By comparing the spectra of the two objects to spectra of the well known main-sequence B star HR 6588 (that were obtained with the same equipment) we identified lines of O II, N II, C II (only PG 1704+222), Si II, Si III (only PG 1323-086), Mg II, and Al III (only PG 1323-086). We measured equivalent widths (resp. upper limits) for those lines in the object spectra and in the spectrum of HR 6588.
A differential abundance analysis is performed using HR 6588 as a reference star. This obviously is not an ideal choice, since HR 6588 is a main sequence star. Although its is similar to those of our programme stars, its gravity is considerably higher.
From model atmospheres for the appropriate values of effective temperature and surface gravity (see Table 1) we calculated curves of growth for the elements mentioned above, from which we then derived abundances. The number of spectral lines of any ion is insufficient to derive microturbulent velocities for the programme stars. Therefore we adopted a value of 15 km/sec, which is in good agreement with microturbulent velocities derived for other post-AGB stars (McCausland et al., 1992, see also Table 4). A change of 5 km/sec in the microturbulent velocity results typically in a change of 0.05 dex or less in abundance. Even if we decrease the microturbulent velocity to 5 km/sec the abundances increase by 0.08 to 0.2 dex only. A change in by 1000 K results in abundance changes of less than 0.1 dex (except for O II: 0.2 dex). Table 2 lists the abundances derived from individual lines for each star.
Table 2. The measured equivalent widths and derived abundances for the programme stars and the reference star HR 6588 ( denotes the particle numbers of the respective element with log = log(X/H)+12.00). For HR 6588 we used the same microturbulent velocity as Hambly et al. (1997, 5 km/s).
Table 3. The mean abundances of the programme stars
All measured metals are heavily depleted with respect to solar abundances, carbon showing the largest depletion in both stars, nitrogen and oxygen the lowest depletions. The abundance patterns resemble very closely that of BD +332642 (Napiwotzki et al., 1994), a central star of a halo planetary nebula (halo CSPN), as well as those of PHL 1580 and PHL 147, two post-AGB stars analysed by Conlon et al. (1991) (see Fig. 4). Note, however, that other B type PAGB stars show a much larger scatter in abundance patterns (see Table 4).
Table 4. The abundances of PG 1323-086 and PG 1704+222 compared to those of other PAGB stars of spectral type B.
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998