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) 12 Septembre

John Vaillancourt 12 Sept Overview Polarization by emission in molecular clouds Polarization spectra –  m = GHz Extension to … –Dark clouds and the diffuse ISM –Longer wavelengths: ~ 3 cm, ~ 1 GHz –New instruments (WMAP, Planck, SHARP, HAWC/SOFIA)

John Vaillancourt 12 Sept Absorption vs. Emission: dense & diffuse regions Hildebrand 2002; Heiles 2000; Dotson et al. 2000, 2005 Polarization by absorption UV-Vis-NIR Diffuse ISM Polarization by emission FIR-MM Dense ISM

Grain Alignment See reviews by Roberge 2004, Lazarian & Yan 2004, Astrophysics of Dust, ASP Conf. Ser Larmor precession Step 1: Internal Alignment Step 2: Angular Momentum Alignment Suprathermal rotation (Purcell 1979) E rot » kT gas H 2 ejection (Purcell 1979) radiative torques (Draine & Weingartner 1996)

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

John Vaillancourt 12 Sept Loss of alignment in dense clouds? OMC-1 Schleuning 1998 Polarized Flux (P  F) P abs =   P emis Polarized flux traces aligned grains in dense regions

John Vaillancourt 12 Sept Alignment near embedded stars 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) Schluening et al W51 W3 350  m Vaillancourt et al., in prep. W3A

John Vaillancourt 12 Sept 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)

John Vaillancourt 12 Sept Magnetic field vs. Wavelength 60  m, 100  m, 350  m, 850  m W3 W51 Schleuning et al (350  m grayscale/contours) Dotson et al Dotson et al Chrysostomou 2002

John Vaillancourt 12 Sept Measured Polarization Spectra in Cloud Envelopes (Vaillancourt 2002; Matthews et al. 2002)

T A > T B, p A > p B Expected Polarization Spectra (Hildebrand et al. 1999) Dust emission from a single grain species at a single temperature yields a flat spectrum in the FIR/SMM Dust emission from multiple grain species at multiple temperatures T A > T B, p A < p B

John Vaillancourt 12 Sept 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) T A > T B > T C p A  p C > p B Model of Molecular Clouds ISRF

Polarization (%), Flux (Jy/beam) Temperature (K) Log(Relative Column Density) T= K T 2, Warm Component T= K 28 K 52 K Testing the Mixture Model Spectral Energy Distributions T 1, Cold Component OMC-1 BNKL M42 KHW (Vaillancourt 2002)

John Vaillancourt 12 Sept T= K Declination (arcmin) OMC-1 Relative Polarizing Power p 1 / p 2 BNKL M42 Trapezium KHW p cold /p hot (Vaillancourt 2002) Polarization (%), Flux (Jy/beam) 28 K 52 K OMC-1

John Vaillancourt 12 Sept 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

John Vaillancourt 12 Sept 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 IRAS 100  m N. Gal. Pole (FDS99)

Stratospheric Observatory for Infrared Astronomy HAWC - SOFIA First Light infrared camera (2007) –53, 88, 155, 215  m SuperHAWC ( ?) –HAWC upgraded to polarimeter 1) Split pol. Components and use single array … or 2) Add dual superconducting detector arrays Polarimetry with SOFIA

John Vaillancourt 12 Sept Infrared Cirrus with (Super)HAWC  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?

SHARP - a polarization module for the SHARC-II camera 1’  1’ F.O.V. 12  12 pixels 9’’ at 350  m 11’’ at 450  m M82 SHARP SHARC-II 12  32 detector array SHARC-II August 2005 deployment at Caltech Submm Observatory - Mauna Kea Mars

John Vaillancourt 12 Sept SHARP Science Hertz 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 M82

John Vaillancourt 12 Sept Microwave Polarization Spectrum Extending the Mixture Model Microwave flux excess (1-100 GHz) - “Foreground X” –Kogut et al. 1996, de Oliveira-Costa et al. 1997; Leitch et al –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 (<  m) “spinning dust” grains (Draine & Lazarian 1998) –Magnetic dipole emission from large (  m) magnetic “vibrating dust” grains (Draine & Lazarian 1999) How much does the excess contribute to diffuse flux?How much does the excess contribute to diffuse flux? Is the excess emission polarized ?Is the excess emission polarized ?

John Vaillancourt 12 Sept Microwave excess in dark & diffuse clouds Both spinning & vibrating dust consistent with excess (Hildebrand & Kirby 2004; Finkbeiner 2004) vibrating dust (Finkbeiner, Langston, & Mintner 2004) spinning dust free-free thermal soft-synchrotron free-free Diffuse ISM |b| < 4 o Lynds 1622

John Vaillancourt 12 Sept Polarization of Microwave Excess Intrinsically different polarization spectra allow for test of spinning vs. vibrating dust models Spinning DustVibrating Dust Lazarian & Draine 2000 Draine & Lazarian 1999

John Vaillancourt 12 Sept Wavelength (micron) Frequency (GHz) Thermal Dust (16, 9 K) electric dipole - spinning dust magnetic dipole - vibrating dust free-free Total Flux Relative Polarization warm unpol. cold pol. free-free unpol. spin. dust unpol. spin. dust pol. vib. dust pol. vib. dust unpol. mm mm GHz

John Vaillancourt 12 Sept 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

John Vaillancourt 12 Sept

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

John Vaillancourt 12 Sept Polarization by absorption Slide from A. Goodman: (UV, Visible, NIR)

John Vaillancourt 12 Sept Polarization by emission Slide from A. Goodman: (FIR, SMM, MM)

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

John Vaillancourt 12 Sept Polarization by extinction UV-Visible-IR, diffuse ISM (Heiles 2000)

Polarization by emission FIR-MM dense ISM CSO (e.g. Schleuning 1998, Dotson et al 2005)