JCMT’s Next Generation of Polarimeters:

POL-2 and ROVER

Brenda Matthews

(Herzberg Institute of Astrophysics)

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Polarimetry Targets with SCUBA

Range of target objects: Filaments, cores, galaxies, planetary nebula Non-exhaustive ADS search finds 28 refereed publications with

12 different first authors One consistent problem was the limited field of view

“scan mapping” polarimetry for larger areas never produced robust results

Difficulty in establishing the DC level of the background in the maps for I, Q and U

Ratio of U/Q in calculation of polarization angle makes this critical

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Star-forming RegionsLow Mass/Starless High Mass/Active

Planetary Nebula NGC 7027 Starburst Galaxy M82

Crutcher et al. 2004 Matthews & Wilson 2002 Curran et al. 2004

Greaves 2002 Greaves et al. 2000

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Outstanding Questions in Studies of Polarization of Interstellar Dust What is the role of magnetic fields (strength and

geometry) before and during protostellar collapse? (very few cases studied) Are they the variable which regulates star formation?

YES: Crutcher, Fiege, Stahler, MHD turbulence simulators

NO: Elmegreen, Hartmann, MHD turbulence simulators… hmmm…

What is the origin of the polarization holes?

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POL-2: for SCUBA-2

Advantages over SCUPOL SCUBA-2’s higher sensitivity (3-5 x SCUBA at 850 micron) Larger FOV

not all may be accessible to the polarimeter ~80-90% diameter > 5.6 arcminutes (> 5x SCUBA FOV)

Available all the time Removal of atmospheric effects to first order by rapid

modulation of the waveplate 850 and 450 micron data should be well calibrated

can use calibration polarizer

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Area

?

Larger field of view will greatly facilitate mapping of large and / or filamentary clouds which were a real challenge for SCUPOL.

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Sky Noise

Artefacts of Chopping

Traditional “chopping” of the secondary mirror for a differential measurement will not be an issue with SCUBA-2.

Rely on rapid waveplate modulation to remove sky noise (rotation speed 12.5 Hz) with detectors reading at 200 kHz, binned to 20 Hz.

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POL-2: The Basics

fixed (reflecting half signal)spinning

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POL-2: The Basics

Half-waveplate Orientation (degrees)

Oscillating signal received by SCUBA-2 from a linearly polarized beam as the waveplate rotates

Alignment of waveplate plane of polarization with analyzer

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Polarimeter Construction

Ongoing at the University of Montreal (PI: Pierre Bastien)

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POL-2: Observing Example

Source smaller than SCUBA-2 FOV

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POL-2: Observing Example

sky

Ip

Total signal will consist of the Earth’s atmosphere emission (“sky”), unpolarized light from the source and a modulated signal due to the modulating polarized component.

P% = 100 x (Imax-Imin)/(Imax+Imin)

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Calculating the Components

Imin is unknown unless the sky level can be estimated Estimate from blank sky? Could also be estimated from a measurement without

rotating the waveplate Imin = Iobs – (Ip at waveplate angle)

Which observing mode is adopted will be critical

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So, How Fast Is It? (SCUPOL v. POL-2)

1 FOV to 5 mJy (1 sigma polarized rms) at 850 micron

S x (P/100) -------------- S/N

e.g. 1 Jy source polarized at 2%, requiring a S/N of 4

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So, How Fast Is It? (SCUPOL v. POL-2)

1 FOV to 5 mJy (1 sigma polarized rms) at 850 micron

With SCUBA (jiggle/chop/nod) ~ 10 hours

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So, How Fast Is It? (SCUPOL v. POL-2)

1 FOV to 5 mJy (1 sigma polarized rms) at 850 micron

With SCUBA (jiggle/chop/nod) ~ 10 hoursWith SCUBA-2* (no chop/nod) ~ 3 minutes !

Most known targets will be well detected with an rms of 0.6 mJy/beam (3.5 hours on source)

likely the deepest polarimetry observation

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So, How Fast Is It? (SCUPOL v. POL-2)

1 FOV to 5 mJy (1 sigma polarized rms) at 850 micron

With SCUBA (jiggle/chop) ~ 10 hoursWith SCUBA-2* (no chop/nod) ~ 3 minutes !

Statistically significant numbers of objects will be observable with POL-2e.g. 100 cores in Gould Belt Survey to 1 mJy rms (126 hours) + 10 x 300 sq arcmin fields to 1 mJy rms (80 hours)

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Variable Polarization Targets e.g. Sag A*

Flux density varies from 0.5-5 Jy and is typically polarized around the 10% level 50-500 mJy polarized intensity good angular measure 10 sigma

850 m 450 m

Time interval Pol rms 3 Pol rms 3

1 minute 8.8 26.4 30 90

3 minutes 5.1 15.3 16 48

10 minutes 2.8 8.4 8.8 26.5`

20 minutes 2.0 6.0 6.2 18.6

Sag A* varies on timescales 20 min (Bower et al.)

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POL-2 summary

Allows for observations of many more objects than its predecessor Significantly deeper observations 450 micron observing likely to be common

Faster speed means larger areas* and variable objects will be monitored easily over multiple epochs

* Subject to constraints in mapping methods

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Polarization of Spectral Lines

Goldreich-Kylafis Effect (Goldreich & Kylafis 1981, 1982; Kylafis 1983, 1983, 1983) Theoretical prediction of linear polarization of molecular lines Observationally confirmed in 1997 toward the evolved star

IRC +10126 in CS 2-1 emission (Glenn et al. 1997)

Linear polarization of pure rotational emission arises from molecules in the presence of a magnetic field due to imbalances in the magnetic sublevel populations

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Polarization of Spectral Lines

Polarization levels are only around 1%, making detections very challenging Stronger in lower transitions Stronger in optically thin regimes

Benefits are evident: Regions with different velocities are spectrally

separated Quasi-3D picture of fields in rotating, outflowing or

infalling gas is possible

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Polarization of Spectral Lines

Promising technique to probe fields in outflows, cloud envelopes galaxies

NGC 1333 IRAS 4ABIMA array

Girart et al. (1999)

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ROVER (ROVing polarimetER)

Polarimeter module completed and tested in March 2003

Tested at IRAM 30m in May 2003 Continuous spin timing

accuracy at the millisecond level

“world’s first imaging spectropolarimeter”

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ROVER: for HARP-B

345 GHz range (e.g. CO 3-2 line) 12 of 16 HARP-B beams received without vignetting Design is similar to the SCUBA polarimeter

Halfwave plate, analyzer More flexible motor and controller system for faster data rates Utilize new correlator, ACSIS, with its fastest sampling speed

of 1/20th second

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ROVER & XPOL: SiO Maser R Leo

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Timelines

ROVER: already delivered to Hawaii Commissioning with HARP-B/ACSIS this fall (06B)

POL-2: less definite 3-6 months after SCUBA-2 commissioning Expect earliest availability to users in Spring 2008

(08A) Required for ~200 hours of allocated time on the

“Gould Belt Legacy Survey”