ALMA Observations of Polarization from Dust Scattering in ...
Modeling Dust Polarization from the UV to the Microwave
Transcript of Modeling Dust Polarization from the UV to the Microwave
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
Polarized Extinction 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
Inclusions
Of course, interstellar grains are nonspherical...
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
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
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