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The origin of cosmic rays in our Galaxy remains a subject of active debate. While supernova remnant shocks are often invoked as the sites of acceleration, it is now widely accepted that the difficulties of such sources in reaching PeV energies are daunting and it seems likely that only a subclass of rare remnants can satisfy the necessary conditions. Moreover the spectra of cosmic rays escaping the remnants have a complex shape that is not obviously the same as the spectra observed at the Earth. Here we investigate the process of particle acceleration at the termination shock that develops in the bubble excavated by star clusters' winds in the interstellar medium. While the main limitation to the maximum energy in supernova remnants comes from the need for effective wave excitation upstream so as to confine particles in the near-shock region and speed up the acceleration process, at the termination shock of star clusters the confinement of particles upstream in guaranteed by the geometry of the problem. We develop a theory of diffusive shock acceleration at such shock and we find that the maximum energy may reach the PeV region for powerful clusters in the high end of the luminosity tail for these sources. A crucial role in this problem is played by the dissipation of energy in the wind to magnetic perturbations. Under reasonable conditions the spectrum of the accelerated particles has a power law shape with a slope $4\div 4.3$, in agreement with what is required based upon standard models of cosmic ray transport in the Galaxy., Comment: 11 pages, 5 figures, submitted to MNRAS, comments welcome

To understand how stars can form in the interstellar medium (ISM), it has to be understood how cold (~ 10 K) and dense gas (> 10^{4} cm^{-3}) can emerge during the evolution of the ISM. With the Herschel telescope it was found that most of this dense star forming gas is organised in filamentary structures.To understand how this dense filamentary gas forms, multiple CO transitions were observed towards the Musca filament, which can form low-mass stars, using the APEX telescope. These observations were complemented with [CII] and [OI] observations by the SOFIA telescope. The non-detection of [CII] demonstrates that the Musca cloud is embedded in a weak FUV field (< 1 G0). However, the observed CO(4-3) line with APEX demonstrates the presence of warm (> 50 K) CO gas around the Musca filament which cannot be explained with heating by the FUV radiation field. A comparison of the observed CO(4-3) emission with shock models shows that the emission can be the result of a low-velocity (< 4 km/s) J-type shock. Further analysis of this emission demonstrates that this shock emission resembles the signature of a shock responsible for mass accretion on a filament. This suggests that a low-velocity shock as a result of continuous mass accretion is responsible for the formation of cold and dense gas that can form stars in the Musca filament.The accretion scenario for Musca is further analysed with low-J CO observations from APEX and NANTEN2 to study the large scale gas kinematics. These observations unveil a velocity gradient over the Musca filament crest which is correlated with the velocity field of the nearby ambient gas. This suggests that the velocity gradient is the result of mass accretion from the ambient cloud. Analysing the full Musca cloud demonstrates a spatial and kinematic asymmetry from low- to high-density gas. This asymmetry is seen as a V-shape in the position-velocity (PV) diagram perpendicular to the Musca filament. Including atomic hydrogen (HI) observations in the analysis first of all confirms that Musca is part of a larger HI cloud, the Chamaeleon-Musca complex. It also demontrates that the kinematic asymmetry is seen from the HI cloud down to the filament crest. Furthermore, the CO-HI asymmetry is found for basically all dense regions (Cha I, Cha II, Cha III and Musca) with archival data of Chamaeleon-Musca, while HI shows indications of more than one velocity component. This asymmetric accretion scenario is predicted by magnetised cloud-cloud collision simulations, where the bending of the magnetic field is responsible the observed asymmetric accretion scenario. The filament formation in Musca is thus the result of two intersecting converging flows which are driven by the magnetic field bending due to a large-scale colliding HI flow that triggered the observed star formation in the full Chamaeleon-Musca complex.Finally, the kinematics of the high-mass star forming ridge DR21 and its surrounding gas are studied to compare low- and high-mass star formation. This shows a similar spatial and kinematic asymmetry as in