3/11/09C. D. Dowell, Keck Institute/Dust Polarimetry Opportunities for Dust Polarization Surveys C....

3/11/09 C. D. Dowell, Keck Institute/Dust Polarimetry

Opportunities for Dust Polarization Surveys

C. Darren Dowell (Caltech)

1/30

Outline• magnetic fields in interstellar clouds

• observational and theoretical approaches

• dust composition, shape, and alignment

• sensitivity comparison

• survey approaches

3/11/09 C. D. Dowell, Keck Institute/Dust Polarimetry 2/30


Magnetic Field Strengths in Interstellar Clouds


Galactic Field in AV ≈ 1 Medium

optical polarimetry


Galactic Field in AV ≈ 10 Medium

BICEP: southern sky at = 2 mm, 1º resolution


Galactic Field in AV ≈ 100 MediumFIR-bright cloud cores: no B angle correlation with Gal. plane

data from Dotson et al. (2000, 2008)

Measuring Magnetic Fields

• In addition to the usual problems with line-of-sight averaging and unresolved structure:

• vector B = (Bx, By, Blos)

– Blos: Zeeman, Faraday rotation

– tan-1(By/Bx): synchrotron, dust polarization


Measuring Magnetic Fields

• In addition to the usual problems with line-of-sight averaging and unresolved structure:

• vector B = (Bx, By, Blos)

– Blos: Zeeman, Faraday rotation

– tan-1(By/Bx): synchrotron, dust polarization

– √(Bx2+By

2): dust polarization via Chandrasekhar-Fermi approach?


WMAP all-sky synchrotron map



Berkhuijsen (1997)

complementarity of synchrotron and dust mapping


Blue: Spitzer 8 m red: Bolocam/CSO 1.1 mm purple: VLA 20 cm

Bally et al. (2009)

complementarity of synchrotron and dust polarization


Chuss et al. (2003)

60-350 m


Far-IR polarimetry is an excellent tracer of magnetic fields at densities up to 106 cm-3.

Schleuning (1998)

Rao et al. (1998) Ward-Thompson et al. (2000)

Orion Core

L183 Dark CloudB vec

B vec

Tests of Cloud and Core Formation Theory with FIR Polarimetry

• Ordered structures– flow along field lines– tidal shear– swept-up shells– accretion disks

• Large features resulting from instabilities

• Dispersion in field– Chandrasekhar-Fermi approach



Far-IR polarimetry tests geometrical models of magnetic fields: protostars

“hourglass” field in protostellar envelope

NGC 1333 IRAS 4A

observation w/ interferometer:

Girart, Rao, & Marrone (2000)model:

Fiedler & Mouschovias 1993

B vec


Far-IR polarimetry tests geometrical models of magnetic fields: cloud cores

Kirby (2008)Schleuning (1998); Houde et al. (2004)


Kim & Ostriker (2006)

swing amplifier effect magneto-Jeans instability

Kim & Ostriker (2001)

Far-IR polarimetry tests geometrical models of magnetic fields: cloud formation


Magnetic Field Strength from Chandrasekhar-Fermi Method

• B = x 1/2 v / • x: Ostriker et al.(2001); Padoan et al. (2001); Heitsch

et al. (2001); Falceta-Goncalves et al. (2008)

strong field: small dispersion weak field: large dispersion

Falceta-Gonçalves, et al. (2008)


Magnetic Field Strength from Chandrasekhar-Fermi Method

• B = x 1/2 v / • x: Ostriker et al.(2001); Padoan et al. (2001); Heitsch

et al. (2001); Falceta-Goncalves et al. (2008)

strong field: small dispersion weak field: large dispersion

Falceta-Gonçalves, et al. (2008)

B 80 G

Grain Alignment & Composition• current theory of grain

alignment (Lazarian et al.):– Paramagnetic relaxation

no longer needed.– Instead, radiative torques

on asymmetric grains will do.

– Alignment with polarization perpendicular to field still applies.

• Observational tests possible.


Vaillancourt et al. (2008)

Into the Next Decade• optical/near-IR: big surveys of starlight polarization• FIR:

– BLASTpol: mapping full molecular clouds– SOFIA: precision application of

Chandrasekhar-Fermi• submm:

– Planck: all-sky polarization survey– ALMA: great for circumstellar dust?

• radio: EVLA, GBT


HAWCpol/SOFIA


observation bands 53, 89, 155, 216 m

angular resolution 5 – 22 arcsec

field of view 0.5×1.2 – 1.6×4.3 arcmin2

polarization modulation technique quartz half-wave plate, 15 rpm

minimum flux density to achieve(P) = 0.2% in 5 hour integration

9, 6, 6, 5 Jy

minimum column density to achieve(P) = 0.2% in 5 hour integration

AV = 1, 2, 5, 4

systematic error goal P < 0.2%; < 2°

• started Oct. 2008 on JPL internal funds

• permanent upgrade to HAWC

• good for sources up to ~8’

Required Polarization Sensitivity• typical degree polarization = 3%

• typical intrinsic dispersion = 10° – 30°() = 3°, (P) = 0.3%photometric signal-to-noise of 500


Hildebrand et al. (1999)


Far-IR polarimetry is 106 faster from space.

sensitivity to extended emission

BLASTpol

Giant Steps

• MIDEX-class FIR polarization survey

• SAFIR polarimeter

• SPIRIT polarimeter

• EPIC (CMBPol) with extended high-frequency coverage

• SKA


Dedicated Polarization Survey


• PIREX/M4 (Clemens, P.I.; Goodman; Jones)– satellite proposed to NASA 1990, 1993,

1996– possible reasons for non-selection:

• Polarimetry not a scientific priority for NASA.• Difficult for a cryogenic mission to compete on

the NASA SMEX playing field.

