Martin Lemoine

Transport of cosmic rays in chaotic magnetic fields

The transport of charged particles in disorganised magnetic fields is an important issue which concerns the propagation of cosmic rays of all energies in a variety of astrophysical environments, such as the interplanetary, interstellar and even extra-galactic media, as well as the efficiency of Fermi acceleration processes. We have performed detailed numerical experiments using Monte-Carlo simulations of particle propagation in stochastic magnetic fields in order to measure the parallel and transverse spatial diffusion coefficients and the pitch angle scattering time as a function of rigidity and strength of the turbulent magnetic component. We confirm the extrapolation to high turbulence levels of the scaling predicted by the quasi-linear approximation for the scattering frequency and parallel diffusion coefficient at low rigidity. We show that the widely used Bohm diffusion coefficient does not provide a satisfactory approximation to diffusion even in the extreme case where the mean field vanishes. We find that diffusion also takes place for particles with Larmor radii larger than the coherence length of the turbulence. We argue that transverse diffusion is much more effective than predicted by the quasi-linear approximation, and appears compatible with chaotic magnetic diffusion of the field lines. We provide numerical estimates of the Kolmogorov length and magnetic line diffusion coefficient as a function of the level of turbulence. Finally we comment on applications of our results to astrophysical turbulence and the acceleration of high energy cosmic rays in supernovae remnants, in super-bubbles, and in jets and hot spots of powerful radio-galaxies.

Abundances of Deuterium, Nitrogen, and Oxygen in the Local Interstellar Medium: Overview of First Results from the FUSE Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D i ,N i, and O i along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 to 179 pc. Five of the sight lines are within the Local Bubble, and two penetrate the surrounding H i wall. Reliable values of N(H i) were determined for five of the sight lines from Hubble Space Telescope (HST) data, International Ultraviolet Explorer (IUE) data, and published Extreme Ultraviolet Explorer (EUVE) measurements. The weighted mean of D i/H i for these five sight lines is ð1:52 � 0:08 Þ� 10 � 5 (1 � uncertainty in the mean). It is likely that the D i/H i ratio in the Local Bubble has a single value. The D i/O i ratio for the five sight lines within the Local Bubble is ð3:76 � 0:20 Þ� 10 � 2 .I t is likely that O i column densities can serve as a proxy for H i in the Local Bubble. The weighted mean for O i/H i for the seven FUSE sight lines is ð3:03 � 0:21 Þ� 10 � 4 , comparable to the weighted mean ð3:43 � 0:15 Þ� 10 � 4 reported for 13 sight lines probing larger distances and higher column densities. The FUSE weighted mean of N i/H i for five sight lines is half that reported by Meyer and colleagues for seven sight lines with larger distances and higher column densities. This result combined with the variability of O i/N i (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O i ,N i cannot be used as a proxy for H i or as a metallicity indicator in the LISM. Subject headings: cosmology: observations — Galaxy: abundances — ISM: abundances — ISM: evolution — ultraviolet: ISM

Deuterium Abundance toward WD 2211–495: Results from the FUSE Mission*

We present a deuterium abundance analysis of the line of sight toward the white dwarf WD 2211-495 observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). Numerous interstellar lines are detected on the continuum of the stellar spectrum. A thorough analysis was performed through the simultaneous fit of interstellar absorption lines detected in the four FUSE channels of multiple observations with different slits. We excluded all saturated lines in order to reduce possible systematic errors on the column density measurements. We report the determination of the average interstellar D/O and D/N ratios along this line of sight at the 95% confidence level: D/O = (4.0 ? 1.2) ? 10-2 and D/N = (4.4 ? 1.3) ? 10-1. In conjunction with FUSE observations of other nearby sight lines, the results of this study will allow a deeper understanding of the present-day abundance of deuterium in the local interstellar medium and its evolution with time.

Deuterium Abundance toward G191‐B2B: Results from the FUSE Mission

High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187 A ˚ , were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). These data were used in conjunction with existing high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) obser- vations to evaluate the total H i ,D i ,O i, and N i column densities along the line of sight. Previous determina- tions of N(D i) based upon GHRS and STIS observations were controversial as a result of the saturated strength of the D i Lyline. In the present analysis the column density of D i has been measured using only the unsaturated Lyand Lylines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are log NðD i Þ¼ 13:40 � 0:07, log NðO i Þ¼ 14:86 � 0:07, and log NðN i Þ¼ 13:87 � 0:07, quoted with 2 � uncertainties. The measurement of the H i column density by profile fitting of the Lyline has been found to be uncertain. If additional weak, hot interstellar components are added to the three detected clouds along the line of sight, the H i column den- sity can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Ly� are excellent. The new estimate of N(H i) toward G191-B2B reads log NðH i Þ¼ 18:18 � 0:18 (2 � ), so that the average D/H ratio on the line of sight is D=H ¼ 1:66 þ0:9 � 0:6 � 10 � 5 (2 � ). Subject headings: ISM: abundances — ISM: clouds — stars: individual (G191-B2B) — ultraviolet: ISM

Particle Transport in Tangled Magnetic Fields and Fermi Acceleration at Relativistic Shocks

This Letter presents a new method of Monte Carlo simulations of test particle Fermi acceleration at relativistic shocks. The particle trajectories in tangled magnetic fields are integrated out exactly from entry to exit through the shock, and the conditional probability of return as a function of ingress and egress pitch angles is constructed by Monte Carlo iteration. These upstream and downstream probability laws are then used in conjunction with the energy gain formula at shock crossing to reproduce Fermi acceleration. For pure Kolmogorov magnetic turbulence upstream and downstream, the spectral index is found to evolve smoothly from s = 2.09 ± 0.02 for mildly relativistic shocks with Lorentz factor Γs = 2 to s 2.26 ± 0.04 in the ultrarelativistic limit Γs 1. The energy gain is ~Γ at first shock crossing, and ~2 in all subsequent cycles, as anticipated by Gallant & Achterberg. The acceleration timescale is found to be as short as a fraction of Larmor time when Γs 1.

