A systematic study of radiative torque grain alignment in the diffuse interstellar medium

Context. Analyses of Planck data have demonstrated that the grain alignment efficiency is almost constant in the diffuse and translucent interstellar medium (ISM). Aims. We aim to test whether the radiative torque (RAT) theory is compatible with these new observational constraints on grain alignment. Methods. We combine a numerical magnetohydrodynamical simulation with a state-of-the-art radiative transfer post-processing code POLARIS which incorporates a physical dust model and the detailed physics of grain alignment by RATs. A dust model based on two distinct power-law-sized distributions of spherical graphite grains and oblate silicate grains was designed to reproduce the mean spectral dependence of extinction and polarization observed in the diffuse ISM. From a simulation of interstellar turbulence obtained with the adaptive-mesh-refinement code RAMSES, we extracted a data cube with physical conditions representative of the diffuse ISM. We post-process the RAMSES cube with POLARIS to compute the grain temperature and alignment efficiency in each cell of the cube. Finally, we simulate synthetic dust emission and polarization observations. Results. In our simulation, the grain alignment efficiency is well-correlated with the gas pressure, but not with the radiative torque intensity. Because of the low dust extinction in our simulation, the magnitude of the radiative torque varies little, decreasing only for column densities larger than 1022 cm−2. In comparing our synthetic maps with those obtained assuming a uniform alignment efficiency, we find no systematic difference and very small random differences. The dependencies of the polarization fraction p with the column density NH or with the dispersion in polarization angle S are also similar in both cases. The drop of grain alignment produced by the RAT model in the denser cells of the data cube does not significantly affect the patterns of the synthetic polarization maps, the polarization signal being dominated by the line-of-sight and beam integration of the geometry of the magnetic field. If a star is artificially inserted at the center of the simulation, the polarization fraction is increased everywhere, with no specific pattern around the star. The angle-dependence of the RAT efficiency is not observed in simulated maps and where the magnetic field is artificially set to a uniform configuration in the plane of the sky, it is only seen to be very weak in the optimal configuration. Conclusions. The RAT alignment theory is found to be compatible with the Planck polarization data for the diffuse and translucent ISM in the sense that both uniform alignment and RAT alignment lead to very similar simulated maps. To further test the predictions of the RAT theory in an environment where an important drop of grain alignment is expected, high-resolution polarization observations of dense regions must be confronted with numerical simulations sampling high-column densities (NH > 1022 cm−2) through dense clouds, given a sufficient statistical basis.

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Zooming in on the Formation of Protoplanetary Disks

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313008107 ◽

2013 ◽

Vol 8 (S299) ◽

pp. 131-135 ◽

Cited By ~ 3

Author(s):

Åke Nordlund ◽

Troels Haugbølle ◽

Michael Küffmeier ◽

Paolo Padoan ◽

Aris Vasileiades

Keyword(s):

Magnetic Field ◽

Energy Efficient ◽

Adaptive Mesh Refinement ◽

Accretion Rate ◽

Magnetic Energy ◽

Protoplanetary Disks ◽

Adaptive Mesh ◽

Gravitational Energy ◽

Solar Mass ◽

The Magnetic Field

AbstractWe use the adaptive mesh refinement code RAMSES to model the formation of protoplanetary disks in realistic star formation environments. The resolution scales over up to 29 powers of two (~ 9 orders of magnitude) covering a range from outer scales of 40 pc to inner scales of 0.015 AU. The accretion rate from a 1.5 solar mass envelope peaks near 10−4 M⊙ about 6 kyr after sink particle formation and then decays approximately exponentially, reaching 10−6 M⊙ in 100 kyr. The models suggest universal scalings of physical properties with radius during the main accretion phase, with kinetic and / or magnetic energy in approximate balance with gravitational energy. Efficient accretion is made possible by the braking action of the magnetic field, which nevertheless allows a near-Keplerian disk to grow to a 100 AU size. The magnetic field strength ranges from more than 10 G at 0.1 AU to less than 1 mG at 100 AU, and drives a time dependent bipolar outflow, with a collimated jet and a broader disk wind.

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Magnetized interstellar molecular clouds – II. The large-scale structure and dynamics of filamentary molecular clouds

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz653 ◽

2019 ◽

Vol 485 (4) ◽

pp. 4509-4528 ◽

Cited By ~ 7

Author(s):

Pak Shing Li ◽

Richard I Klein

Keyword(s):

Magnetic Field ◽

Strong Magnetic Field ◽

Adaptive Mesh Refinement ◽

Molecular Clouds ◽

Large Scale ◽

Velocity Structure ◽

Strong Support ◽

Adaptive Mesh ◽

Dark Cloud ◽

The Magnetic Field

Abstract We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shaped configuration that is strikingly similar to the infrared dark cloud SDC13, leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (${{\cal M}_{\rm A}}\sim 1$) is crucial for maintaining a long and slender filamentary cloud for a long period of time ∼0.5 Myr.