Musca, which suggests that DR21 is formed by a giant molecular cloud (GMC) collision. However, it is also found for high-mass star formation in the DR21 cloud that gravity plays an important role on large scales (> 1 pc) while for Musca gravity only starts to dominate locally (r < 0.1-0.2 pc). So, due to the high density in the DR21 cloud after the GMC collision, gravity eventually drives the evolution of the compressed cloud for high-mass star forming regions. Kinematic observations of the full Cygnus-X north region show further indications of two interacting velocity components over the entire region, which indicates that a high-velocity (> 10 km/s) GMC collision can result in the formation of an OB association similar to OB2. These OB stars then form in gravitationally collapsing hubs and ridges due to the compression by the GMC collision.; Pour comprendre la formation des étoiles, il faut étudier les processus physiques qui forment le gaz froid et dense dans le milieu interstellaire. Le télescope spatial Herschel a récemment démontré que la majorité du gaz froid et dense est formée de structures filamentaires (des filaments).Dans cette thèse, plusieurs raies de CO ont été observées avec le télescope APEX autour du filament de Musca. Ces observations ont été complémentées par des observations [CII] et [OI] avec le télescope SOFIA. La non-détection de [CII] démontre que le nuage de Musca est situé dans un champ de radiation UV faible (1 G0). Par contre, les observations de CO(4-3) avec APEX montrent qu'il y a du gaz CO chauffé (> 50 K) autour du filament que l'irradiation UV ne peut pas expliquer. La comparaison avec des modèles de chocs indique que l'émission CO(4-3) doit alors être le résultat d'un choc J à basse vitesse (< 4 km/s). L'analyse du spectre CO(4-3) montre aussi que l'émission venant du choc ressemble à une signature de choc d'accrétion. Cette observation suggère qu'un choc à basse vitesse, dû à une accrétion continue, est responsable de la formation du gaz dense et froid du filament de Musca.Ce scénario d'accrétion du filament de Musca est de plus étudié à grandes échelles dans les raies CO(2-1) et CO(1-0) obtenues avec les télescopes APEX et NANTEN2. Ces observations montrent un gradient de vitesse sur la crête de Musca qui est correlé avec le champ de vitesse autour du filament. L'analyse globale des observations de Musca montre une asymétrie à la fois spatiale et cinématique. Cette asymétrie est vue comme une forme en V dans le diagramme position-vitesse perpendiculaire au filament. L'inclusion d'observations du gaz neutre HI dans l'analyse confirme que Musca fait partie d'un nuage HI plus grand, le complex Chamaeleon-Musca. Le HI montre aussi que l'asymétrie cinématique est présente des grandes échelles du nuage HI jusqu'aux petites échelles de la crête du filament de Musca. En comparant le HI avec les vitesses CO de Cha I, Cha II et Cha III, on constate que l'asymétrie cinématique est présente pour toutes les régions denses du complexe de Chamaeleon-Musca. Ce scénario d'accrétion asymétrique, qui est observé, est reproduit dans des simulations d'une collision de nuages magnétisés. Dans ce scénario, c'est la déformation du champ magnétique qui est responsable de l'accrétion asymétrique. La formation du filament Musca serait ainsi due à la convergence de deux flots de matière guidée par la courbure du champ magnétique provoquée par la collision des nuages HI à grande échelle.Dans la dernière partie, la cinématique du nuage massif DR21, qui forme des étoiles massives, est étudiée pour comparer la formation des étoiles massives à celle des étoiles de faible masse. Le nuage DR21 montre une asymétrie en V similaire à celle de Musca, ce qui indique que le nuage DR21 est aussi formé par une collision de nuages moléculaires mais avec une vitesse de collision plus importante que pour Musca. Les observations indiquent de plus que la formation des étoiles massives dans le nuage DR21 serait la conséquence directe de la prédominance de la gravité à grande échelle (> 1 pc) du gaz dense en contraste avec Musca pour lequel la gravité ne dominerait qu'aux plus petites échelles (< 0.1-0.2 pc). L'analyse cinématique globale de toute la région du Cygne montre que toute la région résulte de la même collision de nuages. Cette observation indique que c'est une collision de nuages à grande vitesse (> 10 km/s) qui pourrait expliquer la formation d'une association d'étoiles OB de plusieurs milliers d'étoiles. Dans ce scénario, les étoiles massives (OB) se formeraient dans les structures denses et massives (hubs et ridges) formées aux convergences dues à la collision à grande vitesse de nuages, et où la gravité à grande échelles domine la cinématique et l'évolution du gaz dense.

