4 Chemical abundances

Seven programme stars (PG 0122+214, PG 1511+367, PG 1533+467, PG 1610+239, PG 2219+094, HS 1914+7139 and SB 357) display highly broadened lines (due to rotation, see Table 2). Only the strongest metal lines (e.g. C  II 4267 Å, Mg  II 4481 Å) could be identified. Therefore it was impossible to perform a detailed abundance analysis.

The equivalent widths were measured employing the nonlinear least-squares Gaussian fitting routines in MIDAS with central wavelength, central intensity and full width at half maximum as adjustable parameters. For metal lines located in the wings of Balmer or helium lines an additional Lorentzian function is used to describe the line wings of the latter.

Metal lines of the species C  II, C  III, N  II, O  II, Ne  I, Mg  II, Al  II, Al  III, Si  II, Si  III, P  III, S  II, S  III, Ar  II and Fe  III were identified in the sharp-lined spectra of BD-15$^\circ$115, PHL 159 and PHL 346. The atomic data for the analysis were taken from several tables:

1.

CNO from Wiese et al. (1996);

2.

Fe from Kurucz (1992) and Ekberg (1993);

3.

Ne, Mg, Al, Si, S, P, Ar from Hirata et al. (1995).

Figure 3: Positions of the programme stars (filled circles) in a ( ${T_{\rm eff}}$, $\log {g}$) diagram with evolutionary tracks calculated by Schaller et al. (1992) for determining the masses and evolution times.

Figure 4: LTE abundances (relative to $\iota$ Her) and errors of the programme stars. Abundances derived from singly ionized elements are shown as filled circles and from doubly ionized ones as filled triangles.

Figure 5: Like Fig. 4: LTE abundances (relative to $\iota$ Her) for PHL 159. Abundances derived from neutral elements are shown as open circles, from singly ionized ones as filled circles and from doubly ionized ones as filled triangles.

The LTE abundances were derived by using the classical curve-of-growth method and the LINFOR program (version of Lemke, see above). In this case the model atmospheres were generated for the appropriate values of effective temperature, gravity and solar helium and metal abundance with the ATLAS9 program of Kurucz (1992).

Then we calculated curves of growth for the observed metal lines, from which abundances were derived. Blends from different ions were omitted from the analysis. In the final step the abundances were determined from a detailed spectrum synthesis (using the LINFOR code described above) of all lines measured before. The results of the LTE abundance analysis and the rms errors for PHL 346 and BD-15$^\circ$115 are shown in Table 4 and compared with other analyses and for PHL 159 in Table 3. Besides the statistical rms errors (given in Tables 3 and 4) the uncertainties in ${T_{\rm eff}}$, $\log {g}$ and microturbulent velocity (see below) contribute to the error budget. In order to minimize the systematic errors we use the B-type star $\iota$ Her as a comparison star. This star has been analysed by Hambly et al. (1997). We redetermined the LTE abundances of $\iota$ Her using the same atomic data, model atmosphere and spectrum synthesis code as for our programme stars and took the equivalent widths measured by Hambly et al. (1997).

Our results for $\iota$ Her agree to within 0.1dex with those of Hambly et al. (1997) except for C  II (0.12dex), Si  III (0.17dex), S  III (0.21dex) and Fe  III (0.36dex). In particular our statistical error for Fe  III is much lower than that of Hambly et al. (1997). These differences can be attributed to different oscillator strengths used.

Results are given in Tables 3 and 4 and systematic errors are adopted for our programme stars as well. These errors are incorporated in the error bars plotted in Figs. 4 and 5.

  

Table 3: LTE abundances of PHL 159 compared with $\iota$ Her as a comparison star and the range of LTE abundances from 21 B-type stars analysed by Kilian (1994). For $\iota$ Her the abundances are determined with the equivalent widths from Hambly et al. (1997). For $\iota$ Her systematic errors due to uncertainties of atmospheric parameters have been determined in this work and are listed in parentheses.

$$\begin{tabular}{\vert l\vert r@{$\,\pm\,$}rr\vert r@{$\,\pm\,... ...) & \multicolumn{3}{c\vert}{7.09--7.80} \\ \hline \end{tabular}$$

References: $\rm K94=Kilian$ (1994),
(1) See text for a discussion of the Ne abundances.

