2. Observations and data reduction
Observations were made on August 13-15, 1997, using the Submilimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (Holland et al. 1999). Observations consisted of simultaneous 450/850µm maps and additional single-pixel photometry at 1350 and 2000µm. The SCUBA array covers a hexagonal field of view with 91 and 37 pixels at 450/850µm respectively. Maps are fully sampled using the `jiggle' mode, in which the telescope beam is moved around a 64-position pattern by the secondary mirror in order to fully sample the sky with the wider intrinsic beam spacing of the array. Observing parameters are given in Table 1. Table 2 gives source positions and distances (from Churchwell et al. 1990b)
Table 1. Observing parameters
Table 2. Source positions and assumed distances (from Churchwell et al. 1990b).
We reduced the data using the SCUBA User Reduction Facility (SURF; Jenness & Lightfoot 1998). The resulting images are shown in Figs. 1. The maps were calibrated from skydips and observations of Uranus as described in Sandell (1997). Fluxes for each source are given in Table 3. At 450 and 850µm we give both the single beam fluxes towards the peak and fluxes integrated over the entire SCUBA field of view, except for G12.21 where the chop throw results in positive and negative components for sources located near the edge of the image, and only the square region encompassing the central source was included. The flux for G10.47 is the total of the three condensations visible in the image. We note that for G10.47 the 350µm and probably also the 800µm fluxes of Hoare et al. (1991) are low compared to our 450 and 850µm fluxes, taking the wavelength difference into account, probably because we have integrated over a larger region. Uncertainties in the fluxes are dominated by uncertainties in the sky transmission, which varied substantially over the course of the observations. Estimates of the flux uncertainties from the Uranus observations are 5% and 20% at 850 and 450µm respectively. At 1350/2000µm, from the opacities uncertainties were 5% on 14 August (G10.47, G43.89) and 20% on 15 August (G12.21, G13.87, G31.41) (due to poorer weather). The beam was measured using observations of Uranus, and consisted of a roughly Gaussian main beam, with FWHM given in Table 1 plus an extended error lobe, which contained roughly 15% and 50% of the flux at 850 and 450µm respectively. Fig. 2 shows images of Uranus at both wavelengths. Because of the extended error lobe, we did not attempt any beam deconvolution. Instead, when comparing the images with models (see Sect. 4) we convolved the models with the maps of Uranus, assuming it to be a point source. In the case of a simple Gaussian deconvolution, the correction for Uranus's finite () diameter to the main beam FWHM of / is less than , and with much of the flux in the extended error beam at m the correction would be difficult to apply.
Table 3. Measured fluxes at 450 and 850µm (integrated over the entire image) and at 450, 850, 1350 and 2000µm (single pixel). Uncertainties are due to variation in the sky transmission (see Sect. 2).
The observations were taken with a chop throw. Given the extended nature of these sources, which are embedded in molecular cloud complexes (Scoville et al. 1987), the off positions will not have been entirely free of emission, and this will have been subtracted from the fluxes and as a background from the images. The fluxes are therefore strictly lower limits on the total flux in the SCUBA field of view. However, as we are most interested in the warmer dust corresponding to the warm cores which are seen by IRAS, the background-subtracted flux measurements are appropriate. As the flux in the images have fallen to a few percent of peak at the edge of the map, and there are no strong negative features, either the emission in the off position is negligible or we have subtracted a roughly constant background flux. An exception is G12.21 where we have clearly self-chopped onto the sources to the southeast of the main peak (Fig. 1). In the case of roughly constant background flux, the radial profiles (Sect. 4) trace only the varying component of density. At large radius, the radial profiles may be slightly distorted by this background subtraction process.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000