Intermediate and massive stars drive fast and powerful isotropic winds that interact with the winds of nearby stars in star clusters and the surrounding interstellar medium (ISM). Wind-ISM collisions generate astrospheres around these stars that contain hot $T\sim 10^7$ K gas that adiabatically expands. As individual bubbles expand and collide they become unstable, potentially driving turbulence in star clusters. In this paper we use hydrodynamic simulations to model a densely populated young star cluster within a homogeneous cloud to study stellar wind collisions with the surrounding ISM. We model a mass-segregated cluster of 20 B-type young main sequence stars with masses ranging from 3--17 $M_{\odot}$. We evolve the winds for $\sim$11 kyrs and show that wind-ISM collisions and over-lapping wind-blown bubbles around B-stars mixes the hot gas and ISM material generating Kolmogorov-like turbulence on small scales early in its evolution. We discuss how turbulence driven by stellar winds may impact the subsequent generation of star formation in the cluster, 12 pages, 5 figures, Accepted for publication in ApJL

A suite of idealised, global, gravitationally unstable, star-forming galactic disc simulations with 2 pc spatial resolution, performed with the adaptive mesh refinement code {\sc ramses} is used in this paper to predict the emergent effects of supernova feedback. The simulations include a simplified prescriptions for formation of single stellar populations of mass $\sim 100 \, M_{\odot}$, radiative cooling, photoelectric heating, an external gravitational potential for a dark matter halo and an old stellar disc, self-gravity, and a novel implementation of supernova feedback. The results of these simulations show that gravitationally unstable discs can generate violent supersonic winds with mass loading factors $\eta \gtrsim 10$, followed by a galactic fountain phase. These violent winds are generated by highly clustered supernovae exploding in dense environments created by gravitational instability, and they are not produced in simulation without self-gravity. The violent winds significantly perturb the vertical structure of the disc, which is later re-established during the galactic fountain phase. Gas resettles into a quasi-steady, highly turbulent disc with volume-weighted velocity dispersion $\sigma > 50 \, {\rm km/s}$. The new configuration drives weaker galactic winds with mass loading factor $\eta \leq 0.1$. The whole cycle takes place in $\leq 10$ dynamical times. Such high time variability needs to be taken into account when interpreting observations of galactic winds from starburst and post-starburst galaxies., Comment: 17 pages, accepted by MNRAS

et al., This paper presents an overview of the polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse maps of dust polarization fraction and polarization angle at 1° resolution, taking into account noise bias and possible systematic effects. The sensitivity of the Planck HFI polarization measurements allows for the first time a mapping of Galactic dust polarized emission on large scales, including low column density regions. We find that the maximum observed dust polarization fraction is high (pmax = 19.8%), in particular in some regions of moderate hydrogen column density (NH < 2 × 1021 cm-2). The polarization fraction displays a large scatter at NH below a few 1021 cm-2. There is a general decrease in the dust polarization fraction with increasing column density above NH ≃ 1 × 1021 cm-2 and in particular a sharp drop above NH ≃ 1.5 × 1022 cm-2. We characterize the spatial structure of the polarization angle using the angle dispersion function. We find that the polarization angle is ordered over extended areas of several square degrees, separated by filamentary structures of high angle dispersion function. These appear as interfaces where the sky projection of the magnetic field changes abruptly without variations in the column density. The polarization fraction is found to be anti-correlated with the dispersion of polarization angles. These results suggest that, at the resolution of 1°, depolarization is due mainly to fluctuations in the magnetic field orientation along the line of sight, rather than to the loss of grain alignment in shielded regions. We also compare the polarization of thermal dust emission with that of synchrotron measured with Planck, low-frequency radio data, and Faraday rotation measurements toward extragalactic sources. These components bear resemblance along the Galactic plane and in some regions such as the Fan and North Polar Spur regions. The poor match observed in other regions shows, however, that dust, cosmic-ray electrons, and thermal electrons generally sample different parts of the line of sight., The development of Planck has been supported by: ESA; CNES and CNRS/INSU-IN2P3-INP (France); ASI, CNR, and INAF (Italy); NASA and DoE (USA); STFC and UKSA (UK); CSIC, MICINN, J.A., and RES (Spain); Tekes, AoF, and CSC (Finland); DLR and MPG (Germany); CSA (Canada); DTU Space (Denmark); SER/SSO (Switzerland); RCN (Norway); SFI (Ireland); FCT/M CTES (Portugal); and PRACE (EU). The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ ERC grant agreement no 267934 and from a joint agreement between University of São Paulo, Brazil, and COFECUB, France (grant nos. USP 2007.1.433.14.2 and COFECUB Uc Te 114/08).

