On-Going Instrumentation: Polarization Modulator

While polarimetry is a powerful diagnostic tool, it requires a large number of photons. The gains that can be realized by coupling the Gemini telescopes to efficient polarimeters should be quite spectacular; both reducing the time that many observations currently require and making other observations possible for the first time.

Polarization measurements will require implementation of a polarization modulator in the Acquisition and Guiding (A&G) unit above the bottom port of the Gemini Instrument Support Structure (ISS). The NIRI and GNIRS Phase I instruments can accommodate Wollaston prism polarization analyzers, however, implementation of polarizing capability in GMOS and HROS will require upgrades to the Phase I instruments.

Performance Guidelines for the Polarization Modulators:

• Optical/IR polarization modulator plates mounted in the A&G unit in front of up-looking ISS port.
• Implementation must be remotely retractable and allow for unvignetted use of bottom port over a minimum 7 arcmin diameter science FOV with a goal of 9 arcmin
• Wavelength coverage: 0.3 - 1.2µm for GMOS, HROS and 1-5µm with a goal of 0.3-2.5µm with a single waveplate
• FOV: minimum of 1 arcmin. Extended FOV for OIWFS must be maintained
• Assessment of possible operating modes with GMOS and HROS
• A minimum of two modulator wheels that can be used in series
• Calibration capability

Science Illustrations:

Spectropolarimetry is highly 'photon-starved' since the percentages of linear polarization are typically a percent or so even for Be stars presenting nearly edge-on discs. In the past this has meant that polarimetry has mostly been applied at broad-band rather than high spectral resolution. Spectropolarimetry at echelle resolution can be a routine option with Gemini.

Magnetic fields almost certainly play a central role in the star formation process. They are important in all stages from the collapse of molecular clouds to the formation and evolution of disks and envelopes. The component of the field in the plane of the sky can be traced through the effects of aligned non-spherical dust grains on the transmission, emission and/or scattering of light. The study of magnetic field directions at high (arcsec) resolution has been confined to the brightest star forming regions. For example, dichroic extinction measurements have been made with imaging polarimetry towards the diffuse unpolarized H₂ emission at 2µm in Orion and through observations of polarized emission and absorption at 17 µm in Orion. As with all polarimetric measurements, Gemini offers very substantial gains but the advantages at these infrared wavelengths are particularly large.

Imaging- and spectro-polarimetry observations can be powerful diagnostics of the emission mechanism(s) and geometry of extragalactic sources (Seyfert galaxies, broad absorption line quasars, Ultra-luminous IRAS sources, and high redshift radio galaxies). The detection of polarized radiation from a source is an indication that either synchrotron emission, scattering (by dust and/or electrons) or dichroic extinction by dust grains is occurring.

Last update June 5, 1998; Ruth A. Kneale