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Astron. Astrophys. 357, 255-267 (2000)

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2. Observations

Imaging polarimetry of [FORMULA] Carinae was obtained with the ADONIS Adaptive Optics instrument mounted at the F/8.1 Cassegrain focus of the ESO 3.6m telescope. ADONIS is the ESO common-user adaptive optics instrument; it employs a 64 element deformable mirror and wave-front sensor (Beuzit et al. 1997). For the observations of [FORMULA] Car, a Reticon detector was used as the wave-front sensor. The camera is SHARP II - a Rockwell 2562 HgCdTe NICMOS 3 array (Hofmann et al. 1995). The pixel scale was chosen as 0.050[FORMULA], giving a field of view of 12.8[FORMULA]12.8". Although this field does not encompass the full extent of the Homunculus nebula it was chosen so that well-sampled diffraction limited imaging would be possible in at least the H and K bands. Table 1 lists the observations; the two orientations, referred to as A and B, had [FORMULA] Car in the lower right and upper left of the array respectively (east is up; north to the right) enabling full coverage of the Homunculus. The narrow band 2.15 µm (henceforth Kc) observations were made with [FORMULA] Car in the centre of the array. For each filter and orientation, exposures were made at 8 position angles of the polarizer: 0.0, 22.5, 45.0, 67.5, 90.0, 112.5, 135.0, 157.5o in sequence. In addition a repeat of the 0o exposure was made with the polarizer angle set to 180o in order to check the photometry and repeatability of the polarimetric measurements. Each full sequence of polarization measurements at the 8+1 position angles was repeated as specified in Column 6 of Table 1. Offset sky chopping was employed and the relative position of the offset sky is listed in Column 5 of Table 1; the exposure on sky was equal to the on-source time. As discussed in Ageorges & Walsh (1999) it was not possible to calculate reliable K band polarimetry; these data will therefore only be discussed in terms of their high resolution imaging. On 1996 March 03 due to a technical problem, the computer control of the chopper malfunctioned and the offset sky had to be observed subsequent to each sequence of polarizer angles, and in some cases the exposure time on source was greater than on background sky. The last column of Table 1 gives an indication of the external seeing as measured by the Differential Image Motion Monitor at La Silla during the observations. Photometric standards were not observed and no attempt has been made to determine accurate magnitudes in the J, H, K and Kc filters.


Table 1. ADONIS Polarimetry Observations of [FORMULA] Carinae

Polarized and unpolarized standards were observed in the course of the observations to determine the instrumental polarization and any rotation of the instrumental plane of polarization. The star HD 64299 which is relatively close to [FORMULA] Car was observed at J, H and K as an unpolarized standard (polarization 0.15% at B (Turnshek et al. 1990) and assumed to be low in the IR, although the actual values are not measured) and a point source for deconvolution. In the Kc filter, the star HD 94510 was observed. These stars were chosen primarily to be bright but not so bright as to saturate the SHARP II camera. ADONIS observations of these stars and the polarized reference sources are fully described in Ageorges & Walsh (1999), where the observational details are also given.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000