The general conditions for the weak and strong regimes of incompressible magnetohydrodynamic turbulence are derived and studied in the framework of the direct interaction approximation. It is shown that in the framework of the weak turbulence theory, the autocorrelation and cascade timescales are always of the same order of magnitude. This means that, contrary to the general belief, any model of turbulence which implies a large number of collisions among wave packets for an efficient energy cascade (such as the Iroshnikov–Kraichnan model), does not represent a model of weak turbulence.

We explore the amplification of magnetic seed fields during the formation of the first stars and galaxies. During gravitational collapse, turbulence is created from accretion shocks, which may act to amplify weak magnetic fields in the protostellar cloud. Numerical simulations showed that such turbulence is sub-sonic in the first star-forming minihalos, and highly supersonic in the first galaxies with virial temperatures larger than 10^4 K. We investigate the magnetic field amplification during the collapse both for Kolmogorov and Burgers-type turbulence with a semi-analytic model that incorporates the effects of gravitational compression and small-scale dynamo amplification. We find that the magnetic field may be substantially amplified before the formation of a disk. On scales of 1/10 of the Jeans length, saturation occurs after ~10^8 yr. Although the saturation behaviour of the small-scale dynamo is still somewhat uncertain, we expect a saturation field strength of the order ~10^{-7} n^{0.5} G in the first star-forming halos, with n the number density in cgs units. In the first galaxies with higher turbulent velocities, the magnetic field strength may be increased by an order of magnitude, and saturation may occur after 10^6 to 10^7 yr. In the Kolmogorov case, the magnetic field strength on the integral scale (i.e. the scale with most magnetic power) is higher due to the characteristic power-law indices, but the difference is less than a factor of 2 in the saturated phase. Our results thus indicate that the precise scaling of the turbulent velocity with length scale is of minor importance. They further imply that magnetic fields will be significantly enhanced before the formation of a protostellar disk, where they may change the fragmentation properties of the gas and the accretion rate., Comment: 11 pages, 9 figures, accepted at A&A

To understand how stars can form in the interstellar medium (ISM), it has to be understood how cold (~ 10 K) and dense gas (> 10^{4} cm^{-3}) can emerge during the evolution of the ISM. With the Herschel telescope it was found that most of this dense star forming gas is organised in filamentary structures.To understand how this dense filamentary gas forms, multiple CO transitions were observed towards the Musca filament, which can form low-mass stars, using the APEX telescope. These observations were complemented with [CII] and [OI] observations by the SOFIA telescope. The non-detection of [CII] demonstrates that the Musca cloud is embedded in a weak FUV field (< 1 G0). However, the observed CO(4-3) line with APEX demonstrates the presence of warm (> 50 K) CO gas around the Musca filament which cannot be explained with heating by the FUV radiation field. A comparison of the observed CO(4-3) emission with shock models shows that the emission can be the result of a low-velocity (< 4 km/s) J-type shock. Further analysis of this emission demonstrates that this shock emission resembles the signature of a shock responsible for mass accretion on a filament. This suggests that a low-velocity shock as a result of continuous mass accretion is responsible for the formation of cold and dense gas that can form stars in the Musca filament.The accretion scenario for Musca is further analysed with low-J CO observations from APEX and NANTEN2 to study the large scale gas kinematics. These observations unveil a velocity gradient over the Musca filament crest which is correlated with the velocity field of the nearby ambient gas. This suggests that the velocity gradient is the result of mass accretion