3. Physical parameters
For both stars Strömgren photometry is available from the literature (Wesemael et al. 1992), which can be used in combination with the observed Balmer line profiles to derive effective temperatures and surface gravities for these objects.
and were derived from the reddening-free indices [c1] and [u-b]. Since post-AGB stars tend to show sub-solar metallicities (see McCausland et al., 1992) we used theoretical colours for metallicity -1 (Napiwotzki & Lemke, priv. comm., a recent extension of the Napiwotzki et al., 1993, calibration to lower metallicities). We also fitted theoretical ATLAS9 Balmer line profiles for metallicity -1 and solar helium abundance to the to lines (keeping fixed), which yielded another set of , values. The intersection of the curves shown in Fig. 2 resulted in the parameters listed in Table 1. The He I lines are fitted very well by these models, suggesting a close to solar helium abundance.
Table 1. The physical parameters for the programme stars and the calibration star.
As a cross check we also derived the physical parameters from the line profiles only, using the routines of Bergeron et al. (1992) and Saffer et al. (1994), which employ a test. We computed model atmospheres using ATLAS9 (Kurucz 1991, priv. comm.) and used the LINFOR program (developed originally by Holweger, Steffen, and Steenbock at Kiel university) to compute a grid of theoretical spectra, which include the Balmer lines to H22 and He I lines. We fitted to and the He I lines 4026, 4121, 4388, 4437, 4471 simultaneously. As can be seen from Table 1 the values are very similar for both procedures.
A third approach to derive is to use the Si II/Si III ionization equilibrium, which is applicable only to PG 1323-086 and results in =17500 K, somewhat higher than the other values. This is a well known phenomenon discussed e.g. by Hambly et al. (1997), who derive 21000 K for HR 6588 from the Si II/Si III ionization equilibrium as opposed to 17700 K from Strömgren photometry.
For the further analysis we use the values derived from Strömgren photometry and Balmer lines, as the temperature information is more reliably described by Strömgren photometry at the low gravities involved, and keep the helium abundance fixed at = -1.00.
In Fig. 3 the physical parameters are compared to post-AGB tracks from Schönberner (1983) and Blöcker & Schönberner (1990). Also shown are post-AGB stars analysed elsewhere (McCausland et al., 1992, Conlon et al., 1993b, and references therein). It can be clearly seen that PG 1323-086 and PG 1704+222 are well described by post-AGB tracks with masses between 0.546 and 0.565 .
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998