Interstellar linear polarization: discovery 1949 William Hiltner, - - PowerPoint PPT Presentation
Tracing magnetic fields through interstellar polarization Nikolai Voshchinnikov Sobolev Astronomical Institute , St. Petersburg University, Russia Preliminaries: Sobolev-Chandrasekhar effect Victor Sobolev (1943/1949) Subramanian Chandrasekhar
Tracing magnetic fields through interstellar polarization
Nikolai Voshchinnikov
Sobolev Astronomical Institute, St. Petersburg University, Russia
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Preliminaries: Sobolev-Chandrasekhar effect
Victor Sobolev (1943/1949) Subramanian Chandrasekhar (1946) Electronic (Thomson) scattering in stellar atmosphere
Limb polarization: P~12.5% (11.713%) How to observe? Eclipsing binaries!
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Interstellar linear polarization: discovery
1949 – William Hiltner, John Hall, Victor Dombrovskii (searching for Sobolev-Chandrasekhar effect)
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Interstellar polarization: observations
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Exti tinction A(
A(λ)
Polarizatio ion
P( P(λ) Pmax λmax
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Polarization: unique phenomenon!
1. Dust grains must be non-spherical 2. Dust grains must have sizes close to the wavelength of incident radiation 3. Dust grains must have specific magnetic properties in order to interact with interstellar magnetic field 4. Dust grains must be aligned 5. The direction of alignment must not coincide with the line of sight 6. The distribution of aligned grains along the line of sight must be rather regular in order to exclude the cancellation of polarization
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Interpretation
Problems: li light sca cattering by by non-spherical par articles alig alignment mec echanism av averaging ove ver rot rotation
TM TE
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W=90deg? 1951ApJ...114..206D Paramagnetic rela relaxation in in magnetic field field (F (Fe e in incl clusions)
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W=90deg? !
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Alignment
alignment function: alignment parameter
alignment function: ? Particles: helical, dielectric Calculations, comparison with
[Whittet et al., 2008] R-Rayleig igh re reductio ion fac facto tor (Greenberg’68)
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Model
Rotating spheroidal grains Parameters: Refractive index (chemical composition) Size (size distribution): rV,min, rV,max, q Shape (a/b – aspect ratio) Degree of alignment: δ0 Direction of alignment (angle between the line of sight and direction of magnetic field): Ω (0deg.≤ Ω ≤90deg.)
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Sp Sphe hero roids: prola prolate/oblate, , a/b a/b=1.1 .1-10 10 Mat ateri rials: : ast astro rono nomical l si sili licate, , am amorp rphous car carbon Ali lignment: Davi avis-Greenstein imperfect (I (IDG)
7 7 stars stars wi with kno known n UV UV pol polariz izatio ion
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Parameters: 4 (C (C) ) + 4 (S (Si) i) + 1 (s (shape) + 2 (a (alig lignment) !
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Stars in Taurus molecular cloud (TMC1/Heiles cloud2) But UV polarization was measured in a few directions. We can search for relations between parameters of P( P(λ) curves (Pmax, lmax, K) ) + R(V (V)(?) and properties of dust grains (size, shape) + degree and direction of alignment.
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Stars in Taurus molecular cloud (TMC1/Heiles cloud2)
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Data fro from Whit itte tet et t al.
(2001)
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(T (TMC1, , Messenger’s Cl Cloud1)
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TMC1, cloud1: prolate (Si!) spheroids, a/b=3, rVmin=0.07mm, rVmax=0.35mm, q=-1.7, d0=3mm, W=15(15)90
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TMC1, cloud1: prolate spheroids, a/b=3, rVmin=0.07mm, rVmax=0.35mm, q=-1.7, d0=3mm, W=15(15)90
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TMC1, cloud1: prolate spheroids, a/b=3, rVmin=0.07mm, rVmax=0.35mm, q=-1.7, d0=3mm, W=15(15)90deg.
W=60 60-90 90deg. W=4 =45-60deg. W=3 =30-45deg.
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TMC1, cloud1: prolate spheroids, a/b=3, rVmin=0.07mm, rVmax=0.35mm, q=-1.7, d0=3mm, W=15(15)90deg.
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Polarizing grains:?
(Fe+Mg)/Si>2 ! C gra rain ins do do not t pro roduce pola larization May ay be be Fe e gra rains?
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Amorphous sili ilicate (oli (olivine ty type) (M (Mg,Fe)2SiO4 (in (include all ll Si Si + Mg g and nd a part rt of
Fe)
Fe e grai rains do do not t pro roduce pola larization! rco
corr rr.=
.=-0.654
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Si i gra rains pro roduce pola larization!
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Some conclusions and future work
polarization for individual objects allows one to get information about the spatial structure
grains and is not produced by C and Fe-rich grains.
(inhomogeneous particles + imperfect alignment) with a reasonable number of parameters is highly appreciated.
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Average values: ppm / Xd/Xcosmic
ppm – parts per million: N(X)/N(H)*10^6
COS OSMIC Sun Sun
C,O,Mg,Si,Fe + vacuum (!)
E(B (B-V)<0.8 ---
diffuse an and d tran translu lucent IS IS clo clouds
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Amorphous sili ilicate (oli (olivine ty type) (M (Mg,Fe)2SiO4 (31 (31 sig ightl tlines with ith Fe + Mg g + Si Si)
(Fe+Mg)/Si>2 !
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100ppm: olivine (O=4xSi) <[Si/H]d> =25ppm
ve very un uncertain! Pri rincipal ele elements
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Interpretation
(first application of the Mie theory)
Jung --- Mg
Sto tokes prin rinciple of f optic tical eq equi uivalence: It is im impossible le to distinguish two beams which are the sum of non-coherent simple waves if they have the same Stokes parameters
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Why?
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Cosmic dust grains
circumstellar shells)
emission)
(grain characteristics)
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Stars in Taurus molecular cloud (TMC1, cloud1)