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Polarized Thermal Emission from Interstellar Dust

Polarized Thermal Emission from Interstellar Dust. John Vaillancourt Roger Hildebrand, Larry Kirby (U. Chicago) Giles Novak, Megan Krejny, Hua-bai Li (Northwestern) Darren Dowell (JPL/Caltech) Jackie Davidson, Jessie Dotson (NASA Ames) Martin Houde (U. Western Ontario)

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Polarized Thermal Emission from Interstellar Dust

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  1. Polarized Thermal Emission from Interstellar Dust John Vaillancourt Roger Hildebrand, Larry Kirby (U. Chicago) Giles Novak, Megan Krejny, Hua-bai Li (Northwestern) Darren Dowell (JPL/Caltech) Jackie Davidson, Jessie Dotson (NASA Ames) Martin Houde (U. Western Ontario) Alex Lazarian (U. Wisconsin - Madison) 12Septembre

  2. Overview • Polarization by emission in molecular clouds • Polarization spectra • 60 - 1000 m = 300 - 5000 GHz • Extension to … • Dark clouds and the diffuse ISM • Longer wavelengths:  ~ 3 cm,  ~ 1 GHz • New instruments (WMAP, Planck, SHARP, HAWC/SOFIA)

  3. Absorption vs. Emission: dense & diffuse regions Polarization by absorption UV-Vis-NIR Diffuse ISM Polarization by emission FIR-MM Dense ISM Hildebrand 2002; Heiles 2000; Dotson et al. 2000, 2005

  4. Step 1: Internal Alignment Step 2: Angular Momentum Alignment Grain Alignment • Suprathermal rotation (Purcell 1979) • Erot » kTgas • H2 ejection (Purcell 1979) • radiative torques • (Draine & Weingartner 1996) Larmor precession See reviews by Roberge 2004, Lazarian & Yan 2004, Astrophysics of Dust, ASP Conf. Ser. 309.

  5. Limits of Polarization by Absorption Visible extinction, AV (mag) Background star polarization near dark clouds: efficiency of grain alignment drops quickly for AV > 1 - 2 mag Consistent with radiative torques from interstellar radiation field (ISRF) (Lazarian, Goodman, & Myers 1997; Arce et al. 1998; Whittet et al. 2001) Polarization (%) • How do we explain FIR polarization in dense clouds with AV ~ 10 - 30 ? • embedded stars • radiative torques more efficient for large grains; up to Av ~ 10 • (Cho & Lazarian 2005 astroph/0505571) Arce et al. 1998

  6. Loss of alignment in dense clouds? Polarized Flux (P  F) Schleuning 1998 OMC-1 • Pabs =  Pemis • Polarized flux traces aligned grains in dense regions

  7. Alignment near embedded stars W3 350 m W51 • Polarized flux (grayscale) vs. total flux (contours) • W51 - PF maximum coincident with total flux maximum • W3 - PF maximum coincident with H II region (W3A), not total flux (H II region is source of UV photons & radiative torques) W3A Vaillancourt et al., in prep. Schluening et al. 2000

  8. Dust properties & Polarization spectra • Extinction & polarization both drop with wavelength in near-IR, but diverge in UV • Large (> 0.1 m) grains are better aligned than small grains • Silicate & water spectral features are polarized • Line shapes  Grain shapes • oblate spheroids, axes ratio 0.3 < a/b < 0.9 • Carbon spectral features unpolarized • Silicate grains are better aligned than graphite (carbon) grains • Far-IR/sub-mm polarization spectrum has a minimum • multiple domains of aligned & unaligned grains at multiple temperatures. See reviews by Whittet (2003, 2005)

  9. Magnetic field vs. Wavelength W3 W51 Dotson et al. 2000 Dotson et al. 2005 Schleuning et al. 2000 (350 m grayscale/contours) Chrysostomou 2002 850 m 60 m, 100 m, 350 m,

  10. Measured Polarization Spectra in Cloud Envelopes (Vaillancourt 2002; Matthews et al. 2002)

  11. Expected Polarization Spectra(Hildebrand et al. 1999) Dust emission from • multiple grain species at • multiple temperatures Dust emission from • a single grain species at • a single temperature yields a flat spectrum in the FIR/SMM TA > TB, pA < pB TA > TB, pA > pB

  12. Model of Molecular Clouds A) Near embedded stars - warm dust, aligned via radiative torques B) Cooler dust away from stars; optically opaque clumps C) Cold surface layers exposed to the interstellar radiation field (ISRF) TA > TB > TC pA pC > pB ISRF C B B A

  13. Testing the Mixture ModelSpectralEnergyDistributions Temperature (K) Log(Relative Column Density) T1, Cold Component T=15 - 35 K OMC-1 T2, Warm Component T=35 - 55 K Polarization (%), Flux (Jy/beam) 28 K BNKL 52 K M42 KHW (Vaillancourt 2002)

  14. OMC-1 Relative Polarizing Power pcold/phot p1 / p2 OMC-1 Declination (arcmin) T=35 - 55 K Polarization (%), Flux (Jy/beam) BNKL 28 K M42 Trapezium 52 K KHW (Vaillancourt 2002)

