## 3. Comparison with spectroscopic parallaxThe analysis of the stellar spectra gives values for the effective
temperature () and the surface gravity where
We have chosen to make the comparison by first converting the Hipparcos distance into a surface gravity. The results are given in Table 3, where the second column gives the Hipparcos distance, the third column the effective temperature of the star. The fourth column gives the gravity determined from Eq. (1), assuming M = 0.6 M . The values of T given are taken from the spectroscopic determinations. The values could have an error of 30%, which is the difference between what Hoare et al. (1995) and Mendez et al. (1992) give for the central star of NGC 1360. Incidentally, Hoare et al. (1995) notice the difference, and explain it by noting that the model profiles for the H line from the = 80 000K and = 110 000K models are almost identical, and state that it was pure chance that Mendez et al. picked the lower temperature. Hoare et al. were able to distinguish between the two values by fitting more than 6 Balmer line profiles. In the case of the other two central stars only a single value of temperature is available. Since in the analysis the temperature and the gravity are coupled, if one is wrong, so is the other. If the gravity should be increased, the temperature should be increased as well.
The values of surface gravity reported in the literature are given in the last column of Table 3. Those from Mendez et al. (1988a and 1992) and Herrero et al. (1990) are found by fitting the observed H line profile to an NLTE model. The range of values given by Hoare et al. (1995) are from comparing ROSAT X-ray measurements with NLTE models, none of which was completely satisfactory. The value given by Hoare et al. (1996) is from a comparison of EUV (70 to 760 ) measurements with NLTE models. The conclusions which can be drawn from Table 3 are subtle.
Each of the PN will be discussed separately. Consider first
NGC 1360. Here the parallax error is large and the gravity has
sufficient error to encompass any of the literature values listed in
Table 3 for this object. However, we feel that the evidence
points to a more likely value of log The parallax of A 36 is better determined although the error is
still large: the distance being between 150 pc and 600 pc. Even at the
largest distance the derived gravity (log © European Southern Observatory (ESO) 1998 Online publication: November 24, 1997 |