Protostars: Forges of cosmic rays? Padovani, M.; Marcowith, A.; Hennebelle, P. ...
Astronomy and astrophysics (Berlin),
06/2016, Letnik:
590
Journal Article
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Context. Galactic cosmic rays are particles presumably accelerated in supernova remnant shocks that propagate in the interstellar medium up to the densest parts of molecular clouds, losing energy and ...their ionisation efficiency because of the presence of magnetic fields and collisions with molecular hydrogen. Recent observations hint at high levels of ionisation and at the presence of synchrotron emission in protostellar systems, which leads to an apparent contradiction. Aims. We want to explain the origin of these cosmic rays accelerated within young protostars as suggested by observations. Methods. Our modelling consists of a set of conditions that has to be satisfied in order to have an efficient cosmic-ray acceleration through diffusive shock acceleration. We analyse three main acceleration sites (shocks in accretion flows, along the jets, and on protostellar surfaces), then we follow the propagation of these particles through the protostellar system up to the hot spot region. Results. We find that jet shocks can be strong accelerators of cosmic-ray protons, which can be boosted up to relativistic energies. Other promising acceleration sites are protostellar surfaces, where shocks caused by impacting material during the collapse phase are strong enough to accelerate cosmic-ray protons. In contrast, accretion flow shocks are too weak to efficiently accelerate cosmic rays. Though cosmic-ray electrons are weakly accelerated, they can gain a strong boost to relativistic energies through re-acceleration in successive shocks. Conclusions. We suggest a mechanism able to accelerate both cosmic-ray protons and electrons through the diffusive shock acceleration mechanism, which can be used to explain the high ionisation rate and the synchrotron emission observed towards protostellar sources. The existence of an internal source of energetic particles can have a strong and unforeseen impact on the ionisation of the protostellar disc, on the star and planet formation processes, and on the formation of pre-biotic molecules.
Supernova remnants are believed to be the main sources of galactic cosmic rays (CR). Within this framework, particles are accelerated at supernova remnant shocks and then released in the interstellar ...medium. The mechanism through which CRs are released and the way in which they propagate still remain open issues. The main difficulty is the high non-linearity of the problem: CRs themselves excite the magnetic turbulence that confines them close to their sources. We solve numerically the coupled differential equations describing the evolution in space and time of the escaping particles and of the waves generated through the CR streaming instability. The warm ionized and warm neutral phases of the interstellar medium are considered. These phases occupy the largest fraction of the disc volume, where most supernovae explode, and are characterized by the significant presence of neutral particles. The friction between those neutrals and ions results in a very effective wave damping mechanism. It is found that streaming instability affects the propagation of CRs even in the presence of ion-neutral friction. The diffusion coefficient can be suppressed by more than a factor of ∼2 over a region of few tens of pc around the remnant. The suppression increases for smaller distances. The propagation of ≈10 GeV particles is affected for several tens of kiloyears after escape, while ≈1 TeV particles are affected for few kiloyears. This might have a great impact on the interpretation of gamma-ray observations of molecular clouds located in the vicinity of supernova remnants.
Context. The propagation of cosmic rays or energetic charged particles in magnetized turbulence is a complex problem which involves non-linear wave-particle interactions and chaotic magnetic field ...lines transport. This problem has been addressed until recently using either analytical calculations or simulations using prescribed turbulence models. With the advent of super computers it is now possible to investigate energetic charged particle propagation using direct computation of electromagnetic fields. This is in particular the case for high-energy particles propagation in magnetohydrodynamic turbulence. Aims. This work has the main objective to provide a detailed investigation of cosmic ray propagation in magnetohydrodynamic turbulent fields generated by forcing the fluid velocity field at large scales. It provides a derivation of the particle mean free path dependences in terms of the turbulence level described by the Alfvénic Mach number and in terms of the particle rigidity. Methods. We use an upgrade version of the magnetohydrodynamic code RAMSES which includes a forcing module and a kinetic module and solve the Lorentz equation for each particle. The simulations are performed using a 3 dimension periodical box in the test-particle and magnetostatic limits. The forcing module is implemented using an Ornstein-Uhlenbeck process. An ensemble average over a large number of particle trajectories is applied to reconstruct the particle mean free paths. Results. We derive the cosmic ray mean free paths in terms of the Alfvénic Mach numbers and particle reduced rigidities in different turbulence forcing geometries. The reduced particle rigidity is ρ = rL/L where rL is the particle Larmor radius and L is the simulation box length related to the turbulence coherence or injection scale Linj by L ~ 5 Linj. We have investigated with a special attention compressible and solenoidal forcing geometries. Conclusions. We find that compressible forcing solutions are compatible with the quasi-linear theory or more advanced non-linear theories which predict a rigidity dependence as ρ1/2 or ρ1/3. Solenoidal forcing solutions at least at low or moderate Alfvénic numbers are not compatible with the above theoretical expectations and require more refined arguments to be interpreted. It appears especially for Alfvénic Mach numbers close to one that the wandering of field lines controls perpendicular mean free path solutions whatever the forcing geometry.
