JCMTs Next Generation of Polarimeters: POL2 and ROVER

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JCMTs 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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Crutcher et al. 2004
Matthews Wilson 2002
Curran et al. 2004
Star-forming Regions
Low Mass/Starless
High Mass/Active
Greaves et al. 2000
Greaves 2002
Planetary Nebula NGC 7027
Starburst Galaxy M82
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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-2s higher sensitivity (3-5 x SCUBA at 850
  micron)
• Larger FOV
• not all may be accessible to the polarimeter
• 80-90 ? diameter gt 5.6 arcminutes (gt 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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POL-2 Limitations

• Linear polarization only (assume Stokes V is zero
  from dust)
• No estimate of field strength from POL-2 data
• May be necessary to perform an independent
  measurement of total intensity (without rotating
  the polarimeter)

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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
Alignment of waveplate plane of polarization with
analyzer
Half-waveplate Orientation (degrees)
Oscillating signal received by SCUBA-2 from a
linearly polarized beam as the waveplate rotates
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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
P 100 x (Imax-Imin)/(ImaxImin)
Total signal will consist of the Earths
atmosphere emission (sky), unpolarized light
from the source and a modulated signal due to the
modulating polarized component.
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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
hours With 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 hours With
SCUBA-2 (no chop/nod) 3 minutes
! Statistically significant numbers of objects
will be observable with POL-2 e.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

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 4A BIMA 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

worlds 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