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Shock-accelerated cosmic rays and streaming instability in the adaptive mesh refinement code Ramses

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936275 ◽

2019 ◽

Vol 631 ◽

pp. A121 ◽

Cited By ~ 3

Author(s):

Yohan Dubois ◽

Benoît Commerçon ◽

Alexandre Marcowith ◽

Loann Brahimi

Keyword(s):

Magnetic Field ◽

Cosmic Rays ◽

Interstellar Medium ◽

Galaxy Evolution ◽

Adaptive Mesh Refinement ◽

Mesh Refinement ◽

Adaptive Mesh ◽

Shock Acceleration ◽

Streaming Instability ◽

Background Magnetic Field

Cosmic rays (CRs) are thought to play a dynamically important role in several key aspects of galaxy evolution, including the structure of the interstellar medium, the formation of galactic winds, and the non-thermal pressure support of halos. We introduce a numerical model solving for the CR streaming instability and acceleration of CRs at shocks with a fluid approach in the adaptive mesh refinement code RAMSES. CR streaming is solved with a diffusion approach and its anisotropic nature is naturally captured. We introduce a shock finder for the RAMSES code that automatically detects shock discontinuities in the flow. Shocks are the loci for CR injection, and their efficiency of CR acceleration is made dependent on the upstream magnetic obliquity according to the diffuse shock acceleration mechanism. We show that the shock finder accurately captures shock locations and estimates the shock Mach number for several problems. The obliquity-dependent injection of CRs in the Sedov solution leads to situations where the supernova bubble exhibits large polar caps (homogeneous background magnetic field), or a patchy structure of the CR distribution (inhomogeneous background magnetic field). Finally, we combine both accelerated CRs with streaming in a simple turbulent interstellar medium box, and show that the presence of CRs significantly modifies the structure of the gas.

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Cosmic magnetic fields with masclet: an application to galaxy clusters

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa877 ◽

2020 ◽

Vol 494 (2) ◽

pp. 2706-2717

Author(s):

Vicent Quilis ◽

José-María Martí ◽

Susana Planelles

Keyword(s):

Magnetic Field ◽

Galaxy Clusters ◽

Adaptive Mesh Refinement ◽

Adaptive Mesh ◽

Pure Compression ◽

Ideal Magnetohydrodynamics ◽

Relative Errors ◽

Cluster Values ◽

The Magnetic Field ◽

The Ideal

ABSTRACT We describe and test a new version of the adaptive mesh refinement cosmological code masclet. The new version of the code includes all the ingredients of its previous version plus a description of the evolution of the magnetic field under the approximation of the ideal magnetohydrodynamics (MHD). To preserve the divergence-free condition of MHD, the original divergence cleaning algorithm of Dedner et al. (2002) is implemented. We present a set of well-known 1D and 2D tests, such as several shock tube problems, the fast rotor, and the Orszag–Tang vortex. The performance of the code in all the tests is excellent with estimated median relative errors of ∇ · B in the 2D tests smaller than 5 × 10−5 for the fast rotor test, and 5 × 10−3 for the Orszag–Tang vortex. As an astrophysical application of the code, we present a simulation of a cosmological box of 40 comoving Mpc side length in which a primordial uniform comoving magnetic field of strength 0.1 nG is seeded. The simulation shows how the magnetic field is channelled along the filaments of gas and is concentrated and amplified within galaxy clusters. Comparison with the values expected from pure compression reveals an additional amplification of the magnetic field caused by turbulence in the central region of the cluster. Values of the order of ∼1µG are obtained in clusters at z ∼ 0 with median relative errors of ∇ · B below 0.4 per cent. The implications of a proper description of the dynamics of the magnetic field and their possible observational counterparts in future facilities are discussed.

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Planck2018 results

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833885 ◽

2020 ◽

Vol 641 ◽

pp. A12 ◽

Cited By ~ 16

Author(s):

◽

N. Aghanim ◽

Y. Akrami ◽

M. I. R. Alves ◽

M. Ashdown ◽

...

