Modelling grain alignment by radiative torques and hydrogen formation torques in reflection nebula

Hoang, Thiem; Lazarían, A.; Andersson, B. G.

doi:10.1093/mnras/stu2758

Cited by 40 publications

(48 citation statements)

References 63 publications

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“…Hoang & Lazarian (2009b) found that RAT alignment is shown to be enhanced by pinwheel torques. The effect of alignment by H 2 torques is observed in reflection nebula IC 63 (Andersson et al 2013), and it is successfully modelled by Hoang et al (2015). Since MAT alignment of irregular shapes is essentially similar to RAT alignment, we predict that pinwheel torques also act to enhance the alignment by lifting the low-J attractor points and create new high-J attractor points.…”

Section: Effect Of Grain Precession Around the Drift Direction Vs Lamentioning

confidence: 70%

Alignment of Irregular Grains by Mechanical Torques

Hoang

Cho

Lazarían

2018

ApJ

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122

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We study the alignment of irregular dust grains by mechanical torques due to the drift of grains through the ambient gas. We first calculate mechanical alignment torques (MATs) resulting from specular reflection of gas atoms for seven irregular shapes: one shape of mirror symmetry, three highly irregular shapes (HIS), and three weakly irregular shapes (WIS). We find that the grain with mirror symmetry experiences negligible MATs due to its mirror-symmetry geometry. Three highly irregular shapes can produce strong MATs which exhibit some generic properties as radiative torques (RATs), while three weakly irregular shapes produce less efficient MATs. We then study grain alignment by MATs for the different angles between the drift velocity and the ambient magnetic field, for paramagnetic and superparamagnetic grains assuming efficient internal relaxation. We find that for HIS grains, MATs can align subsonically drifting grains in the same way as RATs, with low-J and high-J attractors. For supersonic drift, MATs can align grains with low-J and high-J attractors, analogous to RAT alignment by anisotropic radiation. We also show that the joint action of MATs and magnetic torques in grains with iron inclusions can lead to perfect MAT alignment. Our results point out the potential importance of MAT alignment for HIS grains predicted by the analytical model of Lazarian & Hoang (2007b), although more theoretical and observational studies are required due to uncertainty in the shape of interstellar grains. We outline astrophysical environments where MAT alignment is potentially important.

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Section: Effect Of Grain Precession Around the Drift Direction Vs Lamentioning

confidence: 70%

Alignment of Irregular Grains by Mechanical Torques

Hoang

Cho

Lazarían

2018

ApJ

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122

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“…First of all, it was found that the alignment can be enhanced in the presence of H 2 torques (Andersson et al 2013;Hoang et al 2015). This does not contradict to MRAT process, as the effect of H 2 torques on grain alignment is expected to be still applicable because the small size cut-off will shift to a smaller size due to H 2 torques, allowing more grains to be aligned and thus increasing the total polarization.…”

Section: Implication For Observations In the Diffuse Mediummentioning

confidence: 99%

“…The grains outside this parameter space were found to be aligned at the low-J attractor point and were shown to be not perfectly aligned. For this generic feature, the degree of radiative alignment can be modeled by a parameter f hithe fraction of grains aligned at high-J attractor points, which allows quantitative predictions of the polarization by RAT alignment for various astrophysical conditions Hoang et al (2015). Observational evidence for RAT alignment is numerous and becomes increasingly available (Andersson & Potter 2007;Whittet et al 2008;Andersson & Potter 2010;Matsumura et al 2011;Andersson et al 2011;Reissl et al 2016).…”

Section: Introductionmentioning

confidence: 99%

A Unified Model of Grain Alignment: Radiative Alignment of Interstellar Grains With Magnetic Inclusions

Hoang

Lazarían

2016

ApJ

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135

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The radiative torque (RAT) alignment of interstellar grains with ordinary paramagnetic susceptibilities has been supported by earlier studies. The alignment of such grains depends on the so-called RAT parameter q max that is determined by the grain shape. In this paper, we elaborate our model of RAT alignment for grains with enhanced magnetic susceptibility due to iron inclusions, such that RAT alignment is magnetically enhanced for which we term MRAT mechanism. Such grains can get aligned with high angular momentum at the so-called high-J attractor points, achieving a high degree of alignment. Using our analytical model of RATs we derive the critical value of the magnetic relaxation parameter δ m to produce high-J attractor points as functions of q max and the anisotropic radiation angle relative to the magnetic field ψ. We find that if about 10% of total iron abundance present in silicate grains are forming iron clusters, it is sufficient to produce high-J attractor points for all reasonable values of q max . To calculate the degree of grain alignment, we carry out numerical simulations of MRAT alignment by including stochastic excitations from gas collisions and magnetic fluctuations. We show that large grains can achieve perfect alignment when the high-J attractor point is present, regardless of the values of q max . Our obtained results pave the way for physical modeling of polarized thermal dust emission as well as magnetic dipole emission. We also find that millimetersized grains in accretion disks may be aligned with the magnetic field if they are incorporated with iron nanoparticles.

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“…The actual mechanism by which the dust grains align with the magnetic field in a manner described above is still unclear (Lazarian, 2007;Andersson et al, 2015). However, radiative torque mechanism, originally proposed by Dolginov & Mitrofanov (1976), seems to be emerging as the most successful one in explaining the dust alignment in various environments (e.g., Hoang & Lazarian, 2014;Hoang et al, 2015;Andersson et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

Magnetic field structure of IC 63 and IC 59 associated with H ii region Sh 185

Soam

Maheswar

Lee

et al. 2016

Mon. Not. R. Astron. Soc.

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Bright-rimmed clouds (BRCs) are formed at the periphery of H II regions as the radiation from the central star interacts with dense gas. The ionization and resulting compression of the clouds may lead to cloud disruption causing secondary star formation depending on the stellar and gas parameters. Here we use R-band polarimetry to probe the plane-of-the sky magnetic field in the two near-by BRCs IC 59 and IC 63. Both nebulae are illuminated by γ Cas with the direction of ionizing radiation being orientated parallel or perpendicular to the local magnetic field, allowing us to probe the importance of magnetic field pressure in the evolution of BRCs. Because of the proximity of the system (∼200pc) we have acquired a substantial sample of over 500 polarization measurements for stars background to the nebulae. On large scales, the magnetic field geometries of both clouds are anchored to the ambient magnetic field. For IC 63, the magnetic field is aligned parallel to the head-tail morphology of the main condensation, with convex morphology relative to the direction of the ionizing radiation. We estimate the plane of the sky magnetic field strength in IC 63 to be ∼ 90µG. In IC 59, the projected magnetic field follows the M shape morphology of the cloud. Here, field lines present a concave shape with respect to the direction of the ionizing radiation from γ Cas. Comparing our observations to published theoretical models we find good general agreement, supporting the importance of magnetic fields in BRC evolution.

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Modelling grain alignment by radiative torques and hydrogen formation torques in reflection nebula

Cited by 40 publications

References 63 publications

Alignment of Irregular Grains by Mechanical Torques

Alignment of Irregular Grains by Mechanical Torques

A Unified Model of Grain Alignment: Radiative Alignment of Interstellar Grains With Magnetic Inclusions

Magnetic field structure of IC 63 and IC 59 associated with H ii region Sh 185

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