2. PG 1618+563: A new spectral analysis
The binary star PG 1618+563 (hereafter PG 1618) was observed at the Calar Alto Observatory in July 2, 1988; the medium resolution (2.5 Å) spectra obtained cover the range 4100 to 5000 Å. Details on the observations and data reduction were presented by Moehler et al. (1990). The (relatively) flux calibrated spectra of PG 1618+563A and B (the sdB star), hereafter PG 1618A and B, are displayed in Fig. 1. Weather conditions did not allow to derive a reliable absolute flux scale from our narrow slit spectra.
Both stars are equally bright at 4250 Å, which is consistent with their B magnitudes being similar (see Table 1 in the next section). We classify PG 1618A as a F3 star according to the ratio of the G-band and H line strengths. In the spectrum of the sdB star PG 1618B, the Balmer lines H and H can be measured (H is at the edge of the spectrum) and the He I lines 4471 Å, 4713 Å, 4922 Å and He II 4686 Å are detectable. He I 4388 Å is marginally visible.
Table 1. magnitudes
A grid of synthetic spectra derived from H-He line blanketed NLTE model atmospheres (Napiwotzki 1997) was matched to the data (see Fig. 2) to simultaneously determine effective temperature, gravity and helium abundance (Heber et al. 1999b). The results are: =33 900 K, log g =5.80, log (N(He)/N(H))=-1.6. While the formal statistical (1) errors of the fitting procedure are relatively small (()=500 K, (log g )=0.09 dex, (log (N(He)/N(H))=0.1 dex), the fit is not perfect. H and H, as well as He I 4471 Å , are well reproduced by the model spectrum; but He I 4922 Å and He II 4686 Å are weaker than observed. Therefore systematic errors (e.g. flat fielding, continuum placement, neglect of metal line blanketing etc.) contribute significantly to the error budget and the total errors are estimated as: ()=1500K, (log g )=0.2 dex, (log(N(He)/N(H)))=0.2 dex.
Comparing its atmospheric parameters to those of the other pulsating sdB stars, PG 1618B turns out to be almost a twin of PG 1219+534, discovered by Koen et al. (1999b). Heber et al. (2000) carried out a detailed quantitative spectral analysis of the latter, based on high resolution Keck spectra, and derived =33200K, log g =5.93, log (N(He)/N(H))=-1.6 using the same technique and model atmospheres as applied here for PG 1618B. Besides their similar effective temperature and gravity, the helium abundance in both stars is higher than in all other known pulsators. Moreover, Heber et al. (2000) pointed out that there is a mismatch between the Balmer line profiles and the helium lines for PG 1219+534: from the helium ionization equilibrium =35200K, log g =6.03, log (N(He)/N(H))=-1.41 were derived. For PG 1618B the mismatch of He I 4922 Å and He II 4686 Å in our model fit (see above) may point towards a similar problem and the comparison with PG 1219+534 is used to estimate systematic errors of our spectral analysis. However, a high resolution spectrum of PG 1618B is required to address this question.
The results of the spectroscopic analysis place the star well inside the region of the (, log g ) plane, for which pulsation instabilities are predicted (Charpinet et al. 1997) and where most of the known pulsating sdB stars, indeed, have been found (Koen et al. 1999a). Therefore, PG 1618B was selected for our time-series monitoring program from an ongoing spectroscopic study of hot subluminous stars drawn from the Palomar Green (Green, Schmidt & Liebert 1986) and the Hamburg Schmidt (Heber et al. 1999b) surveys.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000