Keyword(s):

Magnetic Field ◽

Molecular Clouds ◽

Dispersion Function ◽

Polarization Angle ◽

Galactic Latitude ◽

Grain Alignment ◽

Gould Belt ◽

Systematic Trend ◽

Dust Temperature ◽

Polarization Fraction

Observations of the submillimetre emission from Galactic dust, in both total intensityIand polarization, have received tremendous interest thanks to thePlanckfull-sky maps. In this paper we make use of such full-sky maps of dust polarized emission produced from the third public release ofPlanckdata. As the basis for expanding on astrophysical studies of the polarized thermal emission from Galactic dust, we present full-sky maps of the dust polarization fractionp, polarization angleψ, and dispersion function of polarization angles 𝒮. The joint distribution (one-point statistics) ofpandNHconfirms that the mean and maximum polarization fractions decrease with increasingNH. The uncertainty on the maximum observed polarization fraction,pmax= 22.0−1.4+3.5% at 353 GHz and 80′ resolution, is dominated by the uncertainty on the Galactic emission zero level in total intensity, in particular towards diffuse lines of sight at high Galactic latitudes. Furthermore, the inverse behaviour betweenpand 𝒮 found earlier is seen to be present at high latitudes. This follows the 𝒮 ∝ p−1relationship expected from models of the polarized sky (including numerical simulations of magnetohydrodynamical turbulence) that include effects from only the topology of the turbulent magnetic field, but otherwise have uniform alignment and dust properties. Thus, the statistical properties ofp,ψ, and 𝒮 for the most part reflect the structure of the Galactic magnetic field. Nevertheless, we search for potential signatures of varying grain alignment and dust properties. First, we analyse the product map 𝒮 × p, looking for residual trends. While the polarization fractionpdecreases by a factor of 3−4 betweenNH = 1020 cm−2andNH = 2 × 1022 cm−2, out of the Galactic plane, this product 𝒮 × ponly decreases by about 25%. Because 𝒮 is independent of the grain alignment efficiency, this demonstrates that the systematic decrease inpwithNHis determined mostly by the magnetic-field structure and not by a drop in grain alignment. This systematic trend is observed both in the diffuse interstellar medium (ISM) and in molecular clouds of the Gould Belt. Second, we look for a dependence of polarization properties on the dust temperature, as we would expect from the radiative alignment torque (RAT) theory. We find no systematic trend of 𝒮 × pwith the dust temperatureTd, whether in the diffuse ISM or in the molecular clouds of the Gould Belt. In the diffuse ISM, lines of sight with high polarization fractionpand low polarization angle dispersion 𝒮 tend, on the contrary, to have colder dust than lines of sight with lowpand high 𝒮. We also compare thePlanckthermal dust polarization with starlight polarization data in the visible at high Galactic latitudes. The agreement in polarization angles is remarkable, and is consistent with what we expect from the noise and the observed dispersion of polarization angles in the visible on the scale of thePlanckbeam. The two polarization emission-to-extinction ratios,RP/pandRS/V, which primarily characterize dust optical properties, have only a weak dependence on the column density, and converge towards the values previously determined for translucent lines of sight. We also determine an upper limit for the polarization fraction in extinction,pV/E(B − V), of 13% at high Galactic latitude, compatible with the polarization fractionp ≈ 20% observed at 353 GHz. Taken together, these results provide strong constraints for models of Galactic dust in diffuse gas.

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Constrained transport and adaptive mesh refinement in the Black Hole Accretion Code

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935559 ◽

2019 ◽

Vol 629 ◽

pp. A61 ◽

Cited By ~ 7

Author(s):

Hector Olivares ◽

Oliver Porth ◽

Jordy Davelaar ◽

Elias R. Most ◽

Christian M. Fromm ◽

...

Keyword(s):

