Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Modeling Dust Polarizationfrom the UV to the Microwave

Brandon HensleyJet Propulsion Laboratory,

California Institute of Technology

In collaboration withBruce Draine (Princeton)

c© 2016All RightsReserved

Multiple Faces of Interstellar DustSeptember 14, 2016

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

The Inverse Problem

• Bohren and Huffman: Determining the composition ofinterstellar dust is like describing a dragon based on itstracks

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

The Inverse Problem

• Bohren and Huffman: Determining the composition ofinterstellar dust is like describing a dragon based on itstracks

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Big Picture

Find observations that constrain:• Abundances

• Extinction (total and polarized)

• Emission (total and polarized)then build a model that satisfies all constraints simultaneously

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Abundances

Solid phase C < 200 ppm, Si < 41 ppm

Element A [ppm] Type ReferencesC 245 ± 34 F & G Stars 3, 5

339 Solar + Enrichment 1, 4O 589 ± 95 F & G Stars 2, 5

Mg 43.4 ± 6.0 F & G Stars 2, 5Si 40.7 ± 4.7 F & G Stars 2, 5

43 Solar + Enrichment 1, 4Fe 40.7 ± 5.6 F & G Stars 2, 5

1: Chiappini, Romano, & Matteucci 20032: Bensby et al 20053: Bensby & Feltzing 20064: Asplund et al 20095: Lodders et al 2009

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Extinction Constraints

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarized Extinction Constraints

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Emission Constraints

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarized Emission Constraints

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarized Emission Constraints

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Draine and Li 2007 Model

• Specified the material properties of astronomical silicatesand carbonaceous material (graphite and PAHs)

• Successful modeling of Galactic dust emission andextinction, as well as dust emission from other galaxies

• Spherical grains– no polarization• Model predicts too much extinction per unit emission

(Planck Int XXIX 2016)

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Draine and Fraisse 2009 Models

• Used Draine and Lidust materials tomake predictions forpolarized emissionin the Planck bands

• Planck observed thepolarization fractionto decrease withincreasingwavelength, unlikethe predictions here

Draine and Fraisse 2009

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

We Need...

• New Models of Interstellar Dust(with Polarization!)

• Hensley & Draine 2016. In prep.

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

A New Model

To construct a new model, we have:• Updated the material properties of the silicate (Draine &

Hensley 2016, in prep) and carbonaceous grains (Draine2016) based on new astronomical observations andlaboratory data

• Added magnetic nanoparticles as an optional modelcomponent

• Using new grain materials, found a best fit size distributionand alignment function

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarized Dust Emission

• Models with silicate and carbonaceous grains alone havedifficulty reproducing the observed decline in thepolarization fraction with increasing wavelength

• A new ingredient– magnetic nanoparticles

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Magnetic Materials

• Ferromagnetic materials,such as metallic Fe, haveall unpaired spins alignedalong a preferred axis

• Preferred direction ofmagnetization implies aminimum energy state withall unpaired spins alignedalong preferred direction

• Ferrimagnetic grains suchas Fe3O4 and γ-Fe2O3 arealso viable

Ferromagnetic spin lattice

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Magnetic Dipole Emission

• Thermal fluctuations can move the spins away from thisstate

• Then magnetization vector precesses about the preferreddirection and produces radiation

Response to a fluctuation

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Emissivities

• Emissivity per unitvolume of 0.01µmgrains heated to 18K

• Emissivity in mmand sub-mm muchstronger thanamorphous silicategrains

Draine and Hensley 2013

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarization

• Polarization depends on whether grains are free-fliers orinclusions in larger grains

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Free-Fliers

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Inclusions

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Inclusions

Of course, interstellar grains are nonspherical...

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarization

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Example

• 2:1 spheroidal silicate grains with 5% iron nanoparticles byvolume as inclusions

• Unaligned carbonaceous grains• PAHs

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Emission

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Extinction

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarized Extinction

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarized Emission

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Polarization Fraction

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Size Distribution

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Alignment Function

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Summary

• Inclusion of iron grains allows us to match thefrequency-dependence of the polarized emission

• We alleviate the tension between emission and extinctionin the Draine and Li 2007 model by making silicates moreemissive at long wavelengths

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

With a Model, We Can...

• Test the model against the Planck sky to see if it canaccount for variations in dust properties

• Predict dust properties at all wavelengths given a model fit

• Simulate different realizations of dust properties and theimplications for component separation

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Predictions

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Predictions

Introduction Previous Models Magnetic Nanoparticles A New Model Conclusions

Conclusions

• Our new models of interstellar dust successfully reproducethe mean properties of dust in the diffuse ISM, including inpolarization

• We make testable predictions of dust properties,particularly in polarization

• The new models enable future work on the properties ofGalactic dust as well as next-generation componentseparation