Context.
The degree of coupling between the gas and the magnetic field during the collapse of a core and the subsequent formation of a disk depends on the assumed dust size distribution.
Aims.
We ...study the impact of grain–grain coagulation on the evolution of magnetohydrodynamic (MHD) resistivities during the collapse of a prestellar core.
Methods.
We use a 1D model to follow the evolution of the dust size distribution, out-of-equilibrium ionisation state, and gas chemistry during the collapse of a prestellar core. To compute the grain–grain collisional rate, we consider models for both random and systematic, size-dependent, velocities. We include grain growth through grain–grain coagulation and ice accretion, but ignore grain fragmentation.
Results.
Starting with a Mathis-Rumpl-Nordsieck (MRN) size distribution (Mathis et al. 1977, ApJ, 217, 425), we find that coagulation in grain–grain collisions generated by hydrodynamical turbulence is not efficient at removing the smallest grains and, as a consequence, does not have a large effect on the evolution of the Hall and ambipolar diffusion MHD resistivities, which still drop significantly during the collapse like in models without coagulation. The inclusion of systematic velocities, possibly induced by the presence of ambipolar diffusion, increases the coagulation rate between small and large grains, removing small grains earlier in the collapse and therefore limiting the drop in the Hall and ambipolar diffusion resistivities. At intermediate densities (
n
H
~ 10
8
cm
−3
), the Hall and ambipolar diffusion resistivities are found to be higher by 1 to 2 orders of magnitude in models with coagulation than in models where coagulation is ignored, and also higher than in a toy model without coagulation where all grains smaller than 0.1
μ
m would have been removed in the parent cloud before the collapse.
Conclusions.
When grain drift velocities induced by ambipolar diffusion are included, dust coagulation happening during the collapse of a prestellar core starting from an initial MRN dust size distribution appears to be efficient enough to increase the MHD resistivities to the values necessary to strongly modify the magnetically regulated formation of a planet-forming disk. A consistent treatment of the competition between fragmentation and coagulation is, however, necessary before reaching firm conclusions.
This article aims at establishing new benchmark scenarios for Galactic cosmic-ray propagation in the GV-TV rigidity range, based on fits to the AMS-02 boron to carbon ratio (B/C) data with the usine ...v3.5 propagation code. We employ a new fitting procedure, cautiously taking into account data systematic error correlations in different rigidity bins and considering Solar modulation potential and leading nuclear cross section as nuisance parameters. We delineate specific low, intermediate, and high-rigidity ranges that can be related to both features in the data and peculiar microphysics mechanisms resulting in spectral breaks. We single out a scenario which yields excellent fits to the data and includes all the presumably relevant complexity, the BIG model. This model has two limiting regimes: (i) the SLIM model, a minimal diffusion-only setup, and (ii) the QUAINT model, a convection-reacceleration model where transport is tuned by nonrelativistic effects. All models lead to robust predictions in the high-energy regime (≳10 GV), i.e., independent of the propagation scenario: at 1σ, the diffusion slope δ is 0.43–0.53, whereas K10, the diffusion coefficient at 10 GV, is 0.26–0.36 kpc2 Myr−1; we confirm the robustness of the high-energy break, with a typical value Δh∼0.2. We also find a hint for a similar (reversed) feature at low rigidity around the B/C peak (∼4 GV) which might be related to some effective damping scale in the magnetic turbulence.
Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: ...magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulæ, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. A particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics.
Aims.
We aim to elucidate cosmic ray (CR) propagation in the weakly ionized environments of supernova remnants (SNRs) basing our analysis on the cosmic ray cloud (CRC) model.
Methods.
We solved two ...transport equations simultaneously: one for the CR pressure and one for the Alfvén wave energy density where CRs are initially confined in the SNR shock. Cosmic rays trigger a streaming instability and produce slab-type resonant Alfvén modes. The self-generated turbulence is damped by ion-neutral collisions and by noncorrelated interaction with Alfvén modes generated at large scales.
Results.
We show that CRs leaking in cold dense phases such as those found in cold neutral medium (CNM) and diffuse molecular medium (DiM) can still be confined over distances of a few tens of parsecs from the CRC center for a few thousand years. At 10 TeV, CR diffusion can be suppressed by two or three orders of magnitude. This effect results from a reduced ion-neutral collision damping in the decoupled regime. We calculate the grammage of CRs in these environments. We find that in both single and multi-phase setups at 10 GeV, CNM and DiM media can produce grammage in the range 10–20 g cm
−2
in the CNM and DiM phases. At 10 TeV, because of nonlinear propagation the grammage increases to values in the range 0.5–20 g cm
−2
in these two phases. We also present preliminary calculations in inhomogeneous interstellar medium combining two or three different phases where we obtain the same trends.
Context. Supernova remnants are believed to be a major source of energetic particles (cosmic rays) on the Galactic scale. Since their progenitors, namely the most massive stars, are commonly found ...clustered in OB associations, one has to consider the possibility of collective effects in the acceleration process. Aims. We investigate the shape of the spectrum of high-energy protons produced inside the superbubbles blown around clusters of massive stars. Methods. We embed simple semi-analytical models of particle acceleration and transport inside Monte Carlo simulations of OB associations timelines. We consider regular acceleration (Fermi 1 process) at the shock front of supernova remnants, as well as stochastic reacceleration (Fermi 2 process) and escape (controlled by magnetic turbulence) occurring between the shocks. In this first attempt, we limit ourselves to linear acceleration by strong shocks and neglect proton energy losses. Results. We observe that particle spectra, although highly variable, have a distinctive shape because of the competition between acceleration and escape: they are harder at the lowest energies (index s<4) and softer at the highest energies (s>4). The momentum at which this spectral break occurs depends on the various bubble parameters, but all their effects can be summarized by a single dimensionless parameter, which we evaluate for a selection of massive star regions in the Galaxy and the LMC. Conclusions. The behaviour of a superbubble in terms of particle acceleration critically depends on the magnetic turbulence: if B is low then the superbubble is simply the host of a collection of individual supernovae shocks, but if B is high enough (and the turbulence index is not too high), then the superbubble acts as a global accelerator, producing distinctive spectra, that are potentially very hard over a wide range of energies, which has important implications on the high-energy emission from these objects.
ABSTRACT
Some core-collapse supernovae are likely to be efficient cosmic ray accelerators up to the PeV range, and therefore, to potentially play an important role in the overall Galactic cosmic ray ...population. The TeV gamma-ray domain can be used to study particle acceleration in the multi-TeV and PeV range. This motivates the study of the detectability of such supernovae by current and future gamma-ray facilities. The gamma-ray emission of core-collapse supernovae strongly depends on the level of the two-photon annihilation process: high-energy gamma-ray photons emitted at the expanding shock wave following the supernova explosion can interact with soft photons from the supernova photosphere through the pair production channel, thereby strongly suppressing the flux of gamma-rays leaving the system. In the case of SN 1993J, whose photospheric and shock-related parameters are well measured, we calculate the temporal evolution of the expected gamma-ray attenuation by accounting for the temporal and geometrical effects. We find the attenuation to be of about 10 orders of magnitude in the first few days after the supernova explosion. The probability of detection of a supernova similar to SN 1993J with the Cherenkov Telescope Array is highest if observations are performed either earlier than 1 d, or later than 10 d after the explosion, when the gamma-ray attenuation decreases to about two orders of magnitude.
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