Subsonic Mechanical Alignment of Irregular Grains

Lazarían, A.; Hoang, Thiem

doi:10.1086/523849

Cited by 115 publications

(174 citation statements)

References 26 publications

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“…In future work, we will relax the assumption that the angular momentum always lies alongâ 1 , enabling a firm conclusion regarding the effectiveness of mechanical torques in aligning grains. Here we attempt to gain some insight by examining the incidence of attractors satisfying the following three conditions that are conducive to alignment: (1) | cos ξ| ≥ 1/3, ensuring that the 'Rayleigh reduction factor' characterizing the alignment effectiveness is positive (Lee & Draine 1985); (2) τ align ≤ 10 6 yr, in order to be competitive with radiative torques (Draine & Weingartner 1997); (3) ω/ω T ≥ 3, so as to avoid disalignment due to collisions with gas particles (Lazarian & Hoang 2007b;Hoang & Lazarian 2008). For each of the 13 grain shapes, five assumptions regarding the outgoing particles, and five values of s d , we consider 100 values of ψ v between 0 and π/2, uniformly spaced in cos ψ v .…”

Section: Resultsmentioning

confidence: 99%

“…Numerous authors have further elaborated and extended this model; see references in Lazarian & Hoang (2007b). While the alignment described by Gold is a stochastic process, Lazarian (2007) and Lazarian & Hoang (2007a) noted that irregularly shaped grains could experience systematic mechanical torques associated with their helicity.…”

Section: Introductionmentioning

confidence: 99%

“…While the alignment described by Gold is a stochastic process, Lazarian (2007) and Lazarian & Hoang (2007a) noted that irregularly shaped grains could experience systematic mechanical torques associated with their helicity. Lazarian & Hoang (2007b) examined the torque on a highly idealized helical grain. They concluded that the resulting alignment can be efficient even for grains moving subsonically, likely dominates over Gold-type alignment, and aligns grains with their long axes perpendicular to the magnetic field.…”

Section: Introductionmentioning

confidence: 99%

“…They concluded that the resulting alignment can be efficient even for grains moving subsonically, likely dominates over Gold-type alignment, and aligns grains with their long axes perpendicular to the magnetic field. Lazarian & Hoang (2007b) noted that detailed studies of the mechanical torques on irregular grains are needed to clarify the efficiency of helicity-related mechanical torques, since the helicities of realistic grain shapes are unknown. That is our aim in this work.…”

Section: Introductionmentioning

confidence: 99%

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Alignment of interstellar grains by mechanical torques: suprathermally rotating Gaussian random spheres

Das

Weingartner

2016

Mon. Not. R. Astron. Soc.

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Collisions of gas particles with a drifting grain give rise to a mechanical torque on the grain. Recent work by Lazarian & Hoang showed that mechanical torques might play a significant role in aligning helical grains along the interstellar magnetic field direction, even in the case of subsonic drift. We compute the mechanical torques on 13 different irregular grains and examine their resulting rotational dynamics, assuming steady rotation about the principal axis of greatest moment of inertia. We find that the alignment efficiency in the subsonic drift regime depends sensitively on the grain shape, with more efficient alignment for shapes with a substantial mechanical torque even in the case of no drift. The alignment is typically more efficient for supersonic drift. A more rigorous analysis of the dynamics is required to definitively appraise the role of mechanical torques in grain alignment.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Alignment of interstellar grains by mechanical torques: suprathermally rotating Gaussian random spheres

Das

Weingartner

2016

Mon. Not. R. Astron. Soc.

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“…The analytical model was the basis for further theoretical studies in (Lazarian & Hoang 2007b;, Hoang & Lazarian 2009bsee also reviews in Lazarian 2007;Lazarian, Andersson, & Hoang 2015;Andersson, Lazarian, & Vaillancourt 2015).…”

Section: Introductionmentioning

confidence: 98%

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

Hoang

Lazarían

Andersson

2015

Monthly Notices of the Royal Astronomical Society

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Reflection nebulae-dense cores-illuminated by surrounding stars offer a unique opportunity to directly test our quantitative model of grain alignment based on radiative torques (RATs) and to explore new effects arising from additional torques. In this paper, we first perform detailed modeling of grain alignment by RATs for the IC 63 reflection nebula illuminated both by a nearby γ Cas star and the diffuse interstellar radiation field. We calculate linear polarization p λ of background stars by radiatively aligned grains and explore the variation of fractional polarization (p λ /A V ) with visual extinction A V across the cloud. Our results show that the variation of p V /A V versus A V from the dayside of IC 63 to its center can be represented by a power-law (p V /A V ∝ A η V ) with different slopes depending on A V . We find a shallow slope η ∼ −0.1 for A V < 3 and a very steep slope η ∼ −2 for A V > 4. We then consider the effects of additional torques due to H 2 formation and model grain alignment by joint action of RATs and H 2 torques. We find that p V /A V tends to increase with an increasing magnitude of H 2 torques. In particular, the theoretical predictions obtained for p V /A V and peak wavelength λ max in this case show an improved agreement with the observational data. Our results reinforce the predictive power of the RAT alignment mechanism in a broad range of environmental conditions and show the effect of pinwheel torques in environments with efficient H 2 formation. Physical parameters involved in H 2 formation may be constrained using detailed modeling of grain alignment combined with observational data. In addition, we discuss implications of our modeling for interpreting latest observational data by Planck and other ground-based instruments.

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Turbulent molecular clouds

Hennebelle

Falgarone

2012

Astron Astrophys Rev

396

398

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Stars form within molecular clouds but our understanding of this fundamental process remains hampered by the complexity of the physics that drives their evolution. We review our observational and theoretical knowledge of molecular clouds trying to confront the two approaches wherever possible. After a broad presentation of the cold interstellar medium and molecular clouds, we emphasize the dynamical processes with special focus to turbulence and its impact on cloud evolution. We then review our knowledge of the velocity, density and magnetic fields. We end by openings towards new chemistry models and the links between molecular cloud structure and star--formation rates.Comment: To be published in AARv, 58 pages, 13 figures (higher resolution figures will be available on line

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Subsonic Mechanical Alignment of Irregular Grains

Cited by 115 publications

References 26 publications

Alignment of interstellar grains by mechanical torques: suprathermally rotating Gaussian random spheres

Alignment of interstellar grains by mechanical torques: suprathermally rotating Gaussian random spheres

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

Turbulent molecular clouds

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