Deficiency of molecular hydrogen in the disk of beta Pictoris.

Molecular hydrogen (H2) is by far the most abundant material from which stars, protoplanetary disks and giant planets form, but it is difficult to detect directly. Infrared emission lines from H2 have recently been reported towards β Pictoris, a star harbouring a young planetary system. This star is surrounded by a dusty ‘debris disk’ that is continuously replenished either by collisions between asteroidal objects or by evaporation of ices on Chiron-like objects. A gaseous disk has also been inferred from absorption lines in the stellar spectrum. Here we present the far-ultraviolet spectrum of β Pictoris, in which H2 absorption lines are not seen. This allows us to set a very low upper limit on the column density of H2: N(H2)u2009≤u20091018u2009cm-2. This non-detection is puzzling when compared to the quantity of H2 inferred from thexa0infrared observations, but it does show that H2 is not in thexa0disk on the direct line of sight. Carbon monoxide (CO) has been seen in absorption against the star, yielding a ratio of CO/H2u2009>u20096u2009×u200910-4. As CO would be destroyed under ambient conditions in about 200 years (refs 9, 11), our result demonstrates that the CO in the disk arises from evaporation of planetesimals.

Anisotropy vs chemical composition at ultra-high energies

This paper proposes and discusses a test of the chemical composition of ultra-high energy cosmic rays that relies on the anisotropy patterns measured as a function of energy. In particular, we show that if one records an anisotropy signal produced by heavy nuclei of charge Z above an energy Ethr, one should record an even stronger (possibly much stronger) anisotropy at energies >Ethr/Z due to the proton component that is expected to be associated with the sources of the heavy nuclei. This conclusion remains robust with respect to the parameters characterizing the sources and it does not depend at all on the modelling of astrophysical magnetic fields. As a concrete example, we apply this test to the most recent data of the Pierre Auger Observatory. Assuming that the anisotropy reported above 55 EeV is not a statistical accident, and that no significant anisotropy has been observed at energies 10 EeV, we show that the apparent clustering toward Cen A cannot be attributed to heavy nuclei. Similar conclusions are drawn regarding the apparent excess correlation with nearby active galactic nuclei. We then discuss a robust lower bound to the magnetic luminosity that a source must possess in order to be able to accelerate particles of charge Z up to 100 EeV, LB 1045 Z−2 erg/s. Using this bound in conjunction with the above conclusions, we argue that the current PAO data does not support the model of cosmic ray origin in active radio-quiet or even radio-loud galaxies. Finally, we demonstrate that the apparent clustering in the direction of Cen A can be explained by the contribution of the last few gamma-ray bursts or magnetars in the host galaxy thanks to the scattering of the cosmic rays on the magnetized lobes.

On electromagnetic instabilities at ultra‐relativistic shock waves

Recent work on Fermi acceleration at ultra-relativistic shock waves has demonstrated the need for strong amplification of the background magnetic field on very short scales. Amplification of the magnetic field by several orders of magnitude has also been suggested by observations of gamma-ray bursts afterglows, both in downstream and upstream plasmas. This paper addresses this issue of magnetic field generation in a relativistic shock precursor through micro-instabilities. In a generic superluminal configuration, the level of magnetization of the upstream plasma turns out to be a crucial parameter, notably because the length scale of the shock precursor is limited by the Larmor rotation of the accelerated particles in the background magnetic field and by the speed of the shock wave. We discuss in detail and calculate the growth rates of the following beam plasma instabilities seeded by the accelerated and reflected particle populations: for an unmagnetized shock, the Weibel and filamentation instabilities, as well as the Cerenkov resonant instabilities with electrostatic modes; for a magnetized shock, the Weibel instability and the resonant Cerenkov instabilities with the longitudinal electrostatic modes, as well as the Alfven, Whisler and extraordinary modes. All these instabilities are generated upstream, then they are transmitted downstream. The modes excited by Cerenkov resonant instabilities take on particular importance with respect to the magnetization of the downstream medium since, being plasma eigenmodes, they have a longer lifetime than the Weibel modes. We discuss the main limitation of the wave growth associated with the length of precursor and the magnetization of the upstream medium for both oblique and parallel relativistic shock waves. We also characterize the proper conditions to obtain Fermi acceleration at ultra-relativistic shock waves: for superluminal shock waves, the Fermi process works for values of the magnetization parameter below some critical value, and there is an intrinsic limitation of the achievable cosmic ray energy depending on the ratio of the magnetization to its critical value. We recover results of most recent particle-in-cell simulations and conclude with some applications to astrophysical cases of interest. In particular, Fermi acceleration in pulsar winds is found to be unlikely whereas its development appears to hinge on the level of upstream magnetization in the case of ultra-relativistic gamma-ray burst external shock waves.

Extragalactic magnetic fields and the second knee in the cosmic-ray spectrum

Recent work suggests that the cosmic-ray spectrum may be dominated by Galactic sources up to

On Fermi acceleration and magnetohydrodynamic instabilities at ultra-relativistic magnetized shock waves

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