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Astron. Astrophys. 348, L25-L28 (1999)

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2. Observation and data reduction

A high resolution optical spectrum of PG 1605+072 was obtained with the HIRES echelle spectrograph (Vogt et al. 1994) on the Keck I telescope on July 20, 1998 using the blue cross disperser to cover the full wavelength region between 3700 Å and 5200 Å at a resolution 0.09 Å.

The exposure time (600s) is longer than the pulsational periods (206-573s). The standard data reduction as described by Zuckerman & Reid (1998) resulted in spectral orders that have a somewhat wavy continuum. In order to remove the waviness we used the spectrum of H1504+65 (a very hot pre-white dwarf devoid of hydrogen and helium, Werner 1991) which was observed in the same night. Its spectrum has only few weak lines of highly ionized metals in the blue (3600-4480 Å) where the strong Balmer lines are found in the sdB stars. Therefore we normalized individual spectral orders 1 to 20 (3600-4480 Å) of the sdB stars by dividing through the smoothed spectrum of H1504+65. The remaining orders were normalized by fitting the continuum with spline functions (interpolated for orders 26 and 27 which contain H[FORMULA]). Judged from the match of line profiles in the overlapping parts of neighboring orders this procedure worked extremely well. Atmospheric parameters determined from individual Balmer lines are found to be consistent with each other except for H[FORMULA]. Therefore, we excluded H[FORMULA] from the fit procedure. Moreover, the resulting [FORMULA] and [FORMULA] are also in excellent agreement with those from the fit of a low resolution spectrum obtained at the ESO NTT (provided by S. Moehler). Weak lines of C, N, O, Ne, Mg, Si, and Fe can also be identified.

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© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999