4. CNO abundances
The abundances for light elements can be found in Table 1 and are displayed in Fig. 3. The column entitled "method" stands for the method of effective temperature and surface gravity determinations and the values are taken from Zboril et al. (1994) and Zboril et al. (1997). Fundamental parameters were derived by means of spectroscopy from hydrogen line profiles and using multi-colour photometry (Geneva, Strmgren, UBV, whichever possible) in the case of high-resolution spectra where the hydrogen line profiles had not been observed. The spectral line profiles used in the abundance analysis were examined for atmospheric parameters sensitivity and turned out to be sensitive (Fig. 2). Therefore, we derived relevant partial derivatives by running a programme to obtain detailed line profiles for a number of model atmospheres. Subsequently, we used the standard formula of error propagation to derive the overall error bars. The gross effect on error bars was the following: 6-7% due to noise in the case of high-resolution data and three times that value for low-resolution data, as well as a decrease of the error on abundance with increasing effective temperature, especially for carbon. The values (1200 K, 0.2) as Teff and error bars were accepted to express the possibilities of current techniques. Remarkably, errors on abundances from low-resolution data have reached considerable values for weak photospheric lines in stellar spectra.
Table 1. Elemental abundances in He-rich stars (upper part of the table) and in normal, reference stars (lower part). The reference normal stars, solar abundances, mean values from Kilian (1992) and ours are listed at the very bottom. The DM numbers obey the HD rule. In the second column, 4-digit numbers are HR ones and 5-digit numbers are HIP ones, unless a letter indicates otherwise; the letter S stands for SAO and N for NGC. Total error bars on abundances in percent. The symbol stands for Walborn's data.
The spread of light-element abundances (C, N, O) is similiar to the one found by previous authors (e.g. Osmer and Peterson 1974). These authors argued on significant scatter of light element abundances in He-rich stars. The spread in our sample in its purest form was checked by a working and temporary plot, namely vs. equivalent width. While the index may change typically up to 0.03 as some He-rich stars are variable, especially for high-resolution data the equivalent width increment for the same colour reaches 40%. However, the error bars on equivalent width are within 6-7%. Further complementary material also covering possible phase variations, as well as magnetic field measurements are necessary to reach more conclusive statements. Heavier elements such as magnesium appear to be insensitive to effective temperature and have a solar abundance except for extreme helium-rich stars where the magnesium spectral line is located in the wing of the helium one. The oxygen abundance has been determined using the 4254 S III +O II blend in the high-resolution data (Fig. 1) and should be considered as a rough estimate only.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999