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Interstellar dust dynamics across timescales from scattering to alignment Joonas HERRANEN 1 , Johannes MARKKANEN 2 , Karri MUINONEN 1 Cosmic Dust and Magnetism, Oct 30 Nov 2 2018, KASI, Daejeon, South Korea 1 University of Helsinki, Finland 2

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Matemaattis-luonnontieteellinen tiedekunta

Interstellar dust dynamics across timescales from scattering to alignment

Joonas HERRANEN1, Johannes MARKKANEN2, Karri MUINONEN1 Cosmic Dust and Magnetism, Oct 30 – Nov 2 2018, KASI, Daejeon, South Korea

1 University of Helsinki, Finland 2 Max Planck Institute for Solar System Research, Gottingen, Germany

Herranen / Grain alignment & timescales / Cosmic Dust and Magnetism 2018

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Matemaattis-luonnontieteellinen tiedekunta

Outline of the talk

1. Scattering dynamics of arbitrary particles 2. Interstellar dust model and different timescales of alignment 3. Automatic alignment pipeline through the timescales and results

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Matemaattis-luonnontieteellinen tiedekunta

Scattering dynamics software v0.7

Based on the JVIE T-matrix code by J. Markkanen
Geometries defined as tetrahedral meshes
Usually Gaussian random spheres/ellipsoids (K. Muinonen)

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Matemaattis-luonnontieteellinen tiedekunta

Light scattering transfers (angular) momentum → scattering dynamics

Expansion of fields in terms of VSWFs Analytical form of torques (and forces) in the expansion

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Matemaattis-luonnontieteellinen tiedekunta

Fast scattering solution allows detailed study of rotation and motion

Example: Optical tweezers

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Matemaattis-luonnontieteellinen tiedekunta

We pursue towards alignment via explicit and exact methods (as far as possible)

A spinning body in free space Forces, torques, and how the equations of motion are solved A full solution of scattering, dynamics and polarization in a realistic interstellar environment

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Matemaattis-luonnontieteellinen tiedekunta

Modeling interstellar dust particles and their environment

Core: Silicates and amorphous hydrogenated carbon (a-C:H) Mantle: Amorphous carbon (a-C) and a-C:H (older dust) Also: Incident field spectrum

GRE = Gaussian Random Ellipsoid

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Matemaattis-luonnontieteellinen tiedekunta

In free space, there are two types of alignment

Internal alignment

= angular momentum || principal axis (major) ≈ “Am I pointing somewhere?”

External alignment

= angular momentum points at certain direction ≈ “Which way is up?”

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Matemaattis-luonnontieteellinen tiedekunta

Grain motion under radiation (internal alignment)

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Matemaattis-luonnontieteellinen tiedekunta

Useful parameter for identifying internal alignment

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Matemaattis-luonnontieteellinen tiedekunta

Radiative torques quickly stabilize q What happens to an ensemble of different shapes?

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Matemaattis-luonnontieteellinen tiedekunta

Internal alignment of a 0.1 μm silicate dust ensemble under ISRF

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Matemaattis-luonnontieteellinen tiedekunta

From internal to external alignment

2) Perfect internal alignment by relaxation effects (slow) 3) Gradual alignment by radiative torques (slow) 1) Spin-up by radiative torques (fast)

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Matemaattis-luonnontieteellinen tiedekunta

External alignment of the silicate ensemble

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Matemaattis-luonnontieteellinen tiedekunta

Adding metallic inclusions enhance alignment

Increase susceptibility by 100x

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Matemaattis-luonnontieteellinen tiedekunta

Conclusions

Explicit integration gives insight on the internal alignment
In certain situations the described pipeline can be useful for

alignment considerations (when timescales are distinct)

The full pipeline input is shape+composition, ultimate output – when

Interstellar dust dynamics across timescales from scattering to alignment

Outline of the talk

1. Scattering dynamics of arbitrary particles 2. Interstellar dust model and different timescales of alignment 3. Automatic alignment pipeline through the timescales and results

Scattering dynamics software v0.7

Light scattering transfers (angular) momentum → scattering dynamics

Expansion of fields in terms of VSWFs Analytical form of torques (and forces) in the expansion

Fast scattering solution allows detailed study of rotation and motion

Example: Optical tweezers

We pursue towards alignment via explicit and exact methods (as far as possible)

A spinning body in free space Forces, torques, and how the equations of motion are solved A full solution of scattering, dynamics and polarization in a realistic interstellar environment

Modeling interstellar dust particles and their environment

Core: Silicates and amorphous hydrogenated carbon (a-C:H) Mantle: Amorphous carbon (a-C) and a-C:H (older dust) Also: Incident field spectrum

In free space, there are two types of alignment

Internal alignment

= angular momentum || principal axis (major) ≈ “Am I pointing somewhere?”

External alignment

= angular momentum points at certain direction ≈ “Which way is up?”

Grain motion under radiation (internal alignment)

Useful parameter for identifying internal alignment

Radiative torques quickly stabilize q What happens to an ensemble of different shapes?

Internal alignment of a 0.1 μm silicate dust ensemble under ISRF

From internal to external alignment

2) Perfect internal alignment by relaxation effects (slow) 3) Gradual alignment by radiative torques (slow) 1) Spin-up by radiative torques (fast)

External alignment of the silicate ensemble

Adding metallic inclusions enhance alignment

Conclusions

alignment considerations (when timescales are distinct)

the dust settles – is the Mueller matrix (for scattered polarization)