from the ambient cloud. Analysing the full Musca cloud demonstrates a spatial and kinematic asymmetry from low- to high-density gas. This asymmetry is seen as a V-shape in the position-velocity (PV) diagram perpendicular to the Musca filament. Including atomic hydrogen (HI) observations in the analysis first of all confirms that Musca is part of a larger HI cloud, the Chamaeleon-Musca complex. It also demontrates that the kinematic asymmetry is seen from the HI cloud down to the filament crest. Furthermore, the CO-HI asymmetry is found for basically all dense regions (Cha I, Cha II, Cha III and Musca) with archival data of Chamaeleon-Musca, while HI shows indications of more than one velocity component. This asymmetric accretion scenario is predicted by magnetised cloud-cloud collision simulations, where the bending of the magnetic field is responsible the observed asymmetric accretion scenario. The filament formation in Musca is thus the result of two intersecting converging flows which are driven by the magnetic field bending due to a large-scale colliding HI flow that triggered the observed star formation in the full Chamaeleon-Musca complex.Finally, the kinematics of the high-mass star forming ridge DR21 and its surrounding gas are studied to compare low- and high-mass star formation. This shows a similar spatial and kinematic asymmetry as in Musca, which suggests that DR21 is formed by a giant molecular cloud (GMC) collision. However, it is also found for high-mass star formation in the DR21 cloud that gravity plays an important role on large scales (> 1 pc) while for Musca gravity only starts to dominate locally (r < 0.1-0.2 pc). So, due to the high density in the DR21 cloud after the GMC collision, gravity eventually drives the evolution of the compressed cloud for high-mass star forming regions. Kinematic observations of the full Cygnus-X north region show further indications of two interacting velocity components over the entire region, which indicates that a high-velocity (> 10 km/s) GMC collision can result in the formation of an OB association similar to OB2. These OB stars then form in gravitationally collapsing hubs and ridges due to the compression by the GMC collision., Pour comprendre la formation des étoiles, il faut étudier les processus physiques qui forment le gaz froid et dense dans le milieu interstellaire. Le télescope spatial Herschel a récemment démontré que la majorité du gaz froid et dense est formée de structures filamentaires (des filaments).Dans cette thèse, plusieurs raies de CO ont été observées avec le télescope APEX autour du filament de Musca. Ces observations ont été complémentées par des observations [CII] et [OI] avec le télescope SOFIA. La non-détection de [CII] démontre que le nuage de Musca est situé dans un champ de radiation UV faible (1 G0). Par contre, les observations de CO(4-3) avec APEX montrent qu'il y a du gaz CO chauffé (> 50 K) autour du filament que l'irradiation UV ne peut pas expliquer. La comparaison avec des modèles de chocs indique que l'émission CO(4-3) doit alors être le résultat d'un choc J à basse vitesse (< 4 km/s). L'analyse du spectre CO(4-3) montre aussi que l'émission venant du choc ressemble à une signature de choc d'accrétion. Cette observation suggère qu'un choc à basse vitesse, dû à une accrétion continue, est responsable de la formation du gaz dense et froid du filament de Musca.Ce scénario d'accrétion du filament de Musca est de plus étudié à grandes échelles dans les raies CO(2-1) et CO(1-0) obtenues avec les télescopes APEX et NANTEN2. Ces observations montrent un gradient de vitesse sur la crête de Musca qui est correlé avec le champ de vitesse autour du filament. L'analyse globale des observations de Musca montre une asymétrie à la fois spatiale et cinématique. Cette asymétrie est vue comme une forme en V dans le diagramme position-vitesse perpendiculaire au filament. L'inclusion d'observations du gaz neutre HI dans l'analyse confirme que Musca fait partie d'un nuage HI plus grand, le complex Chamaeleon-Musca. Le HI montre aussi que l'asymétrie cinématique est présente des grandes échelles du nuage HI jusqu'aux petites