Numerical Aspects of 3D Stellar Winds

This paper explores and compares the pitfalls of modelling the three-dimensional wind of a spherical star with a cartesian grid. Several numerical methods are compared, using either uniform and stretched grid or adaptative mesh refinement (AMR). An additional numerical complication is added, when an orbiting planet is considered. In this case a rotating frame is added to the model such that the orbiting planet is at rest in the frame of work. The three-dimensional simulations are systematically compared to an equivalent two-dimensional, axisymmetric simulation. The comparative study presented here suggests to limit the rotation rate of the rotating frame below the rotating frame of the star and provides guidelines for further three-dimensional modelling of stellar winds in the context of close-in star-planet interactions.AS thanks T. Matsakos for discussions about the modelling of star-planet systems in 3D. This work was supported by the ANR 2011 Blanc Toupies and the ERC project STARS2 (207430). The authors acknowledge CNRS INSU/PNST and CNES/Solar Orbiter fundings. AS acknowledges support from the Canada’s Natural Sciences and Engineering Research Council and from the Canadian Institute of Theoretical Astrophysics (National fellow). We acknowledge access to supercomputers through GENCI (project 1623), Prace, and ComputeCanada infrastructures

Cool Stars and Space Weather

Stellar flares, winds and coronal mass ejections form the space weather. They are signatures of the magnetic activity of cool stars and, since activity varies with age, mass and rotation, the space weather that extra-solar planets experience can be very different from the one encountered by the solar system planets. How do stellar activity and magnetism influence the space weather of exoplanets orbiting main-sequence stars? How do the environments surrounding exoplanets differ from those around the planets in our own solar system? How can the detailed knowledge acquired by the solar system community be applied in exoplanetary systems? How does space weather affect habitability? These were questions that were addressed in the splinter session "Cool stars and Space Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In this paper, we present a summary of the contributions made to this session.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Eds G. van Belle & H. Harris, 13 pages, 1 figur

Modelling the Corona of HD 189733 in 3D

The braking of main sequence stars originates mainly from their stellar wind. The efficiency of this angular momentum extraction depends on the rotation rate of the star, the acceleration profile of the wind and the coronal magnetic field. The derivation of scaling laws parametrizing the stellar wind torque is important for our understanding of gyro-chronology and the evolution of the rotation rates of stars. In order to understand the impact of complex magnetic topologies on the stellar wind torque, we present three-dimensional, dynamical simulations of the corona of HD 189733. Using the observed complex topology of the magnetic field, we estimate how the torque associated with the wind scales with model parameters and compare those trends to previously published scaling laws.AS thank A. Vidotto for discussions about the modelling of the corona of HD 189733. This work was supported by the ANR 2011 Blanc Toupies and the ERC project STARS2 (207430). The authors acknowledge CNRS INSU/PNST and CNES/Solar Orbiter fundings. AS acknowledges support from the Canada’s Natural Sciences and Engineering Research Council and from the Canadian Institute of Theoretical Astrophysics (National fellow). We acknowledge access to supercomputers through GENCI (project 1623), Prace, and ComputeCanada infrastructures

Fermi~I particle acceleration in converging flows mediated by magnetic reconnection

Context. Converging flows with strong magnetic fields of different polarity can accelerate particles through magnetic reconnection. If the particle mean free path is longer than the reconnection layer is thick, but much shorter than the entire reconnection structure, the particle will mostly interact with the incoming flows potentially with a very low escape probability. Aims. We explore, in general and also in some specific scenarios, the possibility of particles to be accelerated in a magnetic reconnection layer by interacting only with the incoming flows. Methods. We characterize converging flows that undergo magnetic reconnection, and derive analytical estimates for the particle energy distribution, acceleration rate, and maximum energies achievable in these flows. We also discuss a scenario, based on jets dominated by magnetic fields of changing polarity, in which this mechanism may operate. Results. The proposed acceleration mechanism operates if the reconnection layer is much thinner than its transversal characteristic size, and the magnetic field has a disordered component. Synchrotron losses may prevent electrons from entering in this acceleration regime. The acceleration rate should be faster, and the energy distribution of particles harder than in standard diffusive shock acceleration. The interaction of obstacles with the innermost region of jets in active galactic nuclei and microquasars may be suitable sites for particle acceleration in converging flows.Comment: 4 pages, 2 figures, Reserch Note, in press, A&A (final version

