptability to the present day constraints, do we notice any new practices which are able to make the notion of public employment, even of public service, evolve ? Do these practises also contribute, in terms of territorial organization, to generate some new modes of functioning - including networks for instance - which might lead to opposite situations of solidarities or territorial competitions, as well as the revitalization or declining of local economies ? The question is of great interest considering the importance of small and medium-sized towns in the French urban system and the current metropolisation process. (1). Facing social demand, the staff of the French terrorial function has been multiplied by three within twenty years, with an increase of up to 65% in the period 1969-1980. (2). In particular, statistical methods -such as the LONGI method for insatnce - will be used

Radiative, magnetic and numerical feedbacks on small-scale fragmentation

Radiative feedback and magnetic field are understood to have a strong impact on the protostellar collapse. We present high resolution numerical calculations of the collapse of a 1 solar mass dense core in solid body rotation, including both radiative transfer and magnetic field. Using typical parameters for low-mass cores, we study thoroughly the effect of radiative transfer and magnetic field on the first core formation and fragmentation. We show that including the two aforementioned physical processes does not correspond to the simple picture of adding them separately. The interplay between the two is extremely strong, via the magnetic braking and the radiation from the accretion shock.Comment: 4 pages, 2 figures ; to appear in "IAU Symposium 270: Computational Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl

Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores

Stars and more particularly massive stars, have a drastic impact on galaxy evolution. Yet the conditions in which they form and collapse are still not fully understood. In particular, the influence of the magnetic field on the collapse of massive clumps is relatively unexplored, it is thus of great relevance in the context of the formation of massive stars to investigate its impact. We perform high resolution, MHD simulations of the collapse of hundred solar masses, turbulent and magnetized clouds, using the adaptive mesh refinement code RAMSES. We compute various quantities such as mass distribution, magnetic field and angular momentum within the collapsing core and study the episodic outflows and the fragmentation that occurs during the collapse. The magnetic field has a drastic impact on the cloud evolution. We find that magnetic braking is able to substantially reduce the angular momentum in the inner part of the collapsing cloud. Fast and episodic outflows are being launched with typical velocities of the order of 3-5 km s$^{-1}$ although the highest velocities can be as high as 30-40 km s$^{-1}$. The fragmentation in several objects, is reduced in substantially magnetized clouds with respect to hydrodynamical ones by a factor of the order of 1.5-2. We conclude that magnetic fields have a significant impact on the evolution of massive clumps. In combination with radiation, magnetic fields largely determine the outcome of massive core collapse. We stress that numerical convergence of MHD collapse is a challenging issue. In particular, numerical diffusion appears to be important at high density therefore possibly leading to an over-estimation of the number of fragments.Comment: accepted for publication in A&

Magnetically self-regulated formation of early protoplanetary discs

The formation of protoplanetary discs during the collapse of molecular dense cores is significantly influenced by angular momentum transport, notably by the magnetic torque. In turn, the evolution of the magnetic field is determined by dynamical processes and non-ideal MHD effects such as ambipolar diffusion. Considering simple relations between various timescales characteristic of the magnetized collapse, we derive an expression for the early disc radius, r \simeq 18 \, {\rm AU} \, \left({\eta_{\rm AD} / 0.1 \, {\rm s}} \right)^{2/9} \left({B_z / 0.1\, {\rm G}} \right) ^{-4/9} \left({M / 0.1 \msol} \right) ^{1/3}, where $M$ is the total disc plus protostar mass, $\eta_\mathrm{AD}$ is the ambipolar diffusion coefficient and $B_z$ is the magnetic field in the inner part of the core. This is about significantly smaller than the discs that would form if angular momentum was conserved. The analytical predictions are confronted against a large sample of 3D, non-ideal MHD collapse calculations covering variations of a factor 100 in core mass, a factor 10 in the level of turbulence, a factor 5 in rotation, and magnetic mass-to-flux over critical mass-to-flux ratios 2 and 5. The disc radius estimates are found to agree with the numerical simulations within less than a factor 2. A striking prediction of our analysis is the weak dependence of circumstellar disc radii upon the various relevant quantities, suggesting weak variations among class-0 disc sizes. In some cases, we note the onset of large spiral arms beyond this radius.This research has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 Grant Agreement no. 247060 and no. 306483). We acknowledge financial support from ”Programme National de Physique Stellaire” (PNPS) of CNRS/INSU, France

Simulations of protostellar collapse using multigroup radiation hydrodynamics. II. The second collapse

15 pages, 11 figures, accepted for publication in A&AStar 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; the protostar is born. This sequence of events entail many physical processes, of which radiative transfer is of paramount importance. 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. 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. 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 solar mass cloud cores. We find that, for a same 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. Our simulations support the idea of a standard (universal) initial second core size of 0.003 AU and mass 0.0014 solar masses

Thermal Jeans fragmentation within 1000 AU in OMC-1S

We present subarcsecond 1.3 mm continuum ALMA observations towards the Orion Molecular Cloud 1 South (OMC-1S) region, down to a spatial resolution of 74 AU, which reveal a total of 31 continuum sources. We also present subarcsecond 7 mm continuum VLA observations of the same region, which allow to further study fragmentation down to a spatial resolution of 40 AU. By applying a Mean Surface Density of Companions method we find a characteristic spatial scale at ~560 AU, and we use this spatial scale to define the boundary of 19 `cores' in OMC-1S as groupings of millimeter sources. We find an additional characteristic spatial scale at ~2900 AU, which is the typical scale of the filaments in OMC-1S, suggesting a two-level fragmentation process. We measured the fragmentation level within each core and find a higher fragmentation towards the southern filament. In addition, the cores of the southern filament are also the densest (within 1100 AU) cores in OMC-1S. This is fully consistent with previous studies of fragmentation at spatial scales one order of magnitude larger, and suggests that fragmentation down to 40 AU seems to be governed by thermal Jeans processes in OMC-1S.Comment: Accepted to Ap