• Unsuccessful Origins Probe proposal to study FIR polarimeter and C+ heterodyne spectrometer on 0.5 – 1 m telescope (Dowell, Langer et al.)

M4:

0.2 m cold telescope


with SAFIR/CALISTO:

5 hours integration time (104 detectors)

5” = 20 pc resolution at = 100 m

likely detection of polarization wherever AV > 0.3

MIPS 24 m (Gordon et al.)

EPIC Polarization Mapping

• EPIC/850 GHz can make accurate polarization maps with 108 resolution elements.

coverage map, (P) ≤ 0.3%

Planck/350GHz 5’ EPIC/850GHz 40K, 5’EPIC/850GHz 4K, 1’

3/11/09 28C. D. Dowell, Keck Institute/Dust Polarimetry


Enabling Technologies for Space Far-IR Polarimetry

• polarization-sensitive detectors• polarization modulation

– low-power cryogenic rotating quartz half-wave plate– scan modulation only (Boomerang Planck)

• Good polarimeters usually make good cameras.– NEP = 10-18 W Hz-1/2 is adequate for detectors.

Antenna-Coupled 200 m TES Detector


work by Peter Day et al. (JPL/Caltech)


“If nuclear and gravitational forces were the only forces at work in the universe, the broad pattern of cosmic evolution would be one of gradual thermal degradation punctuated by occasional explosive events. The cosmos would resemble the serene and monotonous heavens of classical conception. There is, however, a cosmic agitator: the magnetic field. Although only a small part of the available energy in the universe is invested in magnetic fields, they are responsible for most of the continual violent activity of the cosmos, from auroral displays in the earth's atmosphere to stellar flares and X-ray emission, and the massing of clouds of interstellar gas in galaxies.”

E. Parker, Scientific American (1983)

3/11/09 C. D. Dowell, Keck Institute/Dust Polarimetry


Theory and modeling will play an important role in interpretation of polarimetry.

• MHD simulations to get factors of order unity in application of Chandrasekhar-Fermi method

• magnetic field models of specific structures– forming molecular clouds– collapsing cores: magnetic regulation, magnetic braking– circumstellar disks and envelopes– accretion into Galactic center

• dust grain alignment theory to understand environmental effects on and wavelength dependence of polarization


Polarization Power Spectrum: Probe of Turbulence


Magnetic Field Strengths in Interstellar Clouds

Crutcher (2008)

strong field

weak field


Polarization Power Spectrum: Probe of Turbulence

Some expectation of steepening of spectrum at ambipolar diffusion length scale 0.002 pc 1” in Orion (Li & Houde 2008)


Magnetic fields and IR astrophysics

• large-scale gas flows in galaxies

• timescale of cloud and star formation

• shapes of clouds

• turbulence

• disk, wind, jet, and shock physics


Multi-Wavelength Imaging and Polarimetry

warm and cold components in Galactic center

circumnuclear ring

Vaillancourt et al. (2008)

continuum polarization spectrum normalized to 350 m

32 m 450 m

Latvakoski et al. (1999)


Bpos vs. elongation angle in cloud cores

Tassis et al. (2008)

roundelongated

data from Dotson et al. (2000, 2008)


Space far-IR polarimetry with a 5+ m telescope is an essential tool.

• Consider a modest instrument on a 5 m telescope cooled to 4 K (‘CALISTO’):– 1000 polarization-sensitive pixels covering bands at

= 30 - 300 m (most of the mass and luminosity, component separation)

– angular resolution 5˝ at 100 m (protostellar size scale)

CALISTO: design

concept for SAFIR


Prospects for space far-IR polarimetry

• Infrared Space Observatory (1995-1998): polarimeter add-on, limited results• M4: not funded• Spitzer Space Telescope (2003-2009): no polarimetry• Herschel Space Obs. (2009-2013): no polarimetry• Planck (2009-2011): all-sky survey, 300˝ resolution• SPICA (2015-2020): no polarimetry planned

• 2001 Decadal Survey:– “Magnetic fields play a crucial role in … the formation of stars….”– “Polarization” only mentioned in context of CMB.

• Much the same story in NASA science plans.

• Conclusion: Need to start campaigning early for a polarimeter on a big, cold telescope.


Wide Polarization Survey from Space• Map 100 square degrees worth of molecular clouds at

= 100 m (5˝ resolution with 5 m telescope):– Detect polarization where AV ≥ 0.3– Requires 50 hours with 10,000 pixel detector array

• Contours show regions where (P) would be < 0.3%.• Millions of polarization vectors map of magnetic

field structure and strength on almost all relevant angular scales.

Orion A

Orion B

Oph10º

3/11/09C. D. Dowell, Keck Institute/Dust Polarimetry Opportunities for Dust Polarization Surveys C....

Documents

Transcript of 3/11/09C. D. Dowell, Keck Institute/Dust Polarimetry Opportunities for Dust Polarization Surveys C....