échelles de la crête du filament de Musca. En comparant le HI avec les vitesses CO de Cha I, Cha II et Cha III, on constate que l'asymétrie cinématique est présente pour toutes les régions denses du complexe de Chamaeleon-Musca. Ce scénario d'accrétion asymétrique, qui est observé, est reproduit dans des simulations d'une collision de nuages magnétisés. Dans ce scénario, c'est la déformation du champ magnétique qui est responsable de l'accrétion asymétrique. La formation du filament Musca serait ainsi due à la convergence de deux flots de matière guidée par la courbure du champ magnétique provoquée par la collision des nuages HI à grande échelle.Dans la dernière partie, la cinématique du nuage massif DR21, qui forme des étoiles massives, est étudiée pour comparer la formation des étoiles massives à celle des étoiles de faible masse. Le nuage DR21 montre une asymétrie en V similaire à celle de Musca, ce qui indique que le nuage DR21 est aussi formé par une collision de nuages moléculaires mais avec une vitesse de collision plus importante que pour Musca. Les observations indiquent de plus que la formation des étoiles massives dans le nuage DR21 serait la conséquence directe de la prédominance de la gravité à grande échelle (> 1 pc) du gaz dense en contraste avec Musca pour lequel la gravité ne dominerait qu'aux plus petites échelles (< 0.1-0.2 pc). L'analyse cinématique globale de toute la région du Cygne montre que toute la région résulte de la même collision de nuages. Cette observation indique que c'est une collision de nuages à grande vitesse (> 10 km/s) qui pourrait expliquer la formation d'une association d'étoiles OB de plusieurs milliers d'étoiles. Dans ce scénario, les étoiles massives (OB) se formeraient dans les structures denses et massives (hubs et ridges) formées aux convergences dues à la collision à grande vitesse de nuages, et où la gravité à grande échelles domine la cinématique et l'évolution du gaz dense.

International audience; Using data from the Planck satellite, we study the statistical properties of interstellar dust polarization at high Galactic latitudes around the south pole (b < -60°). Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a modelling framework of the polarized dust foreground for use in cosmic microwave background (CMB) component-separation procedures. We examine the Stokes I, Q, and U maps at 353 GHz, and particularly the statistical distribution of the polarization fraction (p) and angle (ψ), in order to characterize the ordered and turbulent components of the Galactic magnetic field (GMF) in the solar neighbourhood. The Q and U maps show patterns at large angular scales, which we relate to the mean orientation of the GMF towards Galactic coordinates (l0,b0) = (70° ± 5°,24° ± 5°). The histogram of the observed p values shows a wide dispersion up to 25%. The histogram of ψ has a standard deviation of 12° about the regular pattern expected from the ordered GMF. We build a phenomenological model that connects the distributions of p and ψ to a statistical description of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p0) of dust emission. To compute the Stokes parameters, we approximate the integration along the line of sight (LOS) as a sum over a set of N independent polarization layers, in each of which the turbulent component of the GMF is obtained from Gaussian realizations of a power-law power spectrum. We are able to reproduce the observed p and ψ distributions using a p0 value of 26%, a ratio of 0.9 between the strengths of the turbulent and mean components of the GMF, and a small value of N. The mean value of p (inferred from the fit of the large-scale patterns in the Stokes maps) is 12 ± 1%. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS. We emphasize the simplicity of our model (involving only a few parameters), which can be easily computed on the celestial sphere to produce simulated maps of dust polarization. Our work is an important step towards a model that can be used to assess the accuracy of component-separation methods in present and future CMB experiments designed to search the B mode CMB polarization from primordial gravity waves.