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 (1946) Electronic (Thomson) scattering in stellar atmosphere Limb polarization: P~12.5% (11.713%) How to observe? Eclipsing binaries! IDMC, Pune, 24.12.2011 2

Interstellar linear polarization: discovery 1949 – William Hiltner, John Hall, Victor Dombrovskii (searching for Sobolev-Chandrasekhar effect) IDMC, Pune, 24.12.2011 3

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Interstellar polarization: observations IDMC, Pune, 24.12.2011 5

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P max Exti tinction A( A( λ ) λ max Polarizatio ion P( P( λ ) IDMC, Pune, 24.12.2011 8

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 IDMC, Pune, 24.12.2011 9

Interpretation TE TM Problems: li light sca cattering by by non-spherical par articles alig alignment mec echanism av averaging ove ver rot rotation IDMC, Pune, 24.12.2011 10

1951ApJ...114..206D W =90deg? Paramagnetic rela relaxation in in magnetic field field (F (Fe e in incl clusions) IDMC, Pune, 24.12.2011 11

! W =90deg? IDMC, Pune, 24.12.2011 12

Alignment • Radiative torques alignment function: ? • Davis-Greenstein type Particles: helical, dielectric alignment function: alignment parameter Calculations, comparison with observations: ? [Whittet et al., 2008] R -Rayleig igh re reductio ion fac facto tor IDMC, Pune, 24.12.2011 13 (Greenberg’68)

Model Rotating spheroidal grains Parameters: Refractive index (chemical composition) Size (size distribution): r V, min , r V,max , q Shape ( a/b – aspect ratio) Degree of alignment: δ 0 Direction of alignment (angle between the line of sight and direction of magnetic field): Ω (0deg. ≤ Ω ≤ 90deg.) IDMC, Pune, 24.12.2011 14

7 stars 7 stars wi with kno known n ion UV UV pol polariz izatio 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 carbon car Ali lignment: Davi avis-Greenstein imperfect (I (IDG) IDMC, Pune, 24.12.2011 15

! Parameters: 4 (C (C) ) + 4 (S (Si) i) + 1 (s (shape) + 2 (a (alig lignment) IDMC, Pune, 24.12.2011 16

But UV polarization was measured in a few directions. We can search for relations between parameters of P( P( λ ) curves ( P max , l max , K) ) + R(V (V) (?) and properties of dust grains (size, shape) + degree and direction of alignment. Stars in Taurus molecular cloud (TMC1/Heiles cloud2) IDMC, Pune, 24.12.2011 17

Stars in Taurus molecular cloud (TMC1/Heiles cloud2) IDMC, Pune, 24.12.2011 18

Data fro from Whit itte tet et t al. l. (20 (2001) IDMC, Pune, 24.12.2011 19

(T (TMC1, , Messenger’s Cl Cloud1) IDMC, Pune, 24.12.2011 20

TMC1, cloud1: prolate (Si!) spheroids, a/b=3, r Vmin =0.07 m m, r Vmax =0.35 m m, q=-1.7, d 0 =3 m m, W =15(15)90 IDMC, Pune, 24.12.2011 21

TMC1, cloud1: prolate spheroids, a/b=3, r Vmin =0.07 m m, r Vmax =0.35 m m, q=-1.7, d 0 =3 m m, W =15(15)90 IDMC, Pune, 24.12.2011 22

TMC1, cloud1: prolate spheroids, a/b=3, r Vmin =0.07 m m, r Vmax =0.35 m m, q=-1.7, d 0 =3 m m, W =15(15)90deg. W =60 60-90 90deg. W =4 =45-60deg. W =3 =30-45deg. IDMC, Pune, 24.12.2011 23

TMC1, cloud1: prolate spheroids, a/b=3, r Vmin =0.07 m m, r Vmax =0.35 m m, q=-1.7, d 0 =3 m m, W =15(15)90deg. IDMC, Pune, 24.12.2011 24

Polarizing grains: ? C gra rain ins do do not t pro roduce WHY? pola larization May ay be be Fe e gra rains? (Fe+Mg)/Si>2 ! IDMC, Pune, 24.12.2011 25

Amorphous sili ilicate (oli (olivine ty type) (M (Mg,Fe) 2 SiO 4 (in (include all ll Si Si + Mg g and nd a part rt of of Fe) r co rr .= .=-0.654 corr Fe e grai rains do do not t pro roduce pola larization! IDMC, Pune, 24.12.2011 26

Si i gra rains pro roduce pola larization! IDMC, Pune, 24.12.2011 27

Some conclusions and future work • Accurate modelling of interstellar linear polarization for individual objects allows one to get information about the spatial structure of magnetic fields. • More likely: polarization is produced by Si grains and is not produced by C and Fe-rich grains. • Development of appropriate models (inhomogeneous particles + imperfect alignment) with a reasonable number of parameters is highly appreciated. IDMC, Pune, 24.12.2011 28

• THANKS! • शुक्ऱिया • ВСЁ ! IDMC, Pune, 24.12.2011 29

C,O,Mg,Si,Fe + vacuum (!) COS OSMIC  Sun Sun ppm – parts per million: N(X)/N(H)*10^6 Average values: ppm / X d /X cosmic • Fe Mg Si • all stars 30.64 /97.0%(134) 33.13/83.2%(147) 25.01/77.2%(39) • |b|<30deg, 30.86 (78) 34.11 (85) 29.30 (15) • E(B-V)>0.2 • |b|>30deg 28.68 (9) 29.98 (14) 20.03 (10) E(B (B-V)<0.8 --- --- diff diffuse an and d tran translu lucent IS IS clo clouds IDMC, Pune, 24.12.2011 30

Amorphous sili ilicate (oli (olivine ty type) (M (Mg,Fe) 2 SiO 4 (31 (31 sig ightl tlines with ith Fe + Mg g + Si Si) (Fe+Mg)/Si>2 ! IDMC, Pune, 24.12.2011 31

Pri rincipal ele elements O(~120 stars) C(~18 (!) stars) <[Si/H] d > =25ppm 100ppm: olivine (O=4xSi) ve very un uncertain! IDMC, Pune, 24.12.2011 32

Interpretation (first application of the Mie theory) • 1933 Schalen --- Fe • 1934 Schoenberg & Jung --- Mg • Ambiguity! 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 IDMC, Pune, 24.12.2011 33

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Why? IDMC, Pune, 24.12.2011 35

Cosmic dust grains • Observations • Interpretation (grain characteristics) • composition • Interstellar extinction • size • Interstellar polarization • shape • Scattered radiation (nebulae, • structure circumstellar shells) • (surface properties) • Infrared radiation (dust emission) • Dust features • (Element depletions) IDMC, Pune, 24.12.2011 36

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Stars in Taurus molecular cloud (TMC1, cloud1) IDMC, Pune, 24.12.2011 39