Astron. Astrophys. 355, 720-724 (2000)

2. Observations

Low resolution ultraviolet (1150Å to 3200Å) spectra of PC 11 were obtained with the SWP and LWP cameras onboard the IUE satellite using the large entrance aperture (10" 23"). The angular diameter of PC 11, as revealed by HST images available in the Data Archive taken through several narrow filters is 4". Hence, the entire nebula and the central star must have always been observed well centred within the large aperture of the IUE cameras. In fact, during the 1987, 1988 and 1989 observations the optical detector on board IUE (FES) was able to detect this 12.7 mag star, allowing a direct and hence accurate target acquisition into the science aperture. On the other hand, during the 1992 and 1994 observations, the centering of PC 11 into the large apertures was ensured by the Blind Offset technique, checking the relative guide position (found within 0.8") with respect to the nominal target coordinates. Observations were made on six occasions during the period 1987 to 1994 as reported in Table 1.

Table 1. Journal of IUE observations.

All spectra have been re-extracted from the IUE Final Archive at VILSPA which were re-processed using the IUE NEWSIPS pipeline which applies the SWET extraction method as well as the latest flux calibration and close-out camera sensitivity corrections (Garhart et al. 1997). Line-by-line images have been inspected for spurious features. In this work only the image SWP31829 has been used to represent the far-UV spectrum in 1987 since it is best exposed and since the noisy image SWP31820 shows presence of cosmics features at positions of the registered spectrum especially in the region at 1300Å and 1700Å. For the other exposures contaminated pixels on the registered spectrum have been identified (see also Fig. 1).

Fig. 1. The spectral evolution of PC 11 during seven years. From bottom to top, the merged SWP and LWP spectra since 1987 are shown. Spurious features are marked with crosses.

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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