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JCMT’s Next Generation of Polarimeters: POL-2 and ROVER

JCMT’s Next Generation of Polarimeters: POL-2 and ROVER. Brenda Matthews (Herzberg Institute of Astrophysics). Polarimetry Targets with SCUBA. Range of target objects: Filaments, cores, galaxies, planetary nebula

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JCMT’s Next Generation of Polarimeters: POL-2 and ROVER

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  1. JCMT’s Next Generation of Polarimeters: POL-2 and ROVER Brenda Matthews (Herzberg Institute of Astrophysics)

  2. 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 Summer School "Submillimeter Observing Techniques"

  3. Crutcher et al. 2004 Matthews & Wilson 2002 Curran et al. 2004 Low Mass/Starless Star-forming Regions High Mass/Active Greaves et al. 2000 Greaves 2002 Planetary Nebula NGC 7027 Starburst Galaxy M82 Summer School "Submillimeter Observing Techniques"

  4. 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? Summer School "Submillimeter Observing Techniques"

  5. 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 Summer School "Submillimeter Observing Techniques"

  6. Area Larger field of view will greatly facilitate mapping of large and / or filamentary clouds which were a real challenge for SCUPOL. ? Summer School "Submillimeter Observing Techniques"

  7. 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. Summer School "Submillimeter Observing Techniques"

  8. POL-2: The Basics fixed (reflecting half signal) spinning Summer School "Submillimeter Observing Techniques"

  9. 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 Summer School "Submillimeter Observing Techniques"

  10. Polarimeter Construction Ongoing at the University of Montreal (PI: Pierre Bastien) Summer School "Submillimeter Observing Techniques"

  11. POL-2: Observing Example Source smaller than SCUBA-2FOV Summer School "Submillimeter Observing Techniques"

  12. Ip sky POL-2: Observing Example P% = 100 x (Imax-Imin)/(Imax+Imin) 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. Summer School "Submillimeter Observing Techniques"

  13. 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 Summer School "Submillimeter Observing Techniques"

  14. 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 Summer School "Submillimeter Observing Techniques"

  15. 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 Summer School "Submillimeter Observing Techniques"

  16. 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 Summer School "Submillimeter Observing Techniques"

  17. 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) Summer School "Submillimeter Observing Techniques"

  18. 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.) Summer School "Submillimeter Observing Techniques"

  19. 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 Summer School "Submillimeter Observing Techniques"

  20. 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 Summer School "Submillimeter Observing Techniques"

  21. 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 Summer School "Submillimeter Observing Techniques"

  22. Polarization of Spectral Lines • Promising technique to probe fields in • outflows, • cloud envelopes • galaxies NGC 1333 IRAS 4A BIMA array Girart et al. (1999) Summer School "Submillimeter Observing Techniques"

  23. 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” Summer School "Submillimeter Observing Techniques"

  24. 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 Summer School "Submillimeter Observing Techniques"

  25. ROVER & XPOL: SiO Maser R Leo Summer School "Submillimeter Observing Techniques"

  26. 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” Summer School "Submillimeter Observing Techniques"

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