Magnetic Field ◽

Black Hole ◽

Adaptive Mesh Refinement ◽

Mesh Refinement ◽

Adaptive Mesh ◽

Gravitational Fields ◽

Black Hole Accretion ◽

Constrained Transport ◽

The Magnetic Field ◽

Hole Accretion

Context. Worldwide very long baseline radio interferometry (VLBI) arrays are expected to obtain horizon-scale images of supermassive black hole candidates and of relativistic jets in several nearby active galactic nuclei. This, together with the expected detection of electromagnetic counterparts of gravitational-wave signals, motivates the development of models for magnetohydrodynamic flows in strong gravitational fields. Aims. The Black Hole Accretion Code (BHAC) is a publicliy available code intended to aid with the modeling of such sources by performing general relativistic magnetohydrodynamical simulations in arbitrary stationary spacetimes. New additions to the code are required in order to guarantee an accurate evolution of the magnetic field when small and large scales are captured simultaneously. Methods. We discuss the adaptive mesh refinement (AMR) techniques employed in BHAC, which are essential to keep several problems computationally tractable, as well as staggered-mesh-based constrained transport (CT) algorithms to preserve the divergence-free constraint of the magnetic field. We also present a general class of prolongation operators for face-allocated variables compatible with them. Results. After presenting several standard tests for the new implementation, we show that the choice of the divergence-control method can produce qualitative differences in the simulation results for scientifically relevant accretion problems. We demonstrate the ability of AMR to decrease the computational costs of black hole accretion simulations while sufficiently resolving turbulence arising from the magnetorotational instability. In particular, we describe a simulation of an accreting Kerr black hole in Cartesian coordinates using AMR to follow the propagation of a relativistic jet while self-consistently including the jet engine, a problem set up for which the new AMR implementation is particularly advantageous. Conclusions. The CT methods and AMR strategies discussed here are currently being used in the simulations performed with BHAC for the generation of theoretical models for the Event Horizon Telescope collaboration.

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Protostellar birth with ambipolar and ohmic diffusion

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732075 ◽

2018 ◽

Vol 615 ◽

pp. A5 ◽

Cited By ~ 42

Author(s):

N. Vaytet ◽

B. Commerçon ◽

J. Masson ◽

M. González ◽

G. Chabrier

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Adaptive Mesh Refinement ◽

Protoplanetary Disks ◽

Adaptive Mesh ◽

Magnetic Diffusion ◽

Ideal Mhd ◽

The Impact ◽

The Magnetic Field ◽

The Ideal

Context. The transport of angular momentum is fundamental during the formation of low-mass stars; too little removal and rotation ensures stellar densities are never reached, too much and the absence of rotation means no protoplanetary disks can form. Magnetic diffusion is seen as a pathway to resolving this long-standing problem. Aims. We aim to investigate the impact of including resistive magnetohydrodynamics (MHD) in simulations of the gravitational collapse of a 1 M⊙ gas sphere, from molecular cloud densities to the formation of the protostellar seed; the second Larson core. Methods. We used the adaptive mesh refinement code RAMSES to perform two 3D simulations of collapsing magnetised gas spheres, including self-gravity, radiative transfer in the form of flux-limited diffusion, and a non-ideal gas equation of state to describe H2 dissociation which leads to the second collapse. The first run was carried out under the ideal MHD approximation, while ambipolar and ohmic diffusion was incorporated in the second calculation using resistivities computed from an equilibrium chemical network. Results. In the ideal MHD simulation, the magnetic field dominates the energy budget everywhere inside and around the first hydrostatic core, fueling interchange instabilities and driving a low-velocity outflow above and below the equatorial plane of the system. High magnetic braking removes essentially all angular momentum from the second core. On the other hand, ambipolar and ohmic diffusion create a barrier which prevents amplification of the magnetic field beyond 0.1 G in the first Larson core which is now fully thermally supported. A significant amount of rotation is preserved and a small Keplerian-like disk forms around the second core. The ambipolar and ohmic diffusions are effective at radii below 10 AU, indicating that a least ~1 AU is necessary to investigate the angular momentum transfer and the formation of rotationally supported disks. Finally, when studying the radiative efficiency of the first and second core accretion shocks, we found that it can vary by several orders of magnitude over the 3D surface of the cores. Conclusions. This proves that magnetic diffusion is a prerequisite to star formation. Not only does it enable the formation of protoplanetary disks in which planets will eventually form, it also plays a determinant role in the formation of the protostar itself.

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Extreme starlight polarization in a region with highly polarized dust emission

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935266 ◽

2019 ◽

Vol 624 ◽

pp. L8 ◽

Cited By ~ 3

Author(s):

Georgia V. Panopoulou ◽

Brandon S. Hensley ◽

Raphael Skalidis ◽

Dmitry Blinov ◽

Konstantinos Tassis

Keyword(s):