Massive protostars as gamma-ray sources

Massive protostars have associated bipolar outflows with velocities of hundreds of km s$^{-1}$. Such outflows can produce strong shocks when interact with the ambient medium leading to regions of non-thermal radio emission. We aim at exploring under which conditions relativistic particles are accelerated at the terminal shocks of the protostellar jets and can produce significant gamma-ray emission. We estimate the conditions necessary for particle acceleration up to very high energies and gamma-ray production in the non-thermal hot spots of jets associated with massive protostars embedded in dense molecular clouds. We show that relativistic Bremsstrahlung and proton-proton collisions can make molecular clouds with massive young stellar objects detectable by the {\it Fermi}{} satellite at MeV-GeV energies and by Cherenkov telescope arrays in the GeV-TeV range. Gamma-ray astronomy can be used to probe the physical conditions in star forming regions and particle acceleration processes in the complex environment of massive molecular clouds.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in Astronomy and Astrophysic

Massive protostars as gamma-ray sources

Massive protostars have associated bipolar outflows with velocities of hundreds of km s$^{-1}$. Such outflows can produce strong shocks when interact with the ambient medium leading to regions of non-thermal radio emission. We aim at exploring under which conditions relativistic particles are accelerated at the terminal shocks of the protostellar jets and can produce significant gamma-ray emission. We estimate the conditions necessary for particle acceleration up to very high energies and gamma-ray production in the non-thermal hot spots of jets associated with massive protostars embedded in dense molecular clouds. We show that relativistic Bremsstrahlung and proton-proton collisions can make molecular clouds with massive young stellar objects detectable by the {\it Fermi}{} satellite at MeV-GeV energies and by Cherenkov telescope arrays in the GeV-TeV range. Gamma-ray astronomy can be used to probe the physical conditions in star forming regions and particle acceleration processes in the complex environment of massive molecular clouds.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in Astronomy and Astrophysic

The theory of pulsar winds and nebulae

We review current theoretical ideas on pulsar winds and their surrounding nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the striped wind, together with models of magnetic dissipation are discussed. It is shown that the observational signature of this dissipation is likely to be point-like, rather than extended, and that pulsed emission may be produced. The possible pulse shapes and polarisation properties are described. Particle acceleration at the termination shock of the wind is discussed, and it is argued that two distinct mechanisms must be operating, with the first-order Fermi mechanism producing the high-energy electrons (above 1 TeV) and either magnetic annihilation or resonant absorption of ion cyclotron waves responsible for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and pulsars, 40 years after the discovery", ed W.Becke

Magnetic connectivity of the ecliptic plane within 0.5 AU : PFSS modeling of the first PSP encounter

We compare magnetic field measurements taken by the FIELDS instrument on Parker Solar Probe (PSP) during its first solar encounter to predictions obtained by Potential Field Source Surface (PFSS) modeling. Ballistic propagation is used to connect the spacecraft to the source surface. Despite the simplicity of the model, our results show striking agreement with PSPs first observations of the heliospheric magnetic field from 0.5 AU (107.5 Rs) down to 0.16 AU (35.7 Rs). Further, we show the robustness of the agreement is improved both by allowing the photospheric input to the model to vary in time, and by advecting the field from PSP down to the PFSS model domain using in situ PSP/SWEAP measurements of the solar wind speed instead of assuming it to be constant with longitude and latitude. We also explore the source surface height parameter (RSS) to the PFSS model finding that an extraordinarily low source surface height (1.3-1.5Rs) predicts observed small scale polarity inversions which are otherwise washed out with regular modeling parameters. Finally, we extract field line traces from these models. By overlaying these on EUV images we observe magnetic connectivity to various equatorial and mid-latitude coronal holes indicating plausible magnetic footpoints and offering context for future discussions of sources of the solar wind measured by PSP

Radiation from sub-Larmor scale magnetic fields

Spontaneous rapid growth of strong magnetic fields is rather ubiquitous in high-energy density environments ranging from astrophysical sources (e.g., gamma-ray bursts and relativistic shocks), to reconnection, to laser-plasma interaction laboratory experiments, where they are produced by kinetic streaming instabilities of the Weibel type. Relativistic electrons propagating through these sub-Larmor-scale magnetic fields radiate in the jitter regime, in which the anisotropy of the magnetic fields and the particle distribution have a strong effect on the produced radiation. Here we develop the general theory of jitter radiation, which includes (i) anisotropic magnetic fields and electron velocity distributions, (ii) the effects of trapped electrons and (iii) extends the description to large deflection angles of radiating particles thus establishing a cross-over between the classical jitter and synchrotron regimes. Our results are in remarkable agreement with the radiation spectra obtained from particle-in-cell simulations of the classical Weibel instability. Particularly interesting is the onset of the field growth, when the transient hard synchrotron-violating spectra are common as a result of the dominant role of the trapped population. This effect can serve as a distinct observational signature of the violent field growth in astrophysical sources and lab experiments. It is also interesting that a system with small-scale fields tends to evolve toward the small-angle jitter regime, which can, under certain conditions, dominate the overall emission of a source.Comment: 13 pages, 12 figures, matches published versio