  15. Dense vs. Diffuse ISM • Molecular clouds • Dense, hot, turbulent • Power sources: stars, turbulence, ISRF • IR Cirrus (Milky Way & external galaxies) • Diffuse, cool, little/no turbulence • Power source: ISRF, isotropic • All grains exposed to same environment

  16. IRAS 100m N. Gal. Pole (FDS99) Infrared Cirrus Clouds • Finkbeiner, Davis, & Schlegel (FDS99) -- high latitude dust • T = 9.5 K,  = 1.7 (silicate) • T = 16 K,  = 2.7 (graphite) • If silicate is polarized and graphite unpolarized then … • Polarization spectrum rises with increasing wavelength

  17. Polarimetry with SOFIA • Stratospheric Observatory for Infrared Astronomy • HAWC - SOFIA First Light infrared camera (2007) • 53, 88, 155, 215 m • SuperHAWC (2009 - 2010 ?) • HAWC upgraded to polarimeter 1) Split pol. Components and use single array … or 2) Add dual superconducting detector arrays http://astro.uchicago.edu/hawc http://www.sofia.usra.edu

  18. Infrared Cirrus with (Super)HAWC • 10-100 MJy/sr @ 100-200 m • Scientific Objectives • Are grains aligned / polarized? • Polarization Spectrum - which dust components are aligned ? • Is cirrus gravitationally bound? • Are clumps & filaments sub- or super- critical?

  19. SHARP - a polarization module for the SHARC-II camerahttp://lennon.astro.northwestern.edu/CSOpol August 2005 deployment at Caltech Submm Observatory - Mauna Kea • 1’  1’ F.O.V. • 12  12 pixels • 9’’ at 350 m • 11’’ at 450 m SHARC-II M82 SHARC-II 12  32 detector array SHARP Mars

  20. M82 SHARP Science • Sensitivity: 1% pol. error in 5 hours: • 2.7 Jy point source • 0.46 Jy / pixel ~ 150 MJy/sr over 1’ FOV • Brightest cirrus clouds approach 300 MJy/sr at 350 m • Find polarization spectra minimum (350 & 450 m) • Greater than, less than, or equal to 350 m ? • And more … • Dust and magnetic fields in external galaxies • Sgr A* and Galactic center • Turbulence in giant molecular clouds • Low-mass star formation Hertz

  21. Microwave Polarization SpectrumExtending the Mixture Model • Microwave flux excess (1-100 GHz) - “Foreground X” • Kogut et al. 1996, de Oliveira-Costa et al. 1997; Leitch et al. 1997 • Excess in microwave flux beyond contributions from thermal dust, free-free, and synchrotron emission • Strong correlation with dust emission (IRAS-100, DIRBE-240) • Possible sources • Electric dipole emission from small (< 0.001 m) “spinning dust” grains (Draine & Lazarian 1998) • Magnetic dipole emission from large (0.1 - 10 m) magnetic “vibrating dust” grains (Draine & Lazarian 1999) • How much does the excess contribute to diffuse flux? • Is the excess emission polarized ?

  22. Microwave excess in dark & diffuse clouds Lynds 1622 Diffuse ISM |b| < 4o vibrating dust Both spinning & vibrating dust consistent with excess thermal free-free free-free soft-synchrotron spinning dust (Hildebrand & Kirby 2004; Finkbeiner 2004) (Finkbeiner, Langston, & Mintner 2004)

  23. Polarization of Microwave Excess Spinning Dust Vibrating Dust Intrinsically different polarization spectra allow for test of spinning vs. vibrating dust models Lazarian & Draine 2000 Draine & Lazarian 1999

  24. Frequency (GHz) GHz • electric dipole - spinning dust • magnetic dipole - vibrating dust Total Flux Thermal Dust (16, 9 K) free-free m GHz vib. dust pol. cold pol. vib. dust unpol. Relative Polarization warm unpol. free-free unpol. spin. dust unpol. spin. dust pol. m Wavelength (micron)

  25. Summary • Polarization in dense clouds -- magnetic alignment of dust • Support for radiative torques to achieve suprathermal rotation • FIR/SMM polarization spectrum -- multiple domains of aligned & unaligned grains at multiple temperatures • Need polarization observations of diffuse ISM clouds & extension of spectrum to microwave • Possible with new instruments in next several years (WMAP, Planck, SHARP, HAWC/SOFIA) • Polarization spectrum can distinguish between grain emission models

  26. Frequency (GHz) • New • Old Beamsize (arcminutes) Wavelength (m) Future of FIR-MM Spectropolarimetry

  27. Polarization by absorption (UV, Visible, NIR) Slide from A. Goodman: http://cfa-www.harvard.edu/~agoodman/ppiv/

  28. Polarization by emission (FIR, SMM, MM) Slide from A. Goodman: http://cfa-www.harvard.edu/~agoodman/ppiv/

  29. SHARP Science • Predicted sensitivities • 1% pol. error in 5 hours: • 2.7 Jy point source • 0.46 Jy / pixel • 150 MJy/sr over 1’ FOV SuperHAWC M82 ~1 arcmin HAWC SHARP Hertz ~1 arcmin

  30. Polarization by extinction UV-Visible-IR, diffuse ISM (Heiles 2000)

  31. Polarization by emissionFIR-MM dense ISM Hertz @ CSO (e.g. Schleuning 1998, Dotson et al 2005)

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