Magnetic Field ◽

Dust Emission ◽

Wavelength Dependence ◽

Joint Analysis ◽

Grain Alignment ◽

Upper Limit ◽

Field Geometry ◽

Optical Wavelengths ◽

Polarization Fraction ◽

The Magnetic Field

Context. Galactic dust emission is polarized at unexpectedly high levels, as revealed by Planck. Aims. The origin of the observed ≃20% polarization fractions can be identified by characterizing the properties of optical starlight polarization in a region with maximally polarized dust emission. Methods. We measure the R-band linear polarization of 22 stars in a region with a submillimeter polarization fraction of ≃20%. A subset of 6 stars is also measured in the B, V, and I bands to investigate the wavelength dependence of polarization. Results. We find that starlight is polarized at correspondingly high levels. Through multiband polarimetry we find that the high polarization fractions are unlikely to arise from unusual dust properties, such as enhanced grain alignment. Instead, a favorable magnetic field geometry is the most likely explanation, and is supported by observational probes of the magnetic field morphology. The observed starlight polarization exceeds the classical upper limit of [pV/E(B−V)]max = 9% mag−1 and is at least as high as 13% mag−1, as inferred from a joint analysis of Planck data, starlight polarization, and reddening measurements. Thus, we confirm that the intrinsic polarizing ability of dust grains at optical wavelengths has long been underestimated.

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Turbulent structure and star formation in a stratified, supernova-driven, interstellar medium

Proceedings of the International Astronomical Union ◽

10.1017/s174392130700172x ◽

2006 ◽

Vol 2 (S237) ◽

pp. 358-362

Author(s):

M. K. Ryan Joung ◽

Mordecai-Mark Mac Low

Keyword(s):

Star Formation ◽

Interstellar Medium ◽

Adaptive Mesh Refinement ◽

Formation Rate ◽

Three Dimensional ◽

Adaptive Mesh ◽

Mass Function ◽

Initial Mass Function ◽

Interstellar Turbulence ◽

Self Gravity

AbstractWe report on a study of interstellar turbulence driven by both correlated and isolated supernova explosions. We use three-dimensional hydrodynamic models of a vertically stratified interstellar medium run with the adaptive mesh refinement code Flash at a maximum resolution of 2 pc, with a grid size of 0.5 × 0.5 × 10 kpc. Cold dense clouds form even in the absence of self-gravity due to the collective action of thermal instability and supersonic turbulence. Studying these clouds, we show that it can be misleading to predict physical properties such as the star formation rate or the stellar initial mass function using numerical simulations that do not include self-gravity of the gas. Even if all the gas in turbulently Jeans unstable regions in our simulation is assumed to collapse and form stars in local freefall times, the resulting total collapse rate is significantly lower than the value consistent with the input supernova rate. The amount of mass available for collapse depends on scale, suggesting a simple translation from the density PDF to the stellar IMF may be questionable. Even though the supernova-driven turbulence does produce compressed clouds, it also opposes global collapse. The net effect of supernova-driven turbulence is to inhibit star formation globally by decreasing the amount of mass unstable to gravitational collapse.

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Particle re-acceleration and Faraday-complex structures in the RXC J1314.4-2515 galaxy cluster

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2408 ◽

2019 ◽

Cited By ~ 1

Author(s):

C Stuardi ◽

A Bonafede ◽

D Wittor ◽

F Vazza ◽

A Botteon ◽

...

Keyword(s):

Magnetic Field ◽

Galaxy Clusters ◽

Galaxy Cluster ◽

Complex Structures ◽

Very Large Array ◽

Rotation Measure ◽

Low Levels ◽

Hydrodynamical Simulations ◽

Polarization Fraction ◽

The Magnetic Field

Abstract Radio relics are sites of electron (re)acceleration in merging galaxy clusters but the mechanism of acceleration and the topology of the magnetic field in and near relics are yet to be understood. We are carrying out an observational campaign on double relic galaxy clusters starting with RXC J1314.4-2515. With Jansky Very Large Array multi-configuration observations in the frequency range 1-4 GHz, we perform both spectral and polarization analyses, using the Rotation Measure synthesis technique. We use archival XMM-Newton observations to constrain the properties of the shocked region. We discover a possible connection between the activity of a radio galaxy and the emission of the eastern radio relic. In the northern elongated arc of the western radio relic, we detect polarized emission with an average polarization fraction of 31 % at 3 GHz and we derive the Mach number of the underlying X-ray shock. Our observations reveal low levels of fractional polarization and Faraday-complex structures in the southern region of the relic, which point to the presence of thermal gas and filamentary magnetic field morphology inside the radio emitting volume. We measured largely different Rotation Measure dispersion from the two relics. Finally, we use cosmological magneto-hydrodynamical simulations to constrain the magnetic field, viewing angle, and to derive the acceleration efficiency of the shock. We find that the polarization properties of RXC J1314.4-2515 are consistent with a radio relic observed at 70○ with respect to the line of sight and that efficient re-acceleration of fossil electrons has taken place.

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