Astron. Astrophys. 347, 942-948 (1999)

7. The visual spectrum

Many spectra have been taken in the "visual". The principal difficulty is that the precise position is often not given, and the aperture is usually different, or not given. Thus if the spectrum varies strongly with position, it is difficult to form a composite spectrum. Happily, the spectra prove to be relatively uniform.

The 6 most extensive spectral measurements in the visual are listed in Table 6. The positions and diaphragms used are as follows. Danziger et al. (1973) used a 17" 34" diaphragm centered at the brightest point. It was aligned east-west and thus included the entire "core" region measured by ISO and the IUE. In fact, as shown in Table 7, the Danziger et al. measurements represent about 60% of the total nebular emission (as represented by the measurements of Copetti (1990) which had a diaphragm which included the entire nebula). The measurements of Bohigas (1994) were made with a 10" 10" diaphragm, centered both at the "core" and at the brightest region. In Table 6, the measurements made at the "core" are given, but there is no difference between the spectra at the two positions to within the accuracy of the measurements. The spectra taken by Acker et al. (1989b) refer to a 4" 4" region, probably at a bright position. As can be seen from Table 7, only about 10% of the nebular radiation falls within this diaphragm. The spectrum reported by Oliva (1998, private communication, see also Reconditi & Oliva, 1993 and Oliva et al., 1996) was taken with a slit, whose width was 1.5 arcsec. The intensity extracted was averaged over the central 7 arcsec. Aller et al. (1981) measured with a slit in the "brightest region", but further details are not given. In de Freitas Pacheco et al. (1991) a position is reported which is so far from the PN that it must be wrong. The actual position, as well as the diaphragm used, is thus unknown.

Table 6. Observed Visual Spectrum. The Normalized Unreddened Intensity is in units of 10^-12 erg cm^{- 2} s^-1

Table 7. Measured H and OIII line intensities

In spite of the variations in position measured and diaphragm size used, a surprisingly good agreement in the measured spectrum is seen in Table 6. This fact, coupled with the measurement by Bohigas that no substantial spectral differences between the "core" and the brightest region are found, provide the basis for giving an "average" visual spectrum. This average spectrum has been corrected for extinction and normalized in the same way as described in the previous section for the IUE spectrum. The corrected, normalized spectrum is given in the last column of Table 6.

© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999
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