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Astron. Astrophys. 337, 253-260 (1998)

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4. Position on the HR diagram

The luminosity of the stars is calculated by using the relation given by Maciel and Cazetta (1994):

[EQUATION]

For the sdO nucleus of LoTr5, we adopt a value of 14.9 for the magnitude. We give the value of the luminosity for the 13 hot nuclei (Table 4, col. 5) with an estimation of the uncertainty, due essentially to the errors on the distance.


[TABLE]

Table 4. Physical parameters of the central stars of planetary nebulae ordered by decreasing temperatures. The mark S designs small PN (diam.[FORMULA] 5 arcsec). The mark * is given for PN with negative parallaxes. The last 6 lines refer to the cool companions of the true central stars and the pulsating nucleus of He1-5, and are not plotted on Fig. 2.


The temperatures T* of the central stars are taken from different sources. We use the temperatures determined by the Zanstra-method (Gleizes et al., 1989), or/and the the energy-balance method (Preite-Martinez et al., 1991). For the G-K nuclei, we adopt the value T = 5300[FORMULA]200 K given by Thevenin and Jasniewicz (1997) for A 35, and log T = 3.7 by van Genderen et al. (1995) for FG Sge after 1980. For LoTr 5, Jasniewicz et al. (1994 a,b) give the temperature both of the sdO star (150 000 K) and of the G star (5 030 K).

Kinematic ages are derived from the diameter and the expansion velocity, taken from Acker et al., 1992 completed with velocities determined by Hajian et al. (1995), by Masson (1989) for He 2-438, by Miranda (1995) for Hu 2-1. The expansion velocities show a large spread. As found by Weinberger (1989) and Gurzadyan and Terzian (1991), the distribution of the velocities shows a maximum near 15 km[FORMULA]s-1; PN at high galactic latitude and distance z show faster expansion (30-40 km[FORMULA]s-1). For PHL 932, SaSt 2-12 and He 2-36, we adopt the usual value of 15 km[FORMULA]s-1 for the expansion velocity.

Theoretical ages are deduced from the T/L diagram (Fig. 2). Note that, for BD+[FORMULA] 3639, an exhaustive study done by Siebenmorgen et al. (1994) leads to the theoretical age of 900 years after the end of the AGB mass-loss.

[FIGURE] Fig. 2. HR diagram of the central stars of PN (data from Table 4). The theoretical curves are taken from Blocker (1995). The lines of constant age are shown and marked in units of 103 years. Small nebulae are identified by filled squares. Negative parallaxes are identified by crosses.

Table 4 presents the 7 hot nuclei with a distance given by Hipparcos, 4 nuclei of small PN, the 2 [late-WC] nuclei, 6 cool stars belonging mostly to binary systems.

The following data are given:

  • col 1: name of the PN;

  • col 2: spectral type of the central stars;

  • col 3: [FORMULA], taken from Tylenda et al, 1992;

  • col 4: log[FORMULA];

  • col 5: luminosity;

  • col 6: expansion velocity in km[FORMULA]s-1;

  • col 7: diameter (pc) of the nebula, deduced from the angular diameter;

  • col 8: kinematic age (in [FORMULA] yrs), and, in bracket, the theoretical age.

The following comments could be given:

  • NGC 246, NGC 1360, NGC 2392 show high temperatures and luminosities, which should imply fairly massive nuclei ([FORMULA]), and relatively short theoretical ages. We see that the kinematic ages, calculated with a constant expansion velocity assumption, appear longer for these bright nuclei, surrounded by large nebulae. This should imply that the expansion is faster in the first years, after the onset of the fast wind.

  • SwSt 1 and Hu 2-1, as discussed, have very small nebulae. The case of PHL 932 was already discussed in Pottasch and Acker (1998): following Mendez et al. (1988), this star surrounded by a large nebula is not a post-AGB star and its mass must be much lower than the mass of any post-AGB star.

  • The luminosity of the cool stars agrees with the previous spectral classification, excepting SaSt 2-12. This star shows a too low luminosity for a main sequence star with a temperature of about 8 000 K, and could be a metal deficient subdwarf, companion of the ionizing nucleus.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998
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