IR Polarization

I did IR polarimetry in the early and mid 1980ís.  Unfortunately I donít have any photos of the polarimeter. It was simple device, however, consisting of a sheet of HR Polaroid (HN-38). The sheet was mounted directly in front of the detector entrance window. The mount for the polaroid allowed it to be rotated to six different position angles. Since the detectors in those days were single devices, raster scan images were made at each of the position angles.  The result was a set of six images of an IR reflection nebula from which Stokes parameters and ultimately the polarization vector field could be determined. 

Using this technique, the image below shows the IR Nebula GSS30 as I published in 1985 in the Astrophysical Journal, Volume 290, p.261-272.  This is the first IR map of IR nebulosity in a dark cloud and was used to show for the first time the scattering nature of the nebula.


A few years later, in the early 1990ís, I was lucky enough to be at Kitt Peak when Steve Weintraub and Joel Kastner (Vanderbilt University) were using the 1.3 m (when it was still a KPNO scope) with a near IR polarimeter.  Hereís what we did:

2 Micron Polarization of the Orion Nebula


    The Orion Nebula Bar is a nearly edge-on transition zone between the HII region created by the Trapezium stars and neutral gas to the south.  Polarimetric images centered on the Trapezium at J (1.25 microns), H (1.65 microns), and K (2.2 microns) were obtained on 23 November 1993 using  the Simultaneous Quad-color Infrared Imaging  Device (SQIID; Ellis 1992) and half-wave plate mounted at the Cassegrain focus of the KPNO 1.3-m telescope. Four sets of 5.3 x 5.3 square arcmin images were made with the half-wave plate set at angles  of 0, 22.5, 45, and 67.5 degrees. The half-wave  plate does not transmit at L (3.6 mm), so there is no L data. HD 18881, HD 40335, and HD 203856 were observed to establish photometric calibration.   From field stars in the images of AFGL 437, taken with the same instrumental setup during this same observing run (Weintraub & Kastner, 1995), we determined that the instrumental polarization is less than a few tenths of a 1%.

The near-infrared electric field vectors are parallel to the Bar with the lowest amount of polarization in the Bar and the highest polarizations  occur near the edges of the Bar. The average amount of polarization at J, H, and K is 4%, 6%, and 10%, respectively. The average position angles at J, H, and K  are about 150, 135, and 110, degrees,  respectively. Based on Hubble's relation, we show that neither IRc2/BN nor the Trapezium stars are responsible  for the polarization. Also, we show that it is unlikely that dichroic absorption dominates the near-infrared polarization in the Bar because the time needed to align grains is greater than the time needed to disalign grains. The  characteristics of the polarization are consistent with a scattering model,  however, where the source of illumination may be the Bar itself. The  electric field vectors are parallel to the Bar and the H-K color through the Bar  becomes increasingly blue towards the edges of the Bar where the polarization is highest. You can receive a preprint of the paper upon request.   This paper has not been published, and is now a bit dated, with the data being taken in 1995.

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November 2004. mwc