  

Table 4: Comparison of LTE abundances for PHL 346 and BD-15$^\circ$115 with results from literature. The number of spectral lines used is given in brackets. Errors for the programme stars are statistical errors only.

$$\begin{tabular}{\vert l\vert r@{$\,\pm\,$}rr\vert\vert r@{$\,... ...& 7.10 & 0.04 & (2) & 7.20 & 0.40 & (3) \\ \hline \end{tabular}$$

References: $\rm K94=Kilian$ (1994); $\rm R96=Ryans$ et al. (1996); $\rm C92=Conlon$ et al. (1992).

The determination of elemental abundances is interlocked with the microtubulent velocity $\xi$. This can be derived if a sufficient number of lines of one ion can be measured over a wide range of line strengths. In our programme stars N  II and O  II lines are most suitable for this purpose since many lines of these ions can be identified. Microturbulent velocities of $\xi$ = 8 km $\rm {s^{-1}}$ were found for PHL 159 and BD-15$^\circ$115, while a rather high value of $\xi$ = 23 km $\rm {s^{-1}}$ was deduced for PHL 346. Our results for PHL 346 and BD-15$^\circ$115 are somewhat larger than those derived by Ryans et al. (1996) and Conlon et al. (1992), see Table 4.

Remarkable is the large difference ($\approx$1.0dex) between the Si  II and Si  III abundances. This has been found in several analyses of the comparison star $\iota$ Her as well (Hambly et al. 1997, 0.67dex). In a differential analysis these systematic errors cancel to a large extent. NLTE effects are small for all elements ($\le$0.1dex, Kilian 1994) except for Ne  I. As demonstrated by Auer & Mihalas (1973) LTE calculations overestimate the neon abundance. They carried out NLTE calculations for Ne  I in $\iota$ Her and derived a neon abundance (close to solar) which is lower by 0.60 dex than our LTE result. Therefore our absolute Ne abundances are overestimated. The abundances of the programme stars with respect to $\iota$ Her are plotted in Fig. 4.

1.

BD-15$^\circ$115 All abundances are in good agreement with $\iota$ Her (to within error limits) except for Mg and Fe which are underabundant by about 0.2dex. Our results are in accordance with those of a previous analysis of the star by Conlon et al. (1992, see Table 4), except for Si for which the authors derive a considerably larger abundance.

2.

PHL 346 Abundances of C, N, O, Al, Si, S, Ar and Fe are in good agreement with those in $\iota$ Her. P is enriched by about 0.15dex, Mg is depleted by about 0.15dex. Our results agree well with those derived by Ryans et al. (1996) except for Si  II and Fe  III. For the latter our result has a much smaller error.

3.

PHL 159 Mg, Al and S are significantly depleted and O enriched by 0.3dex, whereas the other elements are in good agreement with those of the comparison star.

Spectral analyses of massive B-type stars in open clusters as well as in the field (e.g. Gies & Lambert 1992; Kilian 1994; Cunha & Lambert 1994) have revealed considerable variations of metal abundances from star to star (even within an open cluster). Kilian (1994) carried out spectral analyses of 21 B-type stars in two open clusters and in the field and determined abundances of C, N, O, Ne, Mg, Al, Si, S, and Fe. We compare our results for PHL 346, BD-15$^\circ$115, PHL 159 and $\iota$ Her to her LTE results in Tables 3 and 4. Since her programme stars are somewhat hotter than ours, the Ne abundance is based on Ne  II lines, whereas we had to use Ne  I lines. Correcting for the significant NLTE effect on Ne  I (0.56 dex, see above) the neon abundance of PHL 159, the only programme star for which it has been measured, is found to be consistent with Kilian's distribution. The abundances we derived for all metals of PHL 346, BD-15$^\circ$115 and $\iota$ Her lie well within Kilian's distribution indicating that they are bona fide main sequence B-type stars. For PHL 159, however the O  II abundance is higher and the Mg  II abundance lower than in Kilian's distribution, whereas the other metals are consistent with that distribution. Therefore PHL 159 might either be a massive B-type star with rather peculiar abundances of the elements O and Mg or an evolved, low mass B-type star that mimics a massive B-type star quite closely.