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Astron. Astrophys. 344, 668-674 (1999)

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2. Observations

The observations were made at the James Clerk Maxwell Telescope (JCMT) in September 1995. The Aberdeen/QMW polarimeter, consisting of a rotating half-waveplate and fixed wire-grid analyser, (Murray 1991, Murray et al. 1992) was used in conjunction with the continuum bolometer instrument UKT14 (Duncan et al. 1990). The observations were made at wavelengths of 800 and 1100 [FORMULA].

The observing procedure has been described by Greaves et al. (1994); briefly, photometry was done at 16 equally spaced waveplate positions in a 360o degree cycle, and the sinusoidal modulation of the signal was used to deduce the Stokes parameters Q and U, and hence p and [FORMULA]. Integration times ranged from 8 to 20 waveplate cycles each of 5 minutes duration. The data were analysed using a least-squares fit in the SIT reduction package (Nartallo 1995). Despiking was used to improve the fits, and a Kolmogorov-Smirnov test (e.g. Press et al. 1988) was used to eliminate fits with discontinuous Q or U, usually caused by changes in sky transmission. Approximately 7% of the data were rejected in the K-S test.

Instrumental polarization (IP) was measured using Saturn (assumed unpolarized), and was subtracted in the data reduction. The IP level was 1.61 [FORMULA] 0.03% at 1100 [FORMULA], and 0.28 [FORMULA] 0.04% at 800 [FORMULA]. The large change in IP percentage as a function of wavelength is due to the effects of the JCMT wind-blind, which has a polarization minimum at around 800 [FORMULA].

The sources observed are listed in Table 1. Central positions were Mon R2-IRS2: RA(1950) = 06h 05m 19.4s, Dec.(1950) = [FORMULA] 22´ 27", Mon R2-IRS3: 06h 05m 21.5s, [FORMULA] 22´ 29" (Aspin & Walther 1990), DR21: 20h 37m 14.5s, +42o 09´ 00" (Minchin & Murray 1994), and W3-IRS4: 02h 21m 43.5s, +61o 52´ 49" (Oldham et al. 1994). For W3-IRS4 and the point offset to the north, the data were combined with approximately 1 hour of observations made in 1991, which slightly improved the signal-to-noise.


Table 1. Polarization percentages and position angles at 800 and 1100 [FORMULA]. The values of p and the errors in [FORMULA] have been corrected for bias due to the positive nature of p (Wardle & Kronberg 1974). Position offsets are in arcseconds; beam sizes were set to 19" by the use of an aperture, apart from the W3 data, where a diffraction-limited 14" beam was used.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999