3. Science Case

1. POL-2: a Polarimeter for SCUBA-2P. Bastien, É. Bissonnette, Université de Montréal, Montréal, Québec, CanadaP. Ade, G. Pisano, G. Savini, Cardiff University, Cardiff, UKT. Jenness, Joint Astronomy Center, Hilo, Hawaii, USAD. Johnstone, B. Matthews, Herzberg Institute of Astrophysics, Victoria, B. C., CanadaJ. Molnar, University of British Columbia, Vancouver, B. C., Canada 1. What is SCUBA-2 SCUBA-2 is a second generation wide-field submillimeter camera under development for the James Clerk Maxwell Telescope. With over 10 000 pixels in two arrays (one at 850 microns and one at 450 microns), SCUBA-2 will map the submillimeter sky up to 1000 times faster than the current SCUBA instrument to the same signal-to-noise. • 4. Basic Requirements For The Polarimeter • Removal of atmospheric effects to first order. Polarization at submillimeter wavelengths, similarly to polarization at optical and near infrared wavelengths, is usually affected by rapid variations of the atmosphere. The proposed polarimeter shall be able to remove such effects to first order. This will be accomplished by rapid modulation of the polarization signal, at a speed faster than typical atmospheric variations. At optical wavelengths, this is at about 10 Hz. This will be used as a guide for atmospheric variations in the submillimeter. • 2) Possibility to calibrate and test the instrument performance while on the telescope. The polarimeter shall allow such calibration with a simple computer command. • 3) Be available all the time. The required optical components can be put into the beam in front of the SCUBA-2 entrance window by computer commands. Many areas of astronomy will benefit from such a highly sensitive survey instrument: from studies of galaxy formation and evolution in the early Universe to understanding star and planet formation in our own Galaxy. Due to be operational in 2006, SCUBA-2 will also act as a “pathfinder” for the new generation of submillimeter interferometers (such as ALMA) by performing large-area surveys to an unprecedented depth. 5. Design Three optical components: 1) Removable polarizer for calibration and/or testing 2) Achromatic rotating half-wave plate 3) Wire-grid polarizer The achromatic waveplate design (5 birefringent plates) will be anti-reflection coated for an average transmission efficiency of ≈ 95% (low-frequency) and ≈ 85% (high-frequency).The waveplate will introduce a phase-shift of ≈180°, and less than 1% average ellipticity in both bands. Figure 1: OMC-3 850 mm polarimetry with SCUBA Matthews, Wilson, & Fiege 2001 ApJ 562, 400 2. A Polarimeter POL-2, a polarimeter to be used with SCUBA-2, is also under development. This polarimeter will be the most sensitive instrument for the detection of polarized radiation in the submillimeter regime. This will be possible by taking advantage of the extra sensitivity, imaging speed and improved image fidelity of the new SCUBA-2 camera. 3. Science Case Dust grains in molecular clouds and in other environments can be aligned by various mechanisms. When they are aligned, the radiation they emit is polarized. The most common alignment mechanism is related to magnetic fields; however, other mechanisms, such as the radiation field, could play a role in some environments (e.g., neighbourhood of H II regions). ► Investigating the geometry of magnetic fields within astronomical sources. These fields are prevalent throughout galaxies, from the largest scales to the small cores that are collapsing to form stars within molecular clouds. Understanding the geometry of these fields, both at a global and a detailed level, is crucial to our understanding of star formation processes and the physics of molecular clouds. Polarimetric maps of dense filamentary clouds in Orion obtained with SCUBA have shown that the magnetic field structure (Figure 1) can be explained with a theoretical model of filamentary clouds with a helical magnetic field (Fiege & Pudritz 2000, MNRAS 311, 85). ► 3-D maps of the field configuration by combining three observational techniques as in the M17 molecular cloud (Figures 2 & 3). The strength of the magnetic field along the line of sight is provided by Zeeman measurements; submm polarimetric measurements give the orientation of the field in the plane of the sky; and the ion-to-neutral molecular line width ratio determines the angle between the magnetic field and the line of sight. POL-2 will provide essential measurements for studying magnetic fields in 3-D in other regions. ►Study magnetic fields around Young Stellar Objects, and determine their influence on the evolution. Many models have been published so far (see Fig. 4 for an illustration) and it is important to provide observational constraints to them. ► Learn more about the physics of grains, by measuring the polarization at various wavelengths. Combining measurements at 450 and 850 mm with those from other instruments (e.g., Hale on SOFIA), it will be possible to measure P(l) and put constraints on grain properties. ► Observations of the magnetic field geometries of many external galaxies will be possible with the increased sensitivity of SCUBA-2. Despite the fact that the resolution of the JCMT is 15'' at 850 mm, Greaves et al. (2000, Nature 404, 732) made the first measurement of the magnetic field configuration in an external galaxy: M82. They concluded that the magnetic field is being forced into the galactic halo by the effects of strong star formation in the centre while at other locations, the field could be helping to funnel gas into the centre, thereby fuelling the star formation. Figure 2: M17 Houde et al 2002, ApJ 569, 803 6. Operation In order to remove atmospheric transparency fluctuations, the wave plate will be rotated. The readings of the arrays need to be done in synchronicity with the rotation of the wave plate. A typical rotation speed would be 12.5 Hz. For each rotation of the wave plate, the signal would be divided into a fixed number of bins (16). The detectors will be read at 20 kHz, and these ‘images’ binned to 200 Hz. Therefore, for 0.96 second of integration, there will be 12 images that would accumulate into each of the 16 ‘image bins’. These bins can accumulate the signals for the whole measurement, for example, a 10 minute integration time. 7. Polarimeter Data Processing As the waveplate rotates each data frame acquired will be tagged with the waveplate position angle. These frames will be processed by the DR pipeline resulting in stacked images corresponding to identical waveplate positions. The frames will then be run processed by the Starlink POLPACK application to calculate the Stokes parameters. To prevent problems with sky rotation the Stokes parameters will be calculated every few seconds and co-added with previous cubes prior to calculating the polarization. The DR pipeline data products will be a cube of IQU Stokes parameters and a FITS catalogue of polarization information. Figure 3: M17 at 350 mm Houde et al 2002, ApJ 569, 803 8. Milestone Dates Conceptual Design Review (CoDR) October 29th, 2003 Complete Select opto-mechanical design ……………………… May 2005 Preliminary Design Review (delta PDR) .................. June 2005 Wave-plate test ……………………………………….. October 2005 Critical Design Review (CDR) ………………………. December 2005 Complete Instrument Test……………………………. April 2006 Acceptance Readiness Review (ARR)……………… May 2006 Delivery to system integration ………………………. June 2006 Installation of opto-mechanical unit ………………… Installed together with SCUBA-2 Cryostat Commissioning………………………………………… 3-6 months after SCUBA-2 commissioning Polarimeter Support…………………………………… until SCUBA-2 decommissioning Figure. 4: Magnetic fields Vallée 2002, AJ 123, 382