Astron. Astrophys. 353, 153-162 (2000)

2. Observations and data reduction

2.1. Near-infrared observations

To get information on the morphology of and extinction within BBW 192E at near-infrared (NIR) wavelengths, we performed imaging in the H and K´ bands using the IRAC2b camera at ESO's 2.2-m telescope on November 15th, 1994. For each filter three individual images (/pixel) shifted against each other by 1/3 of the detector field of view were obtained. The individual images were corrected for detector bias, bad pixels, sky background and sensitivity variations within the detector array by using flat field images derived from the sky. The final mosaics were obtained by combining the individual images using a cross-correlation method. In order to obtain exact reference coordinates, we compared the positions of seven stars in the NIR images with their optical positions in the Digitized Sky Survey (DSS, STScI 1994). With the exception of source  31 (, see Fig. 1a), the coordinates of all H and K´ sources agree within . Since the deviations between the positions of the H and K´ band sources are randomly distributed, we give averaged positions in Table 2. The absolute position accuracy of the presented NIR images is better than 2". The 5 detection limits amount to 15.3 and 14.4 mag for the H and K´ bands, respectively. Photometry for numerous objects within the H and K´ band images was performed using the DAOPHOT package (see Table 2). The DAOPHOT routines allow to separate the flux density contributions and determine the positions of close objects by fitting a point-spread function (PSF) simultaneously to all detected objects. The PSF has to be constructed from suitable objects within the image. In the case of objects which are not embedded in an extended infrared nebula, the quality of such a fit can be checked easily by the residual emission within the images. Due to the surrounding extended emission the photometric error might be higher for objects 20 and 25 (see Fig. 1a). In the cases where the fit of a PSF was not successful (objects  8, 9 and 22), we used the MAG/CIRCLE routine (MIDAS) and a fit of a two-dimensional Gaussian function to estimate the magnitudes and coordinates, respectively. As HK-standards, the stars HD 1274, HD 52467, HD 62998, and HD 218814 were used (see, Carter & Meadows 1995). The photometric accuracy amounts to 0.2 mag. In addition, JHKLM photometry of BBW 192E was obtained using the single-channel InSb photometer (diaphragm of 15") at ESO's 1.0-m telescope on November 27th, 1993.

Fig. 1a-e. NIR observations of BBW 192E are shown in all images (fully or partly). a  K´ band image, overlayed with white optical contours drawn from the DSS (3, 6, 12, 24, 48, 96, 192, 390 ) and the error ellipse of IRAS 08513-4201 (bold black). The elongated optical structure near source 13 is an artefact on the plate.  b  H band image and H contours (15.3, 14.3, 14.0, 13.7, 13.3, 12.6 mag/"),  c  K´ band image and K´ contours (14.4, 13.6, 12.9, 12.5, 12.1, 11.1, 10.9 mag/"). The circle encloses the area which was covered by the 15" aperture of the photometer measurements.  d  K´ band image which was cleaned from point-sources. K´ contours at 14.4, 13.6, 12.9, 12.5, 12.1 mag/".  e  Ks band image and linear polarization vectors. The horizontal bar illustrates 100% polarization. The cross marks the location of the illuminating source.

Near-infrared polarimetric observations were performed with SOFI (Finger et al. 1998) at the ESO-NTT on 1999 March 2nd in the Ks band. For each orientation (0^o/90^o, 45^o/135^o) of the Wollaston prism, five dithered frames were taken at the pixel scale of . The observing conditions during these measurements were excellent, i.e. photometric with very good seeing. The FWHM of the stellar profiles in the final image amounts to . After rebinning by a factor of 3, the linear polarization was derived for those regions where the signal level exceeds . The spatial sampling of the resulting polarization map is .

2.2. N, Q-band observations

MIR images were obtained in May 1996/March 1998 using TIMMI/MANIAC at ESO's 3.6-m/2.2-m telescopes, respectively. The cameras are described in Käufl et al. (1992, TIMMI) and Böker et al. (1997, MANIAC). The observations were performed using the standard chopping/nodding scheme at pixel scales of and for MANIAC and TIMMI, respectively. The spatial resolution of the MANIAC images (FWHM) is (TIMMI: ) and for the N and Q bands, respectively. The 3 detection limits for a "point" source amount to 5.45 mag (TIMMI: 3.8 mag) and 1.64 mag for the N and Q bands, respectively. The N band image from TIMMI was obtained under poor weather conditions which prevented a reliable photometric calibration. Since the images obtained with MANIAC have a better signal-to-noise ratio than those from TIMMI, we mainly present the MANIAC images in this paper. The astrometry of the Q band image (MANIAC) is tight to that of the N band image by standard-star observations. The N and Q band images were taken immediately after each other.

2.3. 1.3 mm continuum observations

The 1.3 mm continuum observations (HPBW: =23") were performed in March 1997 at the 15-m SEST telescope on La Silla, Chile using the single-channel facility bolometer system (Kreysa 1990, Thum et al. 1992). Six separate maps (31 rows and 38 columns) were obtained with the "double beam" technique (Emerson et al. 1979). The map rows were generated by scanning the target region continuously along the direction of the beam separation with scanning velocities and elevation spacings between adjacent scans of 8"/sec and 8", respectively. To generate the dual beams, a focal plane chopper operating at 6 Hz with a chopper throw of 68" was used. After correction of the atmospheric extinction, double-beam maps were created from the raw data scans. The double-beam maps were restored into single-beam maps (5.0´ x 4.0´) using the Emerson-Klein-Haslam algorithm (Emerson et al. 1979). The individual maps were averaged and transformed from the horizontal into the equatorial coordinate system.

Maps of the planet Uranus obtained every day with the same technique and parameters as used for the target served for a primary calibration, adopting a brightness temperature of 96 K (Griffin & Orton 1993). The atmospheric transmission was measured by skydips every two hours. Telescope pointing and focus were checked on quasars in similar time intervals. The pointing was found to be repeatable within 5". The flux density calibration factor was computed by integrating over the main antenna beam in the planet maps. Due to the limited accuracy of the adopted planet temperature and uncertainties in the flux integration procedure, the total relative uncertainty of the flux calibration is 20%. For a distance of 1.2 kpc the linear resolution of the millimetre map is about 0.13 pc